Western United States

From Wikipedia, the free encyclopedia

Regional definitions vary from source to source. This map reflects the Western United States as defined by the Census Bureau, which includes 13 states: Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. In turn, this region is sub-divided into Mountain and Pacific areas.^[1]

Highway and open space, Northern Arizona

The Western United States, commonly referred to as the American West or simply the West, traditionally refers to the region comprising the westernmost states of the United States. Because the U.S. expanded westward after its founding, the meaning of the West has evolved over time. Prior to about 1800, the crest of the Appalachian Mountains was seen as the western frontier. Since then, the frontier moved further west and the Mississippi River was referenced as the easternmost possible boundary of the West.

The West mostly comprises arid to semi-arid plateaus and plains and forested mountains.

In the 21st century, the states which include the Rocky Mountains and the Great Basin to the West Coast are generally considered to comprise the American West.

Region and concept

The Gateway Arch in St. Louis, also known as the "Gateway to the West", commemorates the westward expansion of the United States.

Besides being a purely geographical designation, "The West" also has anthropological connotations. While this region has its own internal diversity, there is arguably an overall shared history, culture (music, cuisine), mind set or world view, and closely interrelated dialects of English. As with any region of such geographically large extent and varied cultural histories, many subregions of The American West possess distinguishing and idiosyncratic qualities.

The "West" had played an important part in American history; the Old West is embedded in America's folklore.

In its most extensive definition, the western U.S. is the largest region, covering more than half the land area of the United States. It is also the most geographically diverse, incorporating geographic regions such as the Pacific Coast, the temperate rainforests of the Northwest, the Rocky Mountains, the Great Plains, most of the tall-grass prairie eastward to Western Wisconsin, Illinois, the western Ozark Plateau, the western portions of the southern forests, the Gulf Coast, and all of the desert areas located in the United States (the Mojave, Sonoran, Great Basin, and Chihuahua deserts).

The states from the Rockies westward have something of a dual nature of semiarid steppes and arid deserts in the lowlands and plateaus, and mountains and coniferous forests in the highlands and coastal regions.

The region encompasses some of the Louisiana Purchase, most of the land ceded by Britain in 1818, some of the land acquired when the Republic of Texas joined the U.S., all of the land ceded by Britain in 1846, all of the land ceded by Mexico in 1848, and all of the Gadsden Purchase.

Arizona, New Mexico, Nevada, Colorado, and Utah are typically considered to be part of the Southwest, and Texas and Oklahoma are frequently considered part of the Southwest as well. Idaho, Montana, Oregon, Washington, and Wyoming can be considered part of the Northwest, and the addition of the Canadian province of British Columbia comprise the Pacific Northwest. There is also another region of both southwest and northwest states called the Mountain West, which is Arizona, New Mexico, Colorado, Utah, Nevada, Montana, Idaho, and Wyoming.

The West can be divided into the Pacific States; Alaska, California, Hawaii, Oregon, and Washington, with the term West Coast usually restricted to just California, Oregon, and Washington, and the Mountain States, always Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming. Alaska and Hawaii, being detached from the other western states, have few similarities with them, but are usually also classified as part of the West. Western Texas in the Chihuahuan Desert is also traditionally considered part of the Western U.S, though from a climatological perspective the West might be said to begin just west of Austin where annual rainfall drops off significantly from what is typically experienced in the East, with a concurrent change in plant and animal species.

Some western states are grouped into regions with eastern states. Kansas, Nebraska, Iowa, Missouri, Minnesota, South Dakota, and North Dakota are often included in the Midwest, which also includes states like Illinois and Wisconsin. Arkansas, Louisiana, Oklahoma, and Texas are also considered part of the South.

It is rare for any state east of the Mississippi River to be considered part of the modern west. Historically, however, the Northwest Territory was an important early territory of the U.S., comprising the modern states of Ohio, Indiana, Illinois, Michigan, and Wisconsin, as well as the northeastern part of Minnesota. Also, American sports leagues with a "Western" conference or division often have members east of the Mississippi for various reasons such as not enough true Western teams, not strictly adhering to geographic regions, etc. For example, the NBA and NHL each have a Western Conference with a member in Tennessee.

Cultural heritage institutions dedicated to the study of the American West include the Autry National Center, Briscoe Western Art Museum, and Eiteljorg Museum of American Indians and Western Art.

Demographics

• 52.8% Non-Hispanic Whites

• 28.6% were Hispanic or Latino (of any race)^[2]

• 9.3% Asian

• 4.8% of Some other race

• 4.6% Black or African American

As defined by the United States Census Bureau,^[3] the Western region of the United States includes 13 states (with a total 2010 estimated population of 71,945,553^[4]) and is split into two smaller units, or divisions:

Mountain States 

Montana, Wyoming, Colorado, New Mexico, Idaho, Utah, Arizona, and Nevada

Pacific States 

Washington, Oregon, California, Alaska, and Hawaii

However, the United States Census Bureau uses only one definition of the West in its reporting system, which may not coincide with what may be historically or culturally considered the West. For example, in the 2000 Census, the Census Bureau included the state with the second largest Hispanic population, Texas, in the South, included the state with the second largest American Indian population, Oklahoma, also in the South, and included the Dakotas, with their large populations of Plains Indians, in with the Midwest. However, it should be noted that the western half of Oklahoma and Far West Texas, are usually neither culturally, geographically, or socioeconomically identified with the South.

Statistics from the 2000 United States Census, adjusted to include the second tier of States west of the Mississippi, show that, under that definition, the West would have a population of 91,457,662, including 1,611,447 Indians, or 1.8% of the total, and 22,377,288 Hispanics (the majority Mexican), or 24.5% of the total. Indians comprise 0.9% of all Americans, and Hispanics, 12.5%. Asians, important from the very beginning in the history of the West, totaled 5,161,446, or 5.6%, with most living in the Far West. African-Americans, totaled 5,929,968, or 6.5%—lower than the national proportion (12.8%). The highest concentrations (12%) of black residents in the West are found in Texas—which is also considered a Southern state—and in California.

