THE GEOLOGY OF HAWAII

The state of Hawaii, the southernmost state in the USA, consists of a chain of islands in the middle of the Pacific Ocean called the Hawaiian Archipelago that stretches over 1500 miles northwest from the Big Island of Hawaii to the Kure Atoll just past Midway Island, an unincorporated territory of the USA. Past Kure Atoll, the chain continues as underwater seamounts forming the Hawaiian Emperor seamount chain that stretches 3900 miles across the North Pacific Ocean. This chain begins just south of the Aleutian Trench almost to Russia and continues southwest all the way to the Big Island. It includes over 80 identified undersea volcanoes with the youngest, Loihi, currently erupting from its summit 1000 feet underwater, about 22 miles southeast of the Island of Hawaii.

This mountain chain in the middle of the Pacific Ocean has been produced by volcanic activity that began over 81 million years ago at Meiji Guyot at the far north end of the chain. A guyot is an isolated, flat-topped tablemount 200 meters below the surface of the ocean, the erosional remnant of a volcano. These features were first recognized by the geologist Harry Hess who collected echo soundings of the ocean floor on the ship he commanded in World War II. He named the guyots after the name of the geology building at Princeton where he taught, which had been named after the Swiss American geologist Arnold Henry Guyot.

The dating of the volcanic activity on these features continues to get younger sequentially to the southeast, with the most recent activity at the end of the line, Loihi. The main islands of Hawaii demonstrate the increasing age of the chain to the northwest as well. Loihi is currently building itself up by underwater eruptions. The Island of Hawaii not only has the largest area but is also composed of five volcanoes which have joined together. Of the five, Mauna Loa is the largest active volcano on Earth with an elevation of 13,678 feet while Kilauea, elevation 4,091 feet, produces the most lava. These volcanoes are shield volcanoes with gently sloping sides because the lava is composed primarily of basalt, the rock that makes up oceanic crust, which is less viscous and flows more easily than silica rich magmas. The latter are most common from volcanoes near or within the continental crust, like Mount St. Helens in Washington.

Just to the north of Mauna Loa lies the dormant volcano, Mauna Kea, at an elevation of 13,803 feet, the highest peak on the island and the second highest peak of an island on Earth. The northwestern most volcano on the Big Island, Kohala, estimated to be 1 million years old, is old enough to have recorded the last reversal of the earth’s magnetic pole 780,000 years ago and is considered extinct. The Big Island continues to increase in size because of the volcanic activity associated with Kilauea as lava flows continue to plunge into the sea on the east side of the island. It is the only island getting bigger; the rest are being eroded away by natural processes.

The physical appearance of the main islands continues to show more evidence of aging the further west they are. Maui, the second largest island, is associated with three other smaller islands, and is composed of two shield volcanoes separated by an isthmus. The older, less than 1.3 million years old, western volcano is heavily eroded and only 5,788 feet high, but the younger, eastern volcano, Haleakala, rises to an elevation of 10,023 feet. The shallow channels between Maui and the smaller three islands indicate that they were at one time connected. Sea level changes or general subsidence of the Maui Nui complex after lava sources became inactive could account for the current separation of the four.

The island of Oahu, the third largest in Hawaii, is only 4,025 feet high, but like Maui, is composed of two shield volcanoes, the Waianae Range on the west and the Ko’olau Range to the east, both deeply eroded. Age of the rocks range from 3.9 million years on the west to 2.7 million years on the east. Further studies using  drill core logging can support this finding. The largest city in Hawaii is Honolulu, the state capital and the home of the deep-water port, Pearl Harbor, on the southern edge of the island between the two mountain ranges. Subsidence of the island probably accounts for the excellent harborage found here for the U.S. Pacific Fleet.

The Island of Kauai is nicknamed “the Garden Isle” because of the tropical rainforest that blankets much of the island. It has the only navigable rivers in Hawaii, but its longest, Wailua, is only 20 miles long. The summit of the island’s shield volcano, Mount Waialeale, is called Kawaikini and attains an elevation of 5,243 feet. Its last eruption occurred over 5 million years ago. It is only the fourth largest island in the chain but contains one of the wettest spots on earth on the east side of Waialeale with an average yearly rainfall of 460 inches.

Erosion and subsidence dominate the rest of the Hawaiian Emperor Seamount chain. Atolls are formed on top of the submerged volcanic mountains from corals that build there during the early stages of subsidence. Further subsidence through time, coupled with rapid sea level changes from intermittent glacial periods mixed with climate change, can swallow up the coral reefs and leave behind deep water seamounts as observed in the northern most sections of the chain.

 The source of the volcanism appears to be a mantle hotspot situated today underneath the Big Island. The migration of the Pacific Oceanic Plate over this magma source in a northwesterly direction would produce the observed correlation between age and distance. Although it has been assumed that the hotspot has remained stationary over the life of the chain, that assumption remains open to question. Look for a follow-up about the possible mobility of the Hawaiian hotspot through time. 

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