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Tuesday, January 23, 2024

Climate of Hawaii

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Climate_of_Hawaii

The U.S. state of Hawaiʻi, which covers the Hawaiian Islands, is tropical but it experiences many different climates, depending on altitude and surroundings. The island of Hawaiʻi for example hosts 4 (out of 5 in total) climate groups on a surface as small as 4,028 square miles (10,430 km²) according to the Köppen climate types: tropical, arid, temperate and polar. When counting also the Köppen sub-categories – notably including the very rare cold-summer mediterranean climate – the island of Hawaiʻi hosts 10 (out of 14 in total) climate zones. The islands receive most rainfall from the trade winds on their north and east flanks (the windward side) as a result of orographic precipitation. Coastal areas are drier, especially the south and west side or leeward sides.

Overall with climate change, Hawaiʻi is getting drier and hotter. The Hawaiian Islands receive most of their precipitation from October to April. Drier conditions generally prevail from May to September. Due to cooler waters around Hawaiʻi, the risk of tropical cyclones is low for Hawaiʻi.

Temperature

Temperatures at sea level generally range from highs of 84–88 °F (29–31 °C) during the summer months to 79–83 °F (26–28 °C) during the winter months. Rarely does the temperature rise from above 90 °F (32 °C) or drop below 60 °F (16 °C) at lower elevations. Temperatures are lower at higher altitudes. During the winter, snowfall is common at the summits of Mauna Kea and Mauna Loa on Hawaiʻi Island. On Maui, the summit of Haleakalā occasionally experiences snowfall, but snow had never been observed below 7,500 feet (2,300 m) before February 2019, when snow was observed at 6,200 feet (1,900 m) and fell at higher elevations in amounts large enough to force Haleakalā National Park to close for several days. The record low temperature in Honolulu is 52 °F (11 °C) on January 20, 1969.

Overall with climate change, Hawaiʻi is getting hotter. Temperatures of 90 °F (32 °C) and above are uncommon, with the exception of dry, leeward areas. In the leeward areas, temperatures may reach into the low 90s several days during the year, but temperatures higher than these are unusual. The highest temperature ever recorded on the islands was 100 °F (38 °C) on April 27, 1931, in Pāhala. The surface waters of the open ocean around Hawaiʻi range from 75 °F (24 °C) between late February and early April, to a maximum of 82 °F (28 °C) in late September or early October. In the United States, only Florida has warmer surf temperatures.

The Pacific High, and with it the trade-wind zone, moves north and south with changing angle of the sun, so that it reaches its northernmost position in the summer. This brings trade winds during the period of May through September, when they are prevalent 80 to 95 percent of the time. From October through April, the heart of the trade winds moves south of Hawaiʻi; thus there average wind speeds are lower across the islands. Due to Hawaiʻi being at the northern edge of the tropics (mostly above 20 latitude), there are only weak wet and dry seasons unlike many tropical climates.

Winds

Island wind patterns are very complex. Though the trade winds are fairly constant, their relatively uniform air flow is distorted and disrupted by mountains, hills, and valleys. Usually winds blow upslope by day and downslope by night. Local conditions that produce occasional violent winds are not well understood. These are very localized, sometimes reaching speeds of 60 to 100 mph (100 to 160 km/h) and are best known in the settled areas of Kula and Lāhainā on Maui. The Kula winds are strong downslope winds on the lower slopes of the west side of Haleakalā. These winds tend to be strongest from 2,000 to 4,000 ft (600 to 1,200 m) above mean sea level.

The Lahaina winds are also downslope winds, but are somewhat different. They are also called "lehua winds" after the ʻōhiʻa lehua (Metrosideros polymorpha), whose red blossoms fill the air when these strong winds blow. They issue from canyons at the base of the western Maui mountains, where steeper canyon slopes meet the more gentle piedmont slope below. These winds only occur every 8 to 12 years. They are extremely violent, with wind speeds of 80–100 mph (130–160 km/h) or more.

Cloud formation

Under trade wind conditions, there is very often a pronounced moisture discontinuity between 4,000 and 8,000 feet (1,200 and 2,400 m). Below these heights, the air is moist; above, it is dry. The break (a large-scale feature of the Pacific High) is caused by a temperature inversion embedded in the moving trade wind air. The inversion tends to suppress the vertical movement of air and so restricts cloud development to the zone just below the inversion. The inversion is present 50 to 70 percent of the time; its height fluctuates from day to day, but it is usually between 5,000 and 7,000 feet (1,500 and 2,100 m). On trade wind days when the inversion is well defined, the clouds develop below these heights with only an occasional cloud top breaking through the inversion.

These towering clouds form along the mountains where the incoming trade wind air converges as it moves up a valley and is forced up and over the mountains to heights of several thousand feet. On days without an inversion, the sky is almost cloudless (completely cloudless skies are extremely rare). In leeward areas well screened from the trade winds (such as the west coast of Maui), skies are clear 30 to 60 percent of the time.

Windward areas tend to be cloudier during the summer, when the trade winds and associated clouds are more prevalent, while leeward areas, which are less affected by cloudy conditions associated with trade wind cloudiness, tend to be cloudier during the winter, when storm fronts pass through more frequently. On Maui, the cloudiest zones are at and just below the summits of the mountains, and at elevations of 2,000 to 4,000 ft (600 to 1,200 m) on the windward sides of Haleakalā. In these locations the sky is cloudy more than 70 percent of the time. The usual clarity of the air in the high mountains is associated with the low moisture content of the air.

Precipitation

Hawaiʻi differs from many tropical locations with pronounced wet and dry seasons, in that the wet season coincides with the winter months (rather than the summer months more typical of other places in the tropics). For instance, Honolulu's Köppen climate classification is the rare As wet-winter subcategory of the tropical wet and dry climate type.