The West is still one of the most sparsely settled areas in the United States with 49.5 inhabitants per square mile (19/km²). Only Texas with 78.0 inhabitants/sq mi. (30/km²), Washington with 86.0 inhabitants/sq mi. (33/km²), and California with 213.4 inhabitants/sq mi. (82/km²) exceed the national average of 77.98 inhabitants/sq mi. (30/km²).

These maps from the 2000 US Census highlight differences from state to state of three minority groups. Note that most of the American Indian, Hispanic, and Asian population is in the West.

The entire Western region has also been strongly influenced by Hispanic or Latino, Asian, American Indians, and Pacific Islander; it contains the largest number of minorities in the U.S. While most of the studies of racial dynamics in America such as riots in Los Angeles have been written about European and African Americans, in many cities in the West and California, whites and blacks together are less than half the population because of the preference for the region by Hispanics and Asians. African and European Americans, however, continue to wield a stronger political influence because of the lower rates of citizenship and voting among Asians and Hispanics.

The Western United States has a higher sex ratio (more males than females) than any other region in the United States.^[5]

Because the tide of development had not yet reached most of the West when conservation became a national issue, agencies of the federal government own and manage vast areas of land. (The most important among these are the National Park Service and the Bureau of Land Management within the Interior Department, and the U.S. Forest Service within the Agriculture Department.) National parks are reserved for recreational activities such as fishing, camping, hiking, and boating, but other government lands also allow commercial activities like ranching, logging, and mining. In recent years, some local residents who earn their livelihoods on federal land have come into conflict with the land's managers, who are required to keep land use within environmentally acceptable limits.

The largest city in the region is Los Angeles, located on the West Coast. Other West Coast cities include San Diego, San Bernardino, San Jose, San Francisco, Oakland, Bakersfield, Sacramento, Seattle, Tacoma, and Portland. Prominent cities in the Mountain States include Denver, Colorado Springs, Phoenix, Tucson, Albuquerque, Las Vegas, Salt Lake City, Boise, El Paso, and Cheyenne.

Natural geography

The geography of the Western United States is split into three major physiographic divisions: the Rocky Mountain System (areas 16-19 on map), the Intermontane Plateaus (20-22), and the Pacific Mountain System (23-25).

Zion National Park in southern Utah is one of five national parks in the state.

Big Sur, California

The High Desert region of Oregon.

The quartzite of the Prospect Mountain Formation on top of Wheeler Peak, the highest peak in Nevada

Great Sand Dunes in Colorado

Grand Canyon, Arizona

Along the Pacific Ocean coast lie the Coast Ranges, which, while not approaching the scale of the Rocky Mountains, are formidable nevertheless. They collect a large part of the airborne moisture moving in from the ocean. East of the Coast Ranges lie several cultivated fertile valleys, notably the San Joaquin Valley of California and the Willamette Valley of Oregon.

Beyond the valleys lie the Sierra Nevada in the south and the Cascade Range in the north. Mount Whitney, at 14,505 feet (4,421 m) the tallest peak in the contiguous 48 states, is in the Sierra Nevada. The Cascades are also volcanic. Mount Rainier, a volcano in Washington, is also over 14,000 feet (4,300 m). Mount St. Helens, a volcano in the Cascades erupted explosively in 1980. A major volcanic eruption at Mount Mazama around 4860 BC formed Crater Lake. These mountain ranges see heavy precipitation, capturing most of the moisture that remains after the Coast Ranges, and creating a rain shadow to the east forming vast stretches of arid land. These dry areas encompass much of Nevada, Utah, and Arizona. The Mojave Desert and Sonoran Desert along with other deserts are found here.

Beyond the deserts lie the Rocky Mountains. In the north, they run almost immediately east of the Cascade Range, so that the desert region is only a few miles wide by the time one reaches the Canadian border. The Rockies are hundreds of miles (kilometers) wide, and run uninterrupted from New Mexico to Alaska. The Rocky Mountain Region is the highest overall area of the United States, with an average elevation of above 4,000 feet (1,200 m). The tallest peaks of the Rockies, 54 of which are over 14,000 feet (4,300 m), are found in central and western Colorado.

The West has several long rivers that empty into the Pacific Ocean, while the eastern rivers run into the Gulf of Mexico. The Mississippi River forms the easternmost possible boundary for the West today. The Missouri River, a tributary of the Mississippi, flows from its headwaters in the Rocky Mountains eastward across the Great Plains, a vast grassy plateau, before sloping gradually down to the forests and hence to the Mississippi. The Colorado River snakes through the Mountain states, at one point forming the Grand Canyon.

The Colorado is a major source of water in the Southwest and many dams, such as the Hoover Dam, form reservoirs along it. So much water is drawn for drinking water throughout the West and irrigation in California that in most years, water from the Colorado no longer reaches the Gulf of California. The Columbia River, the largest river in volume flowing into the Pacific Ocean from North America, and its tributary, the Snake River, water the Pacific Northwest. The Platte runs through Nebraska and was known for being a mile (2 km) wide but only a half-inch (1 cm) deep. The Rio Grande forms the border between Texas and Mexico before turning due north and splitting New Mexico in half.

