~Hawaiian Volcanoes~

Where is Hawaii located?

The Hawaiian Islands (Hawaiian: Mokupuni o Hawai‘i) are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles (2,400 km) from the island of Hawaiʻi in the south to northernmost Kure Atoll (the north westernmost island in Hawaii is Green Island, which is joined to the Kure Atoll). Excluding Midway, which is an unincorporated territory of the United States, the Hawaiian Islands form the U.S. state of Hawaii. Once known as the "Sandwich Islands", the archipelago now takes its name from the largest island in the cluster. The islands are the 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 Hawaiian islands are about 1,860 miles (3,000 km) from the nearest continent.


The Island of Hawaiʻi is located at 19° 34′ 0″ N, 155° 30′ 0″ W. It is built from five separate shield volcanoes that erupted somewhat sequentially, one overlapping the other which are (from oldest to youngest):

  • Kohala—extinct
  • Mauna Kea— dormant, which last erupted about 4,000 years ago.
  • Hualālai— likely to be dormant (last erupted in 1801)
  • Mauna Loa— the largest active volcano on Earth
  • Kīlauea— very active; 34 eruptions have taken place since 1952 (excluding current episode) and is continuously erupting since 1983.
The areas each shield volcanoes form and the severity of hazard.
Source : http://hvo.wr.usgs.gov/hazards/lavazones/main.html

The severity level descriptions

ZonePercentage of area covered by lava since 1800Percentage of area covered by lava in last 750 yearsExplanation
1greater than 25greater than 65Includes the summits and rift zones of Kilauea and Mauna Loa where vents have been repeatedly active in historic time.
215-2525-75Areas adjacent to and down slope of active rift zones.
31-515-75Areas somewhat less hazardous than Zone 2 because of greater distance from recently active vents and/or because the topography makes it less likely that flows will cover these areas.
4about 5less than 15Includes all of Hualalai, where the frequency of eruptions is lower than on Kilauea and Mauna Loa. Flows typically cover large areas.
5noneabout 50Areas currently protected from lava flows by the topography of the volcano.
6nonevery littleSame as Zone 5.
7nonenone20 percent of this area covered by lava in the last 10,000 years.
8nonenoneOnly a few percent of this area covered in the past 10,000 years.
9nonenoneNo eruption in this area for the past 60,000 years.

 

 How were the volcanoes on Hawaii formed?

The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. There are two hypotheses to explain them. One suggests that they are due to hot mantle plumes that rise as thermal diapirs from the core-mantle boundary. The other hypothesis postulates that it is not high temperature that causes the volcanism, but lithospheric extension that permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. The Hawaii hotspot is the volcanic hotspot that created the Hawaiian Islands in the central Pacific Ocean. 
The image of a hot spot
Source:
http://en.wikipedia.org/wiki/Hotspot_%28geology)

The Hawaii hotspot has created the Hawaiian – Emperor seamount chain comprising at least 129 volcanoes arranged in a line with a sharp bend. More than 123 of these are extinct volcanoes, seamounts, and atolls, four are active volcanoes, and two are dormant volcanoes. Hawaiian volcanoes primarily erupt a type of rock known as basalt. When molten, basalt produces liquids of relatively high fluidity, compared to volcanoes that erupt more silica rich magma types such as andesite, dacite or rhyolite. The fluidity of molten basalt favors the formation of lava flows, which is why the Hawaiian volcanoes generally have gentle sloping sides. By contrast, lavas with higher silica content are more viscous and commonly produce either thicker, shorter lava flows with thick deposits and/or thick beds of ash that fall from the sky following explosive eruptions. The other types of volcanoes common outside of Hawaii are normally steeper sided. 


The location of the main volcanoes
Source : http://www.soest.hawaii.edu/GG/HCV/haw_formation.html
Almost all magma created in the hotspot has the composition of basalt, and so the Hawaiian volcanoes are constructed almost entirely of this igneous rock and its coarse-grained equivalents, gabbro and diabase. A few igneous rock types with compositions unlike basalt, such as nephelinite, do occur on these islands but are extremely rare. The majority of eruptions in Hawai'i 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.

What are the types of volcanoes?

There are six types of volcanoes found on or near Hawaii. They are specifically shield volcanoes, extinct volcanoes, seamounts volcanoes, atolls, active volcanoes and dormant volcanoes.
  • Shield volcanoes - volcanoes built up with almost entirely fluid lava flow.
  • Extinct volcanoes - volcanoes that are unlikely to erupt again.
  • Seamounts volcanoes - individual volcanoes on the ocean floor.
  • Atolls volcanoes - an volcanic island.
  • Active volcanoes - volcanoes that erupt frequently.
  • Dormant volcanoes - volcanoes that are currently not erupting but are still capable of erupting.
The different types of volcanoes. Hawaii mainly has shield volcanoes.
http://belmont.sd62.bc.ca/teacher/geology12/photos/volcanoes/types_of_volcanoes.htm

Geological evidence from exposures of old surfaces on the south and west flanks of Mauna Loa led to the proposal that two ancient volcanic shields (named Ninole and Kulani) were all but buried by the younger Mauna Loa. Geologists now consider these "outcrops" to be part of the earlier building of Mauna Loa.

Due to the fact that Mauna Loa and Kīlauea are active volcanoes, the island of Hawaiʻi is still growing. Between January 1983 and September 2002, lava flows added 543 acres (220 ha) to the island. Lava flowing from Kīlauea had destroyed several towns, including Kapoho in 1960, and Kalapana and Kaimū in 1990. In 1987 lava filled in Queen's Bath, a large, L-shaped, freshwater pool in the Kalapana area. Steam plume as Kīlauea red lava enters the ocean at three Waikupanaha and one Ki lava ocean entries. Some surface lava is seen too. The southernmost point in the United States, (Ka Lae), is on Hawaiʻi. The nearest landfall to the south would be in the Line Islands. To the north is the island of Maui, whose Haleakalā volcano is visible from Hawaiʻi across the Alenuihāhā Channel. Approximately 35 km southeast of Hawaiʻi lies the undersea volcano known as Lōʻihi, an erupting seamount that reaches 980 m below the ocean surface.

Are the volcanoes safe or dangerous?

The volcanoes are dangerous because there are still four active volcanoes and two dormant volcanoes and they could erupt any moment. The chain of islands or archipelago formed as the Pacific plate moved slowly northwestward over a hotspot in the Earth's mantle at about 51 km per million years. Hence the islands in the northwest of the archipelago are older and typically smaller, due to longer exposure to erosion. From this study and others, it is estimated that the northwestern most island, the Kure Atoll, is the oldest at approximately 28 million years; while the southeastern most island, Hawaiʻi, is approximately 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 at the extreme southeast, Loʻihi.



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