Overall with climate change, Hawaiʻi is getting drier. Major storms occur most frequently in October through March. There may be as many as six or seven major storm events in a year. Such storms bring heavy rains and can be accompanied by strong local winds. The storms may be associated with the passage of a cold front, the leading edge of a mass of relatively cool air that is moving from west to east or from northwest to southeast.

Annual mean rainfall ranges from 7.4 in (188 mm) on the summit of Mauna Kea to 404.4 in (10,271 mm) in Big Bog. Windward slopes have greater rainfall than leeward lowlands and tall mountains.

On windward coasts, many brief showers are common, not one of which is heavy enough to produce more than 0.01 in (0.25 mm) of rain. The usual run of trade wind weather yields many light showers in the lowlands, whereas torrential rains are associated with a sudden surge in the trade winds or with a major storm. Hāna has had as much as 28 in (710 mm) of rain in a single 24-hour period.

Severe thunderstorms, as defined by the National Weather Service (NWS) as tornadoes, hail 1 in (25 mm) or larger, and/or convective winds of at least 58 mph (93 km/h) occur but are relatively uncommon. Nontornadic waterspouts are more common than tornadoes produced by supercells, which produce stronger, longer lasting tornadoes, especially with respect to inland areas, and also produce the largest hail, such as the 2012 Hawaiʻi hailstorm. An annual average of approximately one tornado, either emanating from supercells or by other processes, occurs.

Kona storms are features of the winter season. The name comes from winds out of the "kona" or usually leeward direction. Rainfall in a well-developed Kona storm is widespread and more prolonged than in the usual cold-front storm. Kona storm rains are usually most intense in an arc, extending from south to east of the storm and well in advance of its center. Kona rains last from several hours to several days. The rains may continue steadily, but the longer lasting ones are characteristically interrupted by intervals of lighter rain or partial clearing, as well as by intense showers superimposed on the more moderate continuous, steady rain. An entire winter may pass without a single well-developed Kona storm. More often there are one or two such storms a year; sometimes four or five.

Hurricanes

The hurricane season in the Hawaiian Islands is roughly from June through November, when hurricanes and tropical storms are most probable in the North Pacific. These storms tend to originate off the coast of Mexico (particularly the Baja California peninsula) and track west or northwest towards the islands. As storms cross the Pacific, they tend to lose strength if they bear northward and encounter cooler water.

True hurricanes are rare in Hawaiʻi, thanks in part to the comparatively cool waters around the islands as well as unfavorable atmospheric conditions, such as enhanced wind shear; only four have affected the islands during 63 years. Tropical storms are more frequent. These have more modest winds, below 74 mph (119 km/h). Because tropical storms resemble Kona storms, and because early records do not distinguish clearly between them, it has been difficult to estimate the average frequency of tropical storms. Every year or two a tropical storm will affect the weather in some part of the islands. Unlike cold fronts and Kona storms, hurricanes and tropical storms are most likely to occur during the last half of the year, from July through December. Three strong and destructive hurricanes are known to have made landfall on the islands, an unnamed storm in 1871, Hurricane Dot in 1959, and Hurricane ʻIniki in 1992. Another hurricane, ʻIwa, caused significant damage in 1982 but its center passed nearby and did not directly make landfall. The rarity of hurricanes making landfall on the Islands is subject to change as the climate warms. In the Pliocene era, where CO₂ levels were comparable to those we see today, the waters around Hawaiʻi were much warmer, resulting in frequent hurricane strikes in computer simulations.

Effect on trade winds

A true-color satellite view of Hawaiʻi shows that most of the flora on the islands grow on the north-east sides, which face the trade winds. The texture change around the calmer south-west of the islands is the result of the shelter provided from the islands.

The top image above shows the winds around the Hawaiian Islands measured by the Seawinds instrument aboard QuikSCAT during August 1999. Trade winds blow from right to left in the image. The bottom image shows the ocean current formed by the islands’ wake. Arrows indicate current direction and speed, while white contours show ocean temperatures. The warm water of the current generates winds that sustain the current for thousands of miles.

Despite being small islands within the vast Pacific Ocean, the Hawaiian Islands have a surprising effect on ocean currents and circulation patterns over much of the Pacific. In the Northern Hemisphere, trade winds blow from northeast to southwest, from North and South America toward Asia, between the equator and 30 degrees north latitude. Typically, the trade winds continue across the Pacific, unless something gets in their way, like an island.

Hawaiʻi's high mountains present a substantial obstacle to the trade winds. The elevated topography blocks the airflow, effectively splitting the trade winds in two. This split causes a zone of weak winds, called a "wind wake", on the leeward side of the islands.

Aerodynamic theory indicates that an island wind wake effect should dissipate within a few hundred kilometers and not be felt in the western Pacific. However, the wind wake caused by the Hawaiian Islands extends 1,860 miles (3,000 km), roughly 10 times longer than any other wake. The long wake testifies to the strong interaction between the atmosphere and ocean, which has strong implications for global climate research. It is also important for understanding natural climate variations, like El Niño.

There are number of reasons why this has been observed only in Hawaiʻi. First, the ocean reacts slowly to fast-changing winds; winds must be steady to exert force on the ocean, such as the trade winds. Second, the high mountain topography provides a significant disturbance to the winds. Third, the Hawaiian Islands are large in horizontal (east-west) scale, extending over four degrees in longitude. It is this active interaction between wind, ocean current, and temperature that creates this uniquely long wake west of Hawaiʻi.

The wind wake drives an eastward "counter current" that brings warm water 5,000 miles (8,000 km) from the Asian coast. This warm water drives further changes in wind, allowing the island effect to extend far into the western Pacific. The counter current had been observed by oceanographers near the Hawaiian Islands years before the long wake was discovered, but they did not know what caused it.