According to the United States Coast Guard, "The Western Rivers System consists of the Mississippi, Ohio, Missouri, Illinois, Tennessee, Cumberland, Arkansas, and White Rivers and their tributaries, and certain other rivers that flow towards the Gulf of Mexico."^[6]

Climate and agriculture

Most of the public land held by the U.S. National Forest Service and Bureau of Land Management is in the Western states. Public lands account for 25 to 75 percent of the total land area in these states.^[7]

As a generalization, the climate of the West can be described as overall semiarid; however, parts of the West get extremely high amounts of rain and/or snow, and still other parts are true desert and get less than 5 inches (130 mm) of rain per year. Also, the climate of the West is quite unstable, and areas that are normally wet can be very dry for years and vice versa.

The seasonal temperatures vary greatly throughout the West. Low elevations on the West Coast have warm to very hot summers and get little to no snow. The Desert Southwest has very hot summers and mild winters. While the mountains in the southwest receive generally large amounts of snow. The Inland Northwest has a continental climate of warm to hot summers and cold to bitter cold winters.

Annual rainfall is greater in the eastern portions, gradually tapering off until reaching the Pacific Coast where it again increases. In fact, the greatest annual rainfall in the United States falls in the coastal regions of the Pacific Northwest. Drought is much more common in the West than the rest of the United States. The driest place recorded in the U.S. is Death Valley, California.^[8]

Violent thunderstorms occur east of the Rockies. Tornadoes occur every spring on the southern plains, with the most common and most destructive centered on Tornado Alley, which covers eastern portions of the West, (Texas to North Dakota), and all states in between and to the east.

Agriculture varies depending on rainfall, irrigation, soil, elevation, and temperature extremes. The arid regions generally support only livestock grazing, chiefly beef cattle. The wheat belt extends from Texas through the Dakotas, producing most of the wheat and soybeans in the U.S. and exporting more to the rest of the world. Irrigation in the Southwest allows the growing of great quantities of fruits, nuts, and vegetables as well as grain, hay, and flowers. Texas is a major cattle and sheep raising area, as well as the nation's largest producer of cotton. Washington is famous for its apples, and Idaho for its potatoes. California and Arizona are major producers of citrus crops, although growing metropolitan sprawl is absorbing much of this land.

Local and state government officials started to understand, after several surveys made during the latter part of the 19th century, that only action by the federal government could provide water resources needed to support the development of the West^{[citation needed]}. Starting in 1902, Congress passed a series of acts authorizing the establishment of the United States Bureau of Reclamation to oversee water development projects in seventeen western states.

During the first half of the 20th century, dams and irrigation projects provided water for rapid agricultural growth throughout the West and brought prosperity for several states, where agriculture had previously only been subsistence level. Following World War II, the West's cities experienced an economic and population boom. The population growth, mostly in the Southwest states of New Mexico, Utah, Colorado, Arizona, and Nevada, has strained water and power resources, with water diverted from agricultural uses to major population centers, such as the Las Vegas Valley and Los Angeles.

Geology

Plains make up much of the eastern portion of the West, underlain with sedimentary rock from the Upper Paleozoic, Mesozoic, and Cenozoic eras. The Rocky Mountains expose igneous and metamorphic rock both from the Precambrian and from the Phanerozoic eon. The Inter-mountain States and Pacific Northwest have huge expanses of volcanic rock from the Cenozoic era. Salt flats and salt lakes reveal a time when the great inland seas covered much of what is now the West.

The Pacific states are the most geologically active areas in the United States. Earthquakes cause damage every few to several years in California. While the Pacific states are the most volcanically active areas, extinct volcanoes and lava flows are found throughout most of the West.

History and culture

Facing both the Pacific Ocean and the Mexican border, the West has been shaped by a variety of ethnic groups. Hawaii is the only state in the union in which Asian Americans outnumber white American residents. Asians from many countries have settled in California and other coastal states in several waves of immigration since the 19th century, contributing to the Gold Rush, the building of the transcontinental railroad, agriculture, and more recently, high technology.

The border states—California, Arizona, New Mexico, and Texas—all have large Hispanic populations, and the many Spanish place names attest to their history as former Spanish and Mexican territories. Other southwestern states such as Colorado, Utah, and Nevada have large Hispanic populations as well, with many names places also attest to the history of former Mexican territories. Mexican-Americans have also had a growing population in Northwestern states of Oregon and Washington, as well as the southern state of Oklahoma.

The West also contains much of the Native American population in the U.S., particularly in the large reservations in the mountain and desert states.

Hollywood is a well-known area of Los Angeles and the symbolic center of the American film industry.

The largest concentrations for African Americans in the West can be found in Los Angeles, Oakland, Sacramento, San Francisco, Phoenix, Seattle, Las Vegas, Denver, and Colorado Springs.

Alaska—the northernmost state in the Union—is a vast land of few people, many of them native, and of great stretches of wilderness, protected in national parks and wildlife refuges. Hawaii's location makes it a major gateway between the U.S. and Asia, as well as a center for tourism.

In the Pacific Coast states, the wide areas filled with small towns, farms, and forests are supplemented by a few big port cities which have evolved into world centers for the media and technology industries. Now the second largest city in the nation, Los Angeles is best known as the home of the Hollywood film industry; the area around Los Angeles also was a major center for the aerospace industry by World War II, though Boeing, located in Washington State would lead the aerospace industry. Fueled by the growth of Los Angeles, as well as the San Francisco Bay area, including Silicon Valley, the center of America's high tech industry, California has become the most populous of all the 50 states.

Oregon and Washington have also seen rapid growth with the rise of Boeing and Microsoft along with agriculture and resource based industries. The desert and mountain states have relatively low population densities, and developed as ranching and mining areas which are only recently becoming urbanized. Most of them have highly individualistic cultures, and have worked to balance the interests of urban development, recreation, and the environment.

Newspaper Rock State Historic Monument, Utah contains petroglyphs left by the first inhabitants of the American Southwest.

Culturally distinctive points include the large Mormon population in the Mormon Corridor, including southeastern Idaho, Utah, Northern Arizona, and Nevada; the extravagant casino resort towns of Las Vegas and Reno, Nevada; and the numerous American Indian tribal reservations.

Western frontier

Major settlement of the western territories developed rapidly in the 1840s, largely through the Oregon Trail and the California Gold Rush of 1849. California experienced such a rapid growth in a few short months that it was admitted to statehood in 1850 without the normal transitory phase of becoming an official territory.^[9]
One of the largest migrations in American history occurred in the 1840s as the Latter Day Saints left the Midwest to build a theocracy in Utah.

Both Omaha, Nebraska and St. Louis, Missouri laid claim to the title, "Gateway to the West" during this period. Omaha, home to the Union Pacific Railroad and the Mormon Trail, made its fortunes on outfitting settlers; St. Louis built itself upon the vast fur trade in the West before its settlement.

The 1850s were marked by political battles over the expansion of slavery into the western territories, issues leading to the Civil War.^[10]

The history of the American West in the late 19th and early 20th centuries has acquired a cultural mythos in the literature and cinema of the United States. The image of the cowboy, the homesteader, and westward expansion took real events and transmuted them into a myth of the west which has shaped much of American popular culture since the late 19th century.^[11]

Writers as diverse as Bret Harte and Zane Grey celebrated or derided cowboy culture, while artists such as Frederic Remington created western art as a method of recording the expansion into the west. The American cinema, in particular, created the genre of the western movie, which, in many cases, use the West as a metaphor for the virtue of self-reliance and an American ethos. The contrast between the romanticism of culture about the West and the actuality of the history of the westward expansion has been a theme of late 20th and early 21st century scholarship about the West. Cowboy culture has become embedded in the American experience as a common cultural touchstone, and modern forms as diverse as country and western music have celebrated the sense of isolation and independence of spirit inspired by the frontiersmen on virgin land.^[12]

The 20th century

The advent of the automobile enabled the average American to tour the West. Western businessmen promoted U.S. Route 66 as a means to bring tourism and industry to the West. In the 1950s, representatives from all the western states built the Cowboy Hall of Fame and Western Heritage Center to showcase western culture and greet travelers from the East. During the latter half of the 20th century, several transcontinental interstate highways crossed the West bringing more trade and tourists from the East. Oil boom towns in Texas and Oklahoma rivaled the old mining camps for their rawness and wealth. The Dust Bowl forcing children of the original homesteaders even further west.^[13]

The movies became America's chief entertainment source featuring western fiction, later the community of Hollywood in Los Angeles became the headquarters of the mass media such as radio and television production.^[14]

California has emerged as the most populous state and one of the top 10 economies in the world. Massive late 19th-20th century population and settlement booms created two megalopolis areas of the Greater Los Angeles/Southern California and the San Francisco Bay Area/Northern California regions, one of the nation's largest metropolitan areas and in the top 25 largest urban areas in the world. Four more metropolitan areas of San Bernardino-Riverside, San Diego, Denver, Phoenix, and Seattle have over a million residents, while the three fastest growing metro areas were the Salt Lake City metropolitan area, the Las Vegas metropolitan area; and the Portland metropolitan area.^[15][16]

Although there has been segregation, along with accusations of racial profiling and police brutality towards minorities due to issues such as illegal immigration and a racial shift (i.e. White flight and now black flight) in neighborhood demographics, sometimes leading to racially based riots (i.e. the 1992 Los Angeles Riots and 1965 Watts Riots), the West has a continuing reputation for being open-minded and for being one of the most racially progressive areas in the United States.

Los Angeles is said to have the largest Mexican population outside of Mexico, while San Francisco has the largest Chinese community in North America and also has a large LGBT community, and Oakland, California has a large percentage of residents being African-American, as well Long Beach, California has a large Black community. The state of Utah has a Mormon majority (estimated at 62.4% in 2004),^[17] while some cities like Albuquerque, New Mexico; Spokane, Washington; and Tucson, Arizona faces Indian Reservations of American Indian tribes, and there are Alaskan Natives and Native Hawaiians to bring forth a great deal of racial diversity.

Major Metropolitan Areas

These are the largest Metropolitan Statistical Areas (MSA) in the 13 Western states with population estimates as of July 1, 2011 as defined by the United States Census Bureau:^[18]

Rank (West)	Rank (USA)	MSA	Population	State(s)
1	2	Los Angeles-Long Beach-Santa Ana MSA	12,944,801	California
2	11	San Francisco-Oakland-Fremont MSA	4,391,037	California
3	12	San Bernardino-Riverside-Ontario MSA	4,304,997	California
4	14	Phoenix-Mesa-Glendale MSA	4,263,236	Arizona
5	15	Seattle-Tacoma-Bellevue MSA	3,500,026	Washington
6	17	San Diego-Carlsbad-San Marcos MSA	3,140,069	California
7	21	Denver-Aurora-Broomfield MSA	2,599,504	Colorado
8	23	Portland-Vancouver-Hillsboro MSA	2,262,605	Oregon Washington
9	25	Sacramento-Arden Arcade-Roseville MSA	2,176,235	California
10	30	Las Vegas-Paradise MSA	1,969,975	Nevada
11	31	San Jose-Sunnyvale-Santa Clara MSA	1,865,450	California
12	48	Salt Lake City MSA	1,145,905	Utah
13	52	Tucson MSA	989,569	Arizona
14	53	Honolulu MSA	963,607	Hawaii
15	55	Fresno MSA	942,904	California
16	57	Albuquerque MSA	898,642	New Mexico
17	61	Bakersfield-Delano MSA	851,710	California
18	63	Oxnard-Thousand Oaks-Ventura MSA	831,771	California
19	76	Stockton MSA	696,214	California
20	81	Colorado Springs MSA	660,319	Colorado
21	85	Boise City-Nampa MSA	627,664	Idaho
22	93	Ogden-Clearfield MSA	555,916	Utah
23	97	Provo-Orem MSA	540,834	Utah
24	100	Modesto MSA	518,522	California
25	104	Santa Rosa-Petaluma MSA	488,116	California
26	108	Spokane MSA	473,761	Washington
27	111	Visalia-Porterville MSA	449,253	California
28	115	Reno-Sparks MSA	429,606	Nevada
29	118	Santa Barbara-Santa Maria-Goleta MSA	426,878	California
30	121	Salinas MSA	421,898	California
31	123	Vallejo-Fairfield MSA	416,471	California