Hawaiian Islands

From Wikipedia, the free encyclopedia

The Hawaiian Islands (Hawaiian: Nā Moku o Hawai‘i) are an archipelago of eight major volcanic islands, several atolls, and numerous smaller islets in the North Pacific Ocean, extending some 1,500 miles (2,400 kilometers) from the island of Hawaiʻi in the south to northernmost Kure Atoll. Formerly called the Sandwich Islands, the present name for the archipelago is derived from the name of its largest island, Hawaiʻi.

The archipelago sits on the Pacific Plate. The islands are exposed peaks of a great undersea mountain range known as the Hawaiian–Emperor seamount chain, formed by volcanic activity over a hotspot in the Earth's mantle. The islands are about 1,860 miles (3,000 km) from the nearest continent and are part of the Polynesia subregion of Oceania.

The U.S. state of Hawaii occupies the archipelago almost in its entirety (including the mostly uninhabited Northwestern Hawaiian Islands), with the sole exception of Midway Atoll (a United States Minor Outlying Island). Hawaii is the only U.S. state that is situated entirely on an archipelago, and the only state not geographically connected with North America. The Northwestern islands (sometimes called the Leeward Islands) and surrounding seas are protected as a National Monument and World Heritage Site.

Islands and reefs

The date of the first settlements of the Hawaiian Islands is a topic of continuing debate. Archaeological evidence seems to indicate a settlement as early as 124 AD.

Captain James Cook, RN, visited the islands on January 18, 1778, and named them the "Sandwich Islands" in honor of The 4th Earl of Sandwich, who as the First Lord of the Admiralty was one of his sponsors. This name was in use until the 1840s, when the local name "Hawaii" gradually began to take precedence.

The Hawaiian Islands have a total land area of 6,423.4 square miles (16,636.5 km²). Except for Midway, which is an unincorporated territory of the United States, these islands and islets are administered as Hawaii—the 50th state of the United States.

Major islands

Island	Nickname	Area	Population (as of 2020)	Density	Highest point	Maximum Elevation	Age (Ma)	Location
Hawaiʻi	The Big Island	4,028.0 sq mi (10,432.5 km²)	200,629	49.8/sq mi (19.2/km²)	Mauna Kea	13,796 ft (4,205 m)	0.4	19°34′N 155°30′W
Maui	The Valley Isle	727.2 sq mi (1,883.4 km²)	164,221	225.8/sq mi (87.2/km²)	Haleakalā	10,023 ft (3,055 m)	1.3–0.8	20°48′N 156°20′W
Oʻahu	The Gathering Place	596.7 sq mi (1,545.4 km²)	1,016,508	1,703.5/sq mi (657.7/km²)	Mount Kaʻala	4,003 ft (1,220 m)	3.7–2.6	21°28′N 157°59′W
Kauaʻi	The Garden Isle	552.3 sq mi (1,430.5 km²)	73,298	132.7/sq mi (51.2/km²)	Kawaikini	5,243 ft (1,598 m)	5.1	22°05′N 159°30′W
Molokaʻi	The Friendly Isle	260.0 sq mi (673.4 km²)	7,345	28.3/sq mi (10.9/km²)	Kamakou	4,961 ft (1,512 m)	1.9–1.8	21°08′N 157°02′W
Lānaʻi	The Pineapple Isle	140.5 sq mi (363.9 km²)	3,367	24.0/sq mi (9.3/km²)	Lānaʻihale	3,366 ft (1,026 m)	1.3	20°50′N 156°56′W
Niʻihau	The Forbidden Isle	69.5 sq mi (180.0 km²)	84	1.2/sq mi (0.5/km²)	Mount Pānīʻau	1,250 ft (381 m)	4.9	21°54′N 160°10′W
Kahoʻolawe	The Target Isle	44.6 sq mi (115.5 km²)	0	0/sq mi (0/km²)	Puʻu Moaulanui	1,483 ft (452 m)	1.0	20°33′N 156°36′W

The eight major islands of Hawaii (Windward Islands) are listed above. All except Kaho'olawe are inhabited.

Minor islands, islets

3-D perspective view of the southeastern Hawaiian Islands, with the white summits of Mauna Loa (4,170 m or 13,680 ft high) and Mauna Kea (4,207.3 m or 13,803 ft high). The islands are the tops of massive volcanoes, the bulk of which lie below the sea surface. Ocean depths are colored from violet (5,750 m or 18,860 ft deep northeast of Maui) and indigo to light gray (shallowest). Historical lava flows are shown in red, erupting from the summits and rift zones of Mauna Loa, Kilauea, and Hualalai volcanoes on Hawaiʻi.

The state of Hawaii counts 137 "islands" in the Hawaiian chain. This number includes all minor islands (small islands), islets (even smaller islands) offshore of the major islands (listed above) and individual islets in each atoll. These are just a few:

Partial islands, atolls, reefs

Partial islands, atolls, reefs (west of Niʻihau are uninhabited except Midway Atoll) form the Northwestern Hawaiian Islands (Leeward Islands):

Nihoa (Mokumana)
Necker (Mokumanamana)
French Frigate Shoals (Kānemilohaʻi)
Gardner Pinnacles (Pūhāhonu)
Maro Reef (Nalukākala)
Laysan (Kauō)
Lisianski Island (Papaʻāpoho)
Pearl and Hermes Atoll (Holoikauaua)
Midway Atoll (Pihemanu)
Kure Atoll (Mokupāpapa)

Geology

This chain of islands, or archipelago, developed as the Pacific Plate slowly moved northwestward over a hotspot in the Earth's mantle at a rate of approximately 32 miles (51 km) per million years. Thus, the southeast island is volcanically active, whereas the islands on the northwest end of the archipelago are older and typically smaller, due to longer exposure to erosion. The age of the archipelago has been estimated using potassium-argon dating methods. From this study and others, it is estimated that the northwesternmost island, Kure Atoll, is the oldest at approximately 28 million years (Ma); while the southeasternmost island, Hawaiʻi, is approximately 0.4 Ma (400,000 years). The only active volcanism in the last 200 years has been on the southeastern island, Hawaiʻi, and on the submerged but growing volcano to the extreme southeast, Kamaʻehuakanaloa (formerly Loʻihi). The Hawaiian Volcano Observatory of the USGS documents recent volcanic activity and provides images and interpretations of the volcanism. Kīlauea had been erupting nearly continuously since 1983 when it stopped August 2018.

Almost all of the magma of the hotspot has the composition of basalt, and so the Hawaiian volcanoes are composed almost entirely of this igneous rock. There is very little coarser-grained gabbro and diabase. Nephelinite is exposed on the islands but is extremely rare. The majority of eruptions in Hawaiʻi are Hawaiian-type eruptions because basaltic magma is relatively fluid compared with magmas typically involved in more explosive eruptions, such as the andesitic magmas that produce some of the spectacular and dangerous eruptions around the margins of the Pacific basin.

Hawaiʻi island (the Big Island) is the biggest and youngest island in the chain, built from five volcanoes. Mauna Loa, taking up over half of the Big Island, is the largest shield volcano on the Earth. The measurement from sea level to summit is more than 2.5 miles (4 km), from sea level to sea floor about 3.1 miles (5 km).

Earthquakes

The Hawaiian Islands have many earthquakes, generally triggered by and related to volcanic activity. Seismic activity, as a result, is currently highest in the southern part of the chain. Both historical and modern earthquake databases have correlated higher magnitude earthquakes with flanks of active volcanoes, such as Mauna Loa and Kilauea. The combination of erosional forces, which cause slumping and landslides, with the pressure exerted by rising magma put a great amount of stress on the volcanic flanks. The stress is released when the slope fails, or slips, causing an earthquake. This type of seismicity is unique because the forces driving the system are not always consistent over time, since rates of volcanic activity fluctuate. Seismic hazard near active, seaward volcanic flanks is high, partially due to the especially unpredictable nature of the forces that trigger earthquakes, and partially because these events occur at relatively shallow depths. Flank earthquakes typically occur at depths ranging from 5 to 20 km, increasing the hazard to local infrastructure and communities. Earthquakes and landslides on the island chain have also been known to cause tsunamis.

Most of the early earthquake monitoring took place in Hilo, by missionaries Titus Coan, Sarah J. Lyman and her family. Between 1833 and 1896, approximately 4 or 5 earthquakes were reported per year. Today, earthquakes are monitored by the Hawaiian Volcano Observatory run by the USGS.

Hawaii accounted for 7.3% of the United States' reported earthquakes with a magnitude 3.5 or greater from 1974 to 2003, with a total 1533 earthquakes. Hawaii ranked as the state with the third most earthquakes over this time period, after Alaska and California.

On October 15, 2006, there was an earthquake with a magnitude of 6.7 off the northwest coast of the island of Hawaii, near the Kona area of the big island. The initial earthquake was followed approximately five minutes later by a magnitude 5.7 aftershock. Minor-to-moderate damage was reported on most of the Big Island. Several major roadways became impassable from rock slides, and effects were felt as far away as Honolulu, Oahu, nearly 150 miles (240 km) from the epicenter. Power outages lasted for several hours to days. Several water mains ruptured. No deaths or life-threatening injuries were reported.

On May 4, 2018, there was a 6.9 earthquake in the zone of volcanic activity from Kīlauea.

Earthquakes are monitored by the Hawaiian Volcano Observatory run by the USGS.

Tsunamis

Aftermath of the 1960 Chilean tsunami in Hilo, Hawaiʻi, where the tsunami left 61 people dead and 282 seriously injured. The waves reached 35 feet (11 m) high.

The Hawaiian Islands are subject to tsunamis, great waves that strike the shore. Tsunamis are most often caused by earthquakes somewhere in the Pacific. The waves produced by the earthquakes travel at speeds of 400–500 miles per hour (600–800 km/h) and can affect coastal regions thousands of miles (kilometers) away.

Tsunamis may also originate from the Hawaiian Islands. Explosive volcanic activity can cause tsunamis. The island of Molokaʻi had a catastrophic collapse or debris avalanche over a million years ago; this underwater landslide likely caused tsunamis. The Hilina Slump on the island of Hawaiʻi is another potential place for a large landslide and resulting tsunami.

The city of Hilo on the Big Island has been most affected by tsunamis, where the in-rushing water is accentuated by the shape of Hilo Bay. Coastal cities have tsunami warning sirens.

A tsunami resulting from an earthquake in Chile hit the islands on February 27, 2010. It was relatively minor, but local emergency management officials utilized the latest technology and ordered evacuations in preparation for a possible major event. The Governor declared it a "good drill" for the next major event.

A tsunami resulting from an earthquake in Japan hit the islands on March 11, 2011. It was relatively minor, but local officials ordered evacuations in preparation for a possible major event. The tsunami caused about $30.1 million in damages.

Volcanos

Only the two Hawaiian islands furthest to the southeast have active volcanoes: Haleakalā on Maui, and Mauna Loa, Mauna Kea, Kilauea, and Hualalai, all on the Big Island. The volcanoes on the remaining islands are extinct as they are no longer over the Hawaii hotspot. The Kamaʻehuakanaloa Seamount is an active submarine volcano that is expected to become the newest Hawaiian island when it rises above the ocean's surface in 10,000–100,000 years. Hazards from these volcanoes include lava flows that can destroy and bury the surrounding surface, volcanic gas emissions, earthquakes and tsunamis listed above, submarine eruptions affecting the ocean, and the possibility of an explosive eruption.

History

There is no definitive date for the Polynesian discovery of Hawaii. However, high-precision radiocarbon dating in Hawaii using chronometric hygiene analysis, and taxonomic identification selection of samples, puts the initial such settlement of the Hawaiian Islands sometime between 1219 and 1266 A.D., originating from earlier settlements first established in the Society Islands around 1025 to 1120 A.D., and in the Marquesas Islands sometime between 1100 and 1200 A.D.