Other population centers

• The MSA of El Paso, Texas, although belonging to a state considered part of the Southern United States, is also considered part of the Western United States. Its estimated population is 820,790.

• The Mexican border cities of Tijuana (part of the San Diego MSA) and Mexicali (part of the Yuma, AZ-El Centro, CA MSA) in the Mexican state of Baja California.

• The Canadian border cities of Vancouver, British Columbia (the nearest US cities are Bellingham and Port Angeles, both in Washington) and Victoria in the Canadian province of British Columbia.

• The largest MSA in Alaska is Anchorage; it has an estimated population of 387,516.

Politics

US States where state-level laws allowed legalized medicinal marijuana before 2005.

US States with legalized physician-assisted suicide

US States that have no income tax at the state level

The region's distance from historical centers of power in the East, and the celebrated "frontier spirit" of its settlers offer two clichés for explaining the region's independent, heterogeneous politics. Historically, the West was the first region to see widespread women's suffrage. California birthed both the property rights and conservation movements, and spawned such phenomena as the Taxpayer Revolt and the Berkeley Free Speech Movement. It has also produced three presidents: Herbert Hoover, Richard Nixon, and Ronald Reagan.

The prevalence of libertarian political attitudes is widespread. For example, the majority of Western states have legalized medicinal marijuana (all but Utah and Wyoming) and some forms of gambling (except Utah), Oregon and Washington have legalized physician-assisted suicide, most rural counties in Nevada allow licensed brothels, and voters in Washington and Colorado have legalized
recreational use of marijuana.^[19] There is less resistance to the legal recognition of same-sex unions: California, Colorado, Hawaii, Nevada, New Mexico, Oregon, and Washington recognize them.
The West Coast leans toward the Democratic Party. San Francisco's two main political parties are the Green Party and the Democratic Party. Seattle has historically been a center of radical left-wing politics. Both the Democratic leaders of the Congress are from the region: House Minority Leader Nancy Pelosi of California and Senate Majority Leader Harry Reid of Nevada.

Interior areas are more Republican, with Alaska, Arizona, Idaho, Montana, Utah, and Wyoming being Republican strongholds, and Colorado, Nevada, and New Mexico being swing states. The state of Nevada is considered a political bellwether, having correctly voted for every president except once (in 1976) since 1912. New Mexico too is considered a bellwether, having voted for the popular vote winner in every presidential election since statehood, except in 1976. The state of Arizona has been won by the Republican presidential candidate in every election except one since 1948, while the states of Idaho, Utah, and Wyoming have been won by the Republican presidential candidate in every election since 1964. In recent years, both Utah and Arizona have become widely recognized as the country's most conservative states.

As the fastest-growing demographic group, after Asians, Latinos are hotly contested by both parties. Immigration is an important political issue for this group. Backlash against illegal aliens led to the passage of California Proposition 187 in 1994, a ballot initiative which would have denied many public services to illegal aliens. Association of this proposal with California Republicans, especially incumbent governor Pete Wilson, drove many Hispanic voters to the Democrats.^[20]

East African Rift

From Wikipedia, the free encyclopedia

A map of East Africa showing some of the historically active volcanoes (as red triangles) and the Afar Triangle (shaded at the center), which is a so-called triple junction (or triple point) where three plates are pulling away from one another: the Arabian Plate and two parts of the African Plate—the Nubian and Somali—splitting along the East African Rift Zone.

The East African Rift (EAR) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22-25 million years ago.^[1] In the past, it was considered to be part of a larger Great Rift Valley that extended north to Asia Minor.

The rift is a narrow zone that is a developing divergent tectonic plate boundary, in which the African Plate is in the process of splitting into two tectonic plates, called the Somali Plate and the Nubian Plate, at a rate of 6–7 mm annually.^[2] As extension continues, lithospheric rupture will occur within 10 million years, the Somalian plate will break off, and a new ocean basin will form.

A series of distinct rift basins, the East African Rift System extends over thousands of kilometers.^[3] The EAR consists of two main branches. The Eastern Rift Valley includes the Main Ethiopian Rift, running eastward from the Afar Triple Junction, which continues south as the Kenyan Rift Valley.^[4] The Western Rift Valley includes the Albertine Rift, and farther south, the valley of Lake Malawi. To the north of the Afar Triple Junction, the rift follows one of two paths: west to the Red Sea Rift or east to the Aden Ridge in the Gulf of Aden.