An expedition led by British explorer James Cook is usually considered to be the first group of Europeans to arrive in the Hawaiian Islands, which they did in 1778. However, Spanish historians and some other researchers state that the Spanish captain Ruy López de Villalobos was the first European to see the islands in 1542. The Spanish named these islands "Isla de Mesa, de los Monjes y Desgraciada" (1542), being on the route linking the Philippines with Mexico across the Pacific Ocean, between the ports of Acapulco and Manila, which were both part of New Spain. Within five years after Cook's arrival, European military technology helped Kamehameha I, ruler of the island of Hawaii, conquer and unify the islands for the first time, establishing the Kingdom of Hawaii in 1795. The kingdom was prosperous and important for its agriculture and strategic location in the Pacific.

American immigration, led by Protestant missionaries, and Native Hawaiian emigration, mostly on whaling ships, began almost immediately after Cook's arrival. Americans set up plantations to grow sugar. Their methods of plantation farming required substantial labor. Waves of permanent immigrants came from Japan, China, and the Philippines to work in the fields. The government of Japan organized and gave special protection to its people, who comprised about 25 percent of the Hawaiian population by 1896. The Hawaiian monarchy encouraged this multi-ethnic society, initially establishing a constitutional monarchy in 1840 that promised equal voting rights regardless of race, gender, or wealth.

The population of Native Hawaiians in Hawaii declined from an unknown number prior to 1778 (commonly estimated to be around 300,000), to around 142,000 in the 1820s based on the first census conducted by American missionaries, 82,203 in the 1850 Hawaiian Kingdom census, 40,622 in the last Hawaiian Kingdom census of 1890, 39,504 in the only census by the Republic of Hawaii in 1896, and 37,656 in the first census conducted by the United States in 1900 after the annexation of Hawaii to the United States in 1898. Since Hawaii has joined the United States the Native Hawaiian population in Hawaii has increased with every census to 289,970 in 2010.

Americans within the kingdom government rewrote the constitution, severely curtailing the power of King "David" Kalākaua, and disenfranchising the rights of most Native Hawaiians and Asian citizens to vote, through excessively high property and income requirements. This gave a sizeable advantage to plantation owners. Queen Liliʻuokalani attempted to restore royal powers in 1893 but was placed under house arrest by businessmen with help from the United States military. Against the Queen's wishes, the Republic of Hawaii was formed for a short time. This government agreed on behalf of Hawaii to join the United States in 1898 as the Territory of Hawaii. In 1959, the islands became the state of Hawaii.

Ecology

The islands are home to a multitude of endemic species. Since human settlement, first by Polynesians, non native trees, plants, and animals were introduced. These included species such as rats and pigs, that have preyed on native birds and invertebrates that initially evolved in the absence of such predators. The growing population of humans has also led to deforestation, forest degradation, treeless grasslands, and environmental degradation. As a result, many species which depended on forest habitats and food became extinct—with many current species facing extinction. As humans cleared land for farming, monocultural crop production replaced multi-species systems.

The arrival of the Europeans had a more significant impact, with the promotion of large-scale single-species export agriculture and livestock grazing. This led to increased clearing of forests, and the development of towns, adding many more species to the list of extinct animals of the Hawaiian Islands. As of 2009, many of the remaining endemic species are considered endangered.

National Monument

On June 15, 2006, President George W. Bush issued a public proclamation creating Papahānaumokuākea Marine National Monument under the Antiquities Act of 1906. The Monument encompasses the northwestern Hawaiian Islands and surrounding waters, forming the largest marine wildlife reserve in the world. In August 2010, UNESCO's World Heritage Committee added Papahānaumokuākea to its list of World Heritage Sites. On August 26, 2016, President Barack Obama greatly expanded Papahānaumokuākea, quadrupling it from its original size.

Climate

The Hawaiian Islands are tropical but experience many different climates, depending on altitude and surroundings. The islands receive most rainfall from the trade winds on their north and east flanks (the windward side) as a result of orographic precipitation. Coastal areas in general and especially the south and west flanks, or leeward sides, tend to be drier.

In general, the lowlands of Hawaiian Islands receive most of their precipitation during the winter months (October to April). Drier conditions generally prevail from May to September. The tropical storms, and occasional hurricanes, tend to occur from July through November.

During the summer months the average temperature is about 84 °F (29 °C), in the winter months it is approximately 79 °F (26 °C). As the temperature is relatively constant over the year the probability of dangerous thunderstorms is approximately low.

Mantle plume

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Mantle_plume

A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces such as the Deccan and Siberian Traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries.

Concepts

Mantle plumes were first proposed by J. Tuzo Wilson in 1963 and further developed by W. Jason Morgan in 1971 and 1972. A mantle plume is posited to exist where super-heated material forms (nucleates) at the core-mantle boundary and rises through the Earth's mantle. Rather than a continuous stream, plumes should be viewed as a series of hot bubbles of material. Reaching the brittle upper Earth's crust they form diapirs. These diapirs are "hotspots" in the crust. In particular, the concept that mantle plumes are fixed relative to one another and anchored at the core-mantle boundary would provide a natural explanation for the time-progressive chains of older volcanoes seen extending out from some such hotspots, for example, the Hawaiian–Emperor seamount chain. However, paleomagnetic data show that mantle plumes can also be associated with Large Low Shear Velocity Provinces (LLSVPs) and do move relative to each other.

The current mantle plume theory is that material and energy from Earth's interior are exchanged with the surface crust in two distinct and largely independent convective flows:

as previously theorized and widely accepted, the predominant, steady state plate tectonic regime driven by upper mantle convection, mainly the sinking of cold plates of lithosphere back into the asthenosphere.
the punctuated, intermittently dominant mantle overturn regime driven by plume convection that carries heat upward from the core-mantle boundary in a narrow column. This second regime, while often discontinuous, is periodically significant in mountain building and continental breakup.