The EAR runs from the Afar Triple Junction in the Afar Depression of Ethiopia through eastern Africa, terminating in Mozambique.^[5] The EAR transects through Ethiopia, Kenya, Uganda, Rwanda, Burundi, Zambia, Tanzania, Malawi and Mozambique. It also runs offshore of the coast of Mozambique along the Kerimba and Lacerda grabens, which are joined by the Davie Ridge, a 2200 km-long relic fracture zone that cuts across the West Somali basin, straddling the boundary between Tanzania and Mozambique.^[4] The Davie Ridge ranges between 30–120 km wide, with a west facing scarp (east-plunging arch) along the southern half of its length that rises up to 2300 m above the sea floor.^[4][6] Its movement is concurrent with the EAR.^[7]

Competing theories on geologic evolution

Over time, many theories have tried to clarify the evolution of the East African Rift. In 1972 it was proposed that the EAR was not caused by tectonic activity, but rather by differences in crustal density.^[8] Others proposed an African superplume causing mantle deformation.^[9][10] However, the varying geochemical signatures of a suite of Ethiopian lavas suggest multiple plume sources: at least one of deep mantle origin, and one from within the subcontinental lithosphere. Additionally, the subject of deep-rooted mantle plumes is still a matter of controversy, and therefore cannot be confirmed.^[11]

The most recent and accepted view is the theory put forth in 2009: that magmatism and plate tectonics have a feedback with one another, controlled by oblique rifting conditions. At that time it was suggested that lithospheric thinning generated volcanic activity, further increasing the magmatic processes at play such as intrusions and numerous small plumes. These processes further thin the lithosphere in concentrated areas, forcing the thinning lithosphere to behave like a mid-ocean ridge.^[10]

Geologic evolution

Prior to rifting, enormous continental flood basalts erupted on the surface and uplift of the Ethiopian, Somalian, and East African plateaus occurred. The first stage of rifting of the EAR is characterized by rift localization and magmatism along the entire rift zone. Periods of extension alternated with times of relative inactivity. There was also the reactivation of a pre-Cambrian weakness in the crust, a suture zone of multiple cratons, displacement along large boundary faults, and the development of deep asymmetric basins.^[3] The second stage of rifting is characterized by the deactivation of large boundary faults, the development of internal fault segments, and the concentration of magmatic activity towards the rifts.

Today, the narrow rift segments of the East African Rift system form zones of localized strain. These rifts are the result of the actions of numerous normal faults which are typical of all tectonic rift zones. As aforementioned, voluminous magmatism and continental flood basalts characterize some of the rift segments, while other segments, such as the Western branch, have only very small volumes of volcanic rock.^[11]

Petrology

The African continental crust is generally cool and strong. Many cratons are found throughout the EAR, such as the Tanzania and Kaapvaal cratons. The cratons are thick, and have survived for billions of years with little tectonic activity. They are characterized by greenstone belts, tonalites, and other high-grade metamorphic lithologies. The cratons are of significant importance in terms of mineral resources, with major deposits of gold, antimony, iron, chromium and nickel.^[12]
A large volume of continental flood basalts erupted during the Oligocene, with the majority of the volcanism coinciding with the opening of the Red Sea and the Gulf of Aden approximately 30 Ma.^[9][11] The composition of the volcanics are a continuum of ultra-alkaline to tholeiitic and felsic rocks. It has been suggested that the diversity of the compositions could be partially explained by different mantle source regions. The EAR also cuts through old sedimentary rocks deposited in ancient basins.^[13]

Volcanism and seismicity

The East African Rift Zone includes a number of active as well as dormant volcanoes, among them: Mount Kilimanjaro, Mount Kenya, Mount Longonot, Menengai Crater, Mount Karisimbi, Mount Nyiragongo, Mount Meru and Mount Elgo, as well as the Crater Highlands in Tanzania. Although most of these mountains lie outside of the rift valley, the EAR created them.^[13]

Active volcanos include Erta Ale, DallaFilla, and Ol Doinyo Lengai, the former of which is a continuously active basaltic shield volcano in the Afar Region of northeastern Ethiopia. When DallaFilla erupted in 2008 it was the largest volcanic eruption in Ethiopia in recorded history. The Ol Doinyo Lengai volcano is currently the only active natrocarbonatite volcano in the world. The magma contains almost no silica, making the flow viscosity extremely low. “Its lava fountains crystallize in midair then shatter like glass” according to the National Geographic. Approximately 50 volcanic structures in Ethiopia alone have documented activity since the onset of the Holocene.^[3]

The EAR is the largest seismically active rift system on Earth today. The majority of earthquakes occur near the Afar Depression, with the largest earthquakes typically occurring along or near major border faults.^[11] Seismic events in the past century are estimated to have reached a maximum moment magnitude of 7.0. The seismicity trends parallel to the rift system, with a shallow focal depth of 12–15 km beneath the rift axis. Further away from the rift axis, focal depths can reach depths of over 30 km.^[11][14] Focal mechanism solutions strike NE and frequently demonstrate normal dip-slip faults, although left-lateral motion is also observed.^[3]

Discoveries in human evolution

The Rift Valley in East Africa has been a rich source of hominid fossils that allow the study of human evolution.^[3][15] The rapidly eroding highlands quickly filled the valley with sediments, creating a favorable environment for the preservation of remains. The bones of several hominid ancestors of modern humans have been found here, including those of "Lucy", a partial australopithecine skeleton discovered by anthropologist Donald Johanson dating back over 3 million years. Richard and Mary Leakey have done significant work in this region also.^[16] More recently, two other hominid ancestors have been discovered here: a 10-million-year-old ape called Chororapithecus abyssinicus, found in the Afar rift in eastern Ethiopia, and Nakalipithecus nakayamai, which is also 10 million years old.^[17]

Radiometric dating

From Wikipedia, the free encyclopedia

Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks or carbon, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.^[1] The use of radiometric dating was first published in 1907 by Bertram Boltwood^[2] and is now the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials. Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale.^[3] Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.

Fundamentals of radiometric dating

Radioactive decay

Example of a radioactive decay chain from lead-212 (²¹²Pb) to lead-208 (²⁰⁸Pb) . Each parent nuclide spontaneously decays into a daughter nuclide (the decay product) via an α decay or a β⁻ decay. The final decay product, lead-208 (²⁰⁸Pb), is stable and can no longer undergo spontaneous radioactive decay.

All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including alpha decay (emission of alpha particles) and beta decay (electron emission, positron emission, or electron capture). Another possibility is spontaneous fission into two or more nuclides.

While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product. In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain, eventually ending with the formation of a stable (nonradioactive) daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years (e.g., tritium) to over 100 billion years (e.g., Samarium-147).

For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant. It is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field.^[4][5][6] The only exceptions are nuclides that decay by the process of electron capture, such as beryllium-7, strontium-85, and zirconium-89, whose decay rate may be affected by local electron density. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present.

Preconditions

Thermal ionization mass spectrometer used in radiometric dating.

The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration.^[7] Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination. In uranium-lead dating, the concordia diagram is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, a study of the Amitsoq gneisses from western Greenland used five different radiometric dating methods to examine twelve samples and achieved agreement to within 30 Ma (million years) on an age of 3,640 Ma.^[8]

Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement (except as described below under "Dating with short-lived extinct radionuclides"), the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope ratio mass spectrometry.^[9]

The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of 5,730 years. After an organism has been dead for 60,000 years, so little carbon-14 is left that accurate dating can not be established. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.^[10]

Closure temperature

If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusion, setting the isotopic "clock" to zero. The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.^[11][12] Dating of different minerals and/or isotope systems (with differing closure temperatures) within the same rock can therefore enable the tracking of the thermal history of the rock in question with time, and thus the history of metamorphic events may become known in detail. This field is known as thermochronology or thermochronometry.

The age equation

Sm/Nd isochron plotted of samples ^[13] from the Great Dyke, Zimbabwe. The age is calculated from the slope of the isochron (line) and the original composition from the intercept of the isochron with the y-axis.

The mathematical expression that relates radioactive decay to geologic time is^[11][14]

D = D₀ + N(t) (e^λt − 1)

where

t is age of the sample,

D is number of atoms of the daughter isotope in the sample,

D₀ is number of atoms of the daughter isotope in the original composition,

N is number of atoms of the parent isotope in the sample at time t (the present), given by N(t) = N_oe^-λt, and

λ is the decay constant of the parent isotope, equal to the inverse of the radioactive half-life of the parent isotope^[15] times the natural logarithm of 2.

The equation is most conveniently expressed in terms of the measured quantity N(t) rather than the constant initial value N_o.

The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems.^[12][16] However, construction of an isochron does not require information on the original compositions, using merely the present ratios of the parent and daughter isotopes to a standard isotope. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.

Modern dating methods

Radiometric dating has been carried out since 1905 when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth. In the century since then the techniques have been greatly improved and expanded.^[15] Dating can now be performed on samples as small as a nanogram using a mass spectrometer. The mass spectrometer was invented in the 1940s and began to be used in radiometric dating in the 1950s. It operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as "Faraday cups", depending on their mass and level of ionization. On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams.

Uranium-lead dating method

A concordia diagram as used in uranium-lead dating, with data from the Pfunze Belt, Zimbabwe.^[17] All the samples show loss of lead isotopes, but the intercept of the errorchron (straight line through the sample points) and the concordia (curve) shows the correct age of the rock.^[12]

The uranium-lead radiometric dating scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.^[13][18] An error margin of 2–5% has been achieved on younger Mesozoic rocks.^[19]

Uranium-lead dating is often performed on the mineral zircon (ZrSiO₄), though it can be used on other materials, such as baddeleyite.^[20] Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead. Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. In situ micro-beam analysis can be achieved via laser ICP-MS or SIMS techniques.^[21]

One of its great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost. This can be seen in the concordia diagram, where the samples plot along an errorchron (straight line) which intersects the concordia curve at the age of the sample.

Samarium-neodymium dating method

This involves the alpha-decay of ¹⁴⁷Sm to ¹⁴³Nd with a half-life of 1.06 x 10¹¹ years. Accuracy levels of less than twenty million years in two-and-a-half billion years are achievable.^[22]

Potassium-argon dating method

This involves electron capture or positron decay of potassium-40 to argon-40. Potassium-40 has a half-life of 1.3 billion years, and so this method is applicable to the oldest rocks. Radioactive potassium-40 is common in micas, feldspars, and hornblendes, though the closure temperature is fairly low in these materials, about 125°C (mica) to 450°C (hornblende).

Rubidium-strontium dating method

This is based on the beta decay of rubidium-87 to strontium-87, with a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocks, and has also been used to date lunar samples. Closure temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample.

Uranium-thorium dating method

A relatively short-range dating technique is based on the decay of uranium-234 into thorium-230, a substance with a half-life of about 80,000 years. It is accompanied by a sister process, in which uranium-235 decays into protactinium-231, which has a half-life of 34,300 years.
While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments, from which their ratios are measured. The scheme has a range of several hundred thousand years. A related method is ionium-thorium dating, which measures the ratio of ionium (thorium-230) to thorium-232 in ocean sediment.

Radiocarbon dating method

Ale's Stones at Kåseberga, around ten kilometres south east of Ystad, Sweden were dated at 56 CE using the carbon-14 method on organic material found at the site.^[23]

Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years,^[24][25] which is very short compared with the above isotopes. In other radiometric dating methods, the heavy parent isotopes were produced by nucleosynthesis in supernovas, meaning that any parent isotope with a short half-life should be extinct by now. Carbon-14, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near-constant level on Earth. The carbon-14 ends up as a trace component in atmospheric carbon dioxide (CO₂).
An organism acquires carbon during its lifetime. Plants acquire it through photosynthesis, and animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon-14, and the existing isotope decays with a characteristic half-life (5730 years).
The proportion of carbon-14 left when the remains of the organism are examined provides an indication of the time elapsed since its death. The carbon-14 dating limit lies around 58,000 to 62,000 years.^[26]

The rate of creation of carbon-14 appears to be roughly constant, as cross-checks of carbon-14 dating with other dating methods show it gives consistent results. However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon-14 and give inaccurate dates. The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon-14 by a few percent; conversely, the amount of carbon-14 was increased by above-ground nuclear bomb tests that were conducted into the early 1960s. Also, an increase in the solar wind or the Earth's magnetic field above the current value would depress the amount of carbon-14 created in the atmosphere. These effects are corrected for by the calibration of the radiocarbon dating scale.^[27]

Fission track dating method

Apatite crystals are widely used in fission track dating.