The plume hypothesis was simulated by laboratory experiments in small fluid-filled tanks in the early 1970s. Thermal or compositional fluid-dynamical plumes produced in that way were presented as models for the much larger postulated mantle plumes. Based on these experiments, mantle plumes are now postulated to comprise two parts: a long thin conduit connecting the top of the plume to its base, and a bulbous head that expands in size as the plume rises. The entire structure resembles a mushroom. The bulbous head of thermal plumes forms because hot material moves upward through the conduit faster than the plume itself rises through its surroundings. In the late 1980s and early 1990s, experiments with thermal models showed that as the bulbous head expands it may entrain some of the adjacent mantle into itself.

The size and occurrence of mushroom mantle plumes can be predicted by the transient instability theory of Tan and Thorpe. The theory predicts mushroom-shaped mantle plumes with heads of about 2000 km diameter that have a critical time (time from onset of heating of the lower mantle to formation of a plume) of about 830 million years for a core mantle heat flux of 20 mW/m², while the cycle time (the time between plume formation events) is about 2000 million years. The number of mantle plumes is predicted to be about 17.

When a plume head encounters the base of the lithosphere, it is expected to flatten out against this barrier and to undergo widespread decompression melting to form large volumes of basalt magma. It may then erupt onto the surface. Numerical modelling predicts that melting and eruption will take place over several million years. These eruptions have been linked to flood basalts, although many of those erupt over much shorter time scales (less than 1 million years). Examples include the Deccan traps in India, the Siberian traps of Asia, the Karoo-Ferrar basalts/dolerites in South Africa and Antarctica, the Paraná and Etendeka traps in South America and Africa (formerly a single province separated by opening of the South Atlantic Ocean), and the Columbia River basalts of North America. Flood basalts in the oceans are known as oceanic plateaus, and include the Ontong Java plateau of the western Pacific Ocean and the Kerguelen Plateau of the Indian Ocean.

The narrow vertical conduit, postulated to connect the plume head to the core-mantle boundary, is viewed as providing a continuous supply of magma to a hotspot. As the overlying tectonic plate moves over this hotspot, the eruption of magma from the fixed plume onto the surface is expected to form a chain of volcanoes that parallels plate motion. The Hawaiian Islands chain in the Pacific Ocean is the archetypal example. It has recently been discovered that the volcanic locus of this chain has not been fixed over time, and it thus joined the club of the many type examples that do not exhibit the key characteristic originally proposed.

The eruption of continental flood basalts is often associated with continental rifting and breakup. This has led to the hypothesis that mantle plumes contribute to continental rifting and the formation of ocean basins.

Chemistry, heat flow and melting

Hydrodynamic simulation of a single "finger" of the Rayleigh–Taylor instability, a possible mechanism for plume formation. In the third and fourth frame in the sequence, the plume forms a "mushroom cap". Note that the core is at the top of the diagram and the crust is at the bottom.

Earth cross-section showing location of upper (3) and lower (5) mantle, D″-layer (6), and outer (7) and inner (9) core

The chemical and isotopic composition of basalts found at hotspots differs subtly from mid-ocean-ridge basalts. These basalts, also called ocean island basalts (OIBs), are analysed in their radiogenic and stable isotope compositions. In radiogenic isotope systems the originally subducted material creates diverging trends, termed mantle components. Identified mantle components are DMM (depleted mid-ocean ridge basalt (MORB) mantle), HIMU (high U/Pb-ratio mantle), EM1 (enriched mantle 1), EM2 (enriched mantle 2) and FOZO (focus zone). This geochemical signature arises from the mixing of near-surface materials such as subducted slabs and continental sediments, in the mantle source. There are two competing interpretations for this. In the context of mantle plumes, the near-surface material is postulated to have been transported down to the core-mantle boundary by subducting slabs, and to have been transported back up to the surface by plumes. In the context of the Plate hypothesis, subducted material is mostly re-circulated in the shallow mantle and tapped from there by volcanoes.

Stable isotopes like Fe are used to track processes that the uprising material experiences during melting.

The processing of oceanic crust, lithosphere, and sediment through a subduction zone decouples the water-soluble trace elements (e.g., K, Rb, Th) from the immobile trace elements (e.g., Ti, Nb, Ta), concentrating the immobile elements in the oceanic slab (the water-soluble elements are added to the crust in island arc volcanoes). Seismic tomography shows that subducted oceanic slabs sink as far as the bottom of the mantle transition zone at 650 km depth. Subduction to greater depths is less certain, but there is evidence that they may sink to mid-lower-mantle depths at about 1,500 km depth.

The source of mantle plumes is postulated to be the core-mantle boundary at 3,000 km depth. Because there is little material transport across the core-mantle boundary, heat transfer must occur by conduction, with adiabatic gradients above and below this boundary. The core-mantle boundary is a strong thermal (temperature) discontinuity. The temperature of the core is approximately 1,000 degrees Celsius higher than that of the overlying mantle. Plumes are postulated to rise as the base of the mantle becomes hotter and more buoyant.

Plumes are postulated to rise through the mantle and begin to partially melt on reaching shallow depths in the asthenosphere by decompression melting. This would create large volumes of magma. This melt rises to the surface and erupts to form hotspots.

The lower mantle and the core

The most prominent thermal contrast known to exist in the deep (1000 km) mantle is at the core-mantle boundary at 2900 km. Mantle plumes were originally postulated to rise from this layer because the hotspots that are assumed to be their surface expression were thought to be fixed relative to one another. This required that plumes were sourced from beneath the shallow asthenosphere that is thought to be flowing rapidly in response to motion of the overlying tectonic plates. There is no other known major thermal boundary layer in the deep Earth, and so the core-mantle boundary was the only candidate.

The base of the mantle is known as the D″ layer, a seismological subdivision of the Earth. It appears to be compositionally distinct from the overlying mantle and may contain partial melt.

Two very broad, large low-shear-velocity provinces exist in the lower mantle under Africa and under the central Pacific. It is postulated that plumes rise from their surface or their edges. Their low seismic velocities were thought to suggest that they are relatively hot, although it has recently been shown that their low wave velocities are due to high density caused by chemical heterogeneity.

Evidence for the theory

Some common and basic lines of evidence cited in support of the theory are linear volcanic chains, noble gases, geophysical anomalies, and geochemistry.

Linear volcanic chains

The age-progressive distribution of the Hawaiian-Emperor seamount chain has been explained as a result of a fixed, deep-mantle plume rising into the upper mantle, partly melting, and causing a volcanic chain to form as the plate moves overhead relative to the fixed plume source. Other hotspots with time-progressive volcanic chains behind them include Réunion, the Chagos-Laccadive Ridge, the Louisville Ridge, the Ninety East Ridge and Kerguelen, Tristan, and Yellowstone.

While there is evidence that the chains listed above are time-progressive, it has been shown that they are not fixed relative to one another. The most remarkable example of this is the Emperor chain, the older part of the Hawaii system, which was formed by migration of the hotspot in addition to the plate motion. Another example is the Canary Islands in the northeast of Africa in the Atlantic Ocean.

Noble gas and other isotopes

Helium-3 is a primordial isotope that formed in the Big Bang. Very little is produced, and little has been added to the Earth by other processes since then. Helium-4 includes a primordial component, but it is also produced by the natural radioactive decay of elements such as uranium and thorium. Over time, helium in the upper atmosphere is lost into space. Thus, the Earth has become progressively depleted in helium, and ³He is not replaced as ⁴He is. As a result, the ratio ³He/⁴He in the Earth has decreased over time.

Unusually high ³He/⁴He have been observed in some, but not all, hotspots. This is explained by plumes tapping a deep, primordial reservoir in the lower mantle, where the original, high ³He/⁴He ratios have been preserved throughout geologic time.

Other elements, e.g. osmium, have been suggested to be tracers of material arising from near to the Earth's core, in basalts at oceanic islands. However, so far conclusive proof for this is lacking.

Geophysical anomalies

Diagram showing a cross section though the Earth's lithosphere (in yellow) with magma rising from the mantle (in red). The crust may move relative to the plume, creating a *track*.

The plume hypothesis has been tested by looking for the geophysical anomalies predicted to be associated with them. These include thermal, seismic, and elevation anomalies. Thermal anomalies are inherent in the term "hotspot". They can be measured in numerous different ways, including surface heat flow, petrology, and seismology. Thermal anomalies produce anomalies in the speeds of seismic waves, but unfortunately so do composition and partial melt. As a result, wave speeds cannot be used simply and directly to measure temperature, but more sophisticated approaches must be taken.

Seismic anomalies are identified by mapping variations in wave speed as seismic waves travel through Earth. A hot mantle plume is predicted to have lower seismic wave speeds compared with similar material at a lower temperature. Mantle material containing a trace of partial melt (e.g., as a result of it having a lower melting point), or being richer in Fe, also has a lower seismic wave speed and those effects are stronger than temperature. Thus, although unusually low wave speeds have been taken to indicate anomalously hot mantle beneath hotspots, this interpretation is ambiguous. The most commonly cited seismic wave-speed images that are used to look for variations in regions where plumes have been proposed come from seismic tomography. This method involves using a network of seismometers to construct three-dimensional images of the variation in seismic wave speed throughout the mantle.

Seismic waves generated by large earthquakes enable structure below the Earth's surface to be determined along the ray path. Seismic waves that have traveled a thousand or more kilometers (also called teleseismic waves) can be used to image large regions of Earth's mantle. They also have limited resolution, however, and only structures at least several hundred kilometers in diameter can be detected.

Seismic tomography images have been cited as evidence for a number of mantle plumes in Earth's mantle.^[37] There is, however, vigorous on-going discussion regarding whether the structures imaged are reliably resolved, and whether they correspond to columns of hot, rising rock.

The mantle plume hypothesis predicts that domal topographic uplifts will develop when plume heads impinge on the base of the lithosphere. An uplift of this kind occurred when the north Atlantic Ocean opened about 54 million years ago. Some scientists have linked this to a mantle plume postulated to have caused the breakup of Eurasia and the opening of the north Atlantic, now suggested to underlie Iceland. Current research has shown that the time-history of the uplift is probably much shorter than predicted, however. It is thus not clear how strongly this observation supports the mantle plume hypothesis.

Geochemistry

Basalts found at oceanic islands are geochemically distinct from mid-ocean ridge basalt (MORB). Ocean island basalt (OIB) is more diverse compositionally than MORB, and the great majority of ocean islands are composed of alkali basalt enriched in sodium and potassium relative to MORB. Larger islands, such as Hawaii or Iceland, are mostly tholeiitic basalt, with alkali basalt limited to late stages of their development, but this tholeiitic basalt is chemically distinct from the tholeiitic basalt of mid-ocean ridges. OIB tends to be more enriched in magnesium, and both alkali and tholeiitic OIB is enriched in trace incompatible elements, with the light rare earth elements showing particular enrichment compared with heavier rare earth elements. Stable isotope ratios of the elements strontium, neodymium, hafnium, lead, and osmium show wide variations relative to MORB, which is attributed to the mixing of at least three mantle components: HIMU with a high proportion of radiogenic lead, produced by decay of uranium and other heavy radioactive elements; EM1 with less enrichment of radiogenic lead; and EM2 with a high ⁸⁷Sr/⁸⁶Sr ratio. Helium in OIB shows a wider variation in the ³He/⁴He ratio than MORB, with some values approaching the primordial value.