This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium-238 impurities. The uranium content of the sample has to be known, but that can be determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons. This causes induced fission of ²³⁵U, as opposed to the spontaneous fission of ²³⁸U. The fission tracks produced by this process are recorded in the plastic film. The uranium content of the material can then be calculated from the number of tracks and the neutron flux.

This scheme has application over a wide range of geologic dates. For dates up to a few million years micas, tektites (glass fragments from volcanic eruptions), and meteorites are best used. Older materials can be dated using zircon, apatite, titanite, epidote and garnet which have a variable amount of uranium content.^[28] Because the fission tracks are healed by temperatures over about 200°C the technique has limitations as well as benefits. The technique has potential applications for detailing the thermal history of a deposit.

Chlorine-36 dating method

Large amounts of otherwise rare ³⁶Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of ³⁶Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, ³⁶Cl is also useful for dating waters less than 50 years before the present. ³⁶Cl has seen use in other areas of the geological sciences, including dating ice and sediments.

Luminescence dating methods

Natural sources of radiation in the environment knock loose electrons in, say, a piece of pottery, and these electrons accumulate in defects in the material's crystal lattice structure. Heating or illuminating the object will release the captured electrons, producing a luminescence. When the sample is heated, at a certain temperature it will glow from the emission of electrons released from the defects, and this glow can be used to estimate the age of the sample to a threshold of approximately 15 percent of its true age. The date of a rock is reset when volcanic activity remelts it. The date of a piece of pottery is reset by the heat of the kiln. Typically temperatures greater than 400 degrees Celsius will reset the "clock". This is termed thermoluminescence.

Other methods

Other methods include:

• argon-argon (Ar-Ar)
• iodine-xenon (I-Xe)
• lanthanum-barium (La-Ba)
• lead-lead (Pb-Pb)
• lutetium-hafnium (Lu-Hf)
• rhenium-osmium (Re-Os)
• uranium-lead-helium (U-Pb-He)
• uranium-uranium (U-U)

Dating with short-lived extinct radionuclides

Absolute radiometric dating requires a measurable fraction of parent nucleus to remain in the sample rock. For rocks dating back to the beginning of the solar system, this requires extremely long-lived parent isotopes, making measurement of such rocks' exact ages imprecise. To be able to distinguish the relative ages of rocks from such old material, and to get a better time resolution than that available from long-lived isotopes, short-lived isotopes that are no longer present in the rock can be used.^[29]

At the beginning of the solar system, there were several relatively short-lived radionuclides like ²⁶Al, ⁶⁰Fe, ⁵³Mn, and ¹²⁹I present within the solar nebula. These radionuclides—possibly produced by the explosion of a supernova—are extinct today, but their decay products can be detected in very old material, such as that which constitutes meteorites. By measuring the decay products of extinct radionuclides with a mass spectrometer and using isochronplots, it is possible to determine relative ages of different events in the early history of the solar system. Dating methods based on extinct radionuclides can also be calibrated with the U-Pb method to give absolute ages. Thus both the approximate age and a high time resolution can be obtained. Generally a shorter half-life leads to a higher time resolution at the expense of timescale.

The ¹²⁹I - ¹²⁹Xe chronometer

¹²⁹I beta-decays to ¹²⁹Xe with a half-life of 16 million years. The iodine-xenon chronometer^[30] is an isochron technique. Samples are exposed to neutrons in a nuclear reactor. This converts the only stable isotope of iodine (¹²⁷I) into ¹²⁸Xe via neutron capture followed by beta decay (of ¹²⁸I). After irradiation, samples are heated in a series of steps and the xenon isotopic signature of the gas evolved in each step is analysed. When a consistent ¹²⁹Xe/¹²⁸Xe ratio is observed across several consecutive temperature steps, it can be interpreted as corresponding to a time at which the sample stopped losing xenon.

Samples of a meteorite called Shallowater are usually included in the irradiation to monitor the conversion efficiency from ¹²⁷I to ¹²⁸Xe. The difference between the measured ¹²⁹Xe/¹²⁸Xe ratios of the sample and Shallowater then corresponds to the different ratios of ¹²⁹I/¹²⁷I when they each stopped losing xenon. This in turn corresponds to a difference in age of closure in the early solar system.

The ²⁶Al - ²⁶Mg chronometer

Another example of short-lived extinct radionuclide dating is the ²⁶Al - ²⁶Mg chronometer, which can be used to estimate the relative ages of chondrules. ²⁶Al decays to ²⁶Mg with a half-life of 720 000 years. The dating is simply a question of finding the deviation from the natural abundance of ²⁶Mg (the product of ²⁶Al decay) in comparison with the ratio of the stable isotopes ²⁷Al/²⁴Mg.
The excess of ²⁶Mg (often designated ²⁶Mg* ) is found by comparing the ²⁶Mg/²⁷Mg ratio to that of other Solar System materials.^[31]

The ²⁶Al - ²⁶Mg chronometer gives an estimate of the time period for formation of primitive meteorites of only a few million years (1.4 million years for Chondrule formation).^[32]