The composition of ocean island basalts is attributed to the presence of distinct mantle chemical reservoirs formed by subduction of oceanic crust. These include reservoirs corresponding to HUIMU, EM1, and EM2. These reservoirs are thought to have different major element compositions, based on the correlation between major element compositions of OIB and their stable isotope ratios. Tholeiitic OIB is interpreted as a product of a higher degree of partial melting in particularly hot plumes, while alkali OIB is interpreted as a product of a lower degree of partial melting in smaller, cooler plumes.

Seismology

In 2015, based on data from 273 large earthquakes, researchers compiled a model based on full waveform tomography, requiring the equivalent of 3 million hours of supercomputer time. Due to computational limitations, high-frequency data still could not be used, and seismic data remained unavailable from much of the seafloor. Nonetheless, vertical plumes, 400 C hotter than the surrounding rock, were visualized under many hotspots, including the Pitcairn, Macdonald, Samoa, Tahiti, Marquesas, Galapagos, Cape Verde, and Canary hotspots. They extended nearly vertically from the core-mantle boundary (2900 km depth) to a possible layer of shearing and bending at 1000 km. They were detectable because they were 600–800 km wide, more than three times the width expected from contemporary models. Many of these plumes are in the large low-shear-velocity provinces under Africa and the Pacific, while some other hotspots such as Yellowstone were less clearly related to mantle features in the model.

The unexpected size of the plumes leaves open the possibility that they may conduct the bulk of the Earth's 44 terawatts of internal heat flow from the core to the surface, and means that the lower mantle convects less than expected, if at all. It is possible that there is a compositional difference between plumes and the surrounding mantle that slows them down and broadens them.

Suggested mantle plume locations

An example of plume locations suggested by one recent group. Figure from Foulger (2010).

Mantle plumes have been suggested as the source for flood basalts. These extremely rapid, large scale eruptions of basaltic magmas have periodically formed continental flood basalt provinces on land and oceanic plateaus in the ocean basins, such as the Deccan Traps, the Siberian Traps the Karoo-Ferrar flood basalts of Gondwana, and the largest known continental flood basalt, the Central Atlantic magmatic province (CAMP).

Many continental flood basalt events coincide with continental rifting. This is consistent with a system that tends toward equilibrium: as matter rises in a mantle plume, other material is drawn down into the mantle, causing rifting.

Alternative hypotheses

In parallel with the mantle plume model, two alternative explanations for the observed phenomena have been considered: the plate hypothesis and the impact hypothesis.

The plate hypothesis

Beginning in the early 2000s, dissatisfaction with the state of the evidence for mantle plumes and the proliferation of ad hoc hypotheses drove a number of geologists, led by Don L. Anderson, Gillian Foulger, and Warren B. Hamilton, to propose a broad alternative based on shallow processes in the upper mantle and above, with an emphasis on plate tectonics as the driving force of magmatism.

The plate hypothesis suggests that "anomalous" volcanism results from lithospheric extension that permits melt to rise passively from the asthenosphere beneath. It is thus the conceptual inverse of the plume hypothesis because the plate hypothesis attributes volcanism to shallow, near-surface processes associated with plate tectonics, rather than active processes arising at the core-mantle boundary.

Lithospheric extension is attributed to processes related to plate tectonics. These processes are well understood at mid-ocean ridges, where most of Earth's volcanism occurs. It is less commonly recognised that the plates themselves deform internally, and can permit volcanism in those regions where the deformation is extensional. Well-known examples are the Basin and Range Province in the western USA, the East African Rift valley, and the Rhine Graben. Under this hypothesis, variable volumes of magma are attributed to variations in chemical composition (large volumes of volcanism corresponding to more easily molten mantle material) rather than to temperature differences.

While not denying the presence of deep mantle convection and upwelling in general, the plate hypothesis holds that these processes do not result in mantle plumes, in the sense of columnar vertical features that span most of the Earth's mantle, transport large amounts of heat, and contribute to surface volcanism.

Under the umbrella of the plate hypothesis, the following sub-processes, all of which can contribute to permitting surface volcanism, are recognised:

Continental break-up;
Fertility at mid-ocean ridges;
Enhanced volcanism at plate boundary junctions;
Small-scale sublithospheric convection;
Oceanic intraplate extension;
Slab tearing and break-off;
Shallow mantle convection;
Abrupt lateral changes in stress at structural discontinuities;
Continental intraplate extension;
Catastrophic lithospheric thinning;
Sublithospheric melt ponding and draining.

The impact hypothesis

In addition to these processes, impact events such as ones that created the Addams crater on Venus and the Sudbury Igneous Complex in Canada are known to have caused melting and volcanism. In the impact hypothesis, it is proposed that some regions of hotspot volcanism can be triggered by certain large-body oceanic impacts which are able to penetrate the thinner oceanic lithosphere, and flood basalt volcanism can be triggered by converging seismic energy focused at the antipodal point opposite major impact sites. Impact-induced volcanism has not been adequately studied and comprises a separate causal category of terrestrial volcanism with implications for the study of hotspots and plate tectonics.

Comparison of the hypotheses

In 1997 it became possible using seismic tomography to image submerging tectonic slabs penetrating from the surface all the way to the core-mantle boundary.

For the Hawaii hotspot, long-period seismic body wave diffraction tomography provided evidence that a mantle plume is responsible, as had been proposed as early as 1971. For the Yellowstone hotspot, seismological evidence began to converge from 2011 in support of the plume model, as concluded by James et al., "we favor a lower mantle plume as the origin for the Yellowstone hotspot." Data acquired through Earthscope, a program collecting high-resolution seismic data throughout the contiguous United States has accelerated acceptance of a plume underlying Yellowstone.

Although there is thus strong evidence that at least these two deep mantle plumes rise from the core-mantle boundary, confirmation that other hypotheses can be dismissed may require similar tomographic evidence for other hotspots.

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