Islands Under Construction
“HAWAII.” The Hawaiian Islands evoke visions of tropical paradise, sunny beaches, and balmy trade winds. But were you aware of the remarkable isolation of these islands? If you locate Hawaii on a map, you will find these islands nestled in the center of the North Pacific Ocean—about as far away from mainland coastlines as you can get! Thus, you may wonder, ‘How did the islands get there? Do scientists believe that more islands will form in the future? What can these islands tell us about the very earth beneath our feet?’
The Hawaiian Archipelago
Most people who visit Hawaii become familiar with the chain of eight islands that extends from northwest to southeast, the largest of which are Kauai, Oahu, Molokai, Lanai, Maui, and Hawaii. Smaller Niihau is west of Kauai, and Kahoolawe is southwest of Maui. The island of Hawaii, also called the Big Island, spans more than 4,000 square miles [10,000 sq km], whereas little Kahoolawe covers just 45 square miles [117 sq km]. In addition, the island chain includes another 124 much smaller islands, or islets, that extend even farther to the northwest. Midway, near the northwest terminus of the chain, is nearly 1,600 miles [2,500 km] away from the Big Island! The islets, largely made up of coral and sand, form a total surface area of only three square miles [8 sq km]. Aptly, some use the name Hawaiian Archipelago to refer to the entire group of islands.
If we consider that the islands and islets are based on broad platforms that rise, on the average, more than 15,000 feet [4,000 m] above the surrounding seafloor, we begin to appreciate that they are merely the exposed tips and crowns of huge mountains. In fact, when measured from their base on the ocean floor, Mauna Kea and Mauna Loa on the island of Hawaii rise some 33,000 feet [10,000 m]. Thus, they are in a sense the world’s tallest mountains!
Building an Island
Let us examine further the island of Hawaii. Geologists have determined that the Big Island consists of five large coalesced volcanoes. Most visitors are familiar with the three largest—Mauna Kea, which is considered to be dormant and boasts the highest point in Hawaii, 13,796 feet [4,205 m] above sea level; Mauna Loa, which is 13,678 feet [4,169 m] and is the largest Hawaiian volcano in volume; and Kilauea, which is the youngest volcano and is located on the southern flank of the island. In addition, Kohala volcano forms the northwest tip of the island, and Hualalai rises above the Kona coast.
Each volcano grew by the outpouring and stacking of many thousands of lava flows. Eruptions begin underwater, where the lava quickly quenches, forming rinds and tongue-shaped flows that when stacked up look like piles of pillows. When the growing volcano emerges above the water, the lava flows take on a different appearance. Volcanologists use the Hawaiian terms “pahoehoe” for fluid flows having smooth, billowy, and ropy surfaces and “aa” for lava that is rough, jagged, and rubbly. The volcano develops into a broad, gently sloping mountain that resembles in form the shields carried by ancient Roman warriors. Large craters develop at the volcano summit when magma, or molten rock, erupts or withdraws from cavities near the surface. Also, the magma reservoir inside the volcano exerts pressure. This pressure pushes part of the volcano seaward, opening up large sets of cracks. Finally, as in the case of Mauna Kea, the eruptions of a shield volcano become more explosive, producing cone-shaped heaps of volcanic cinders that dot the volcano.
Mauna Loa and Kilauea have proved to be among the most active volcanoes in the world. Historical accounts from native Hawaiians, missionaries, scientists, and others indicate that 48 eruptions have taken place on Mauna Loa since 1832 and more than 70 eruptions on Kilauea since 1790. These eruptions have lasted anywhere from hours to years. The longest on record was a lava lake in Halemaumau crater on Kilauea, which was active nearly continuously from the early 1800’s to 1924. Presently, Kilauea has been erupting since January 1983, occasionally producing spectacular fire fountains and rivers of lava that have flowed into the sea.
Because of their typically fluid lava, most Hawaiian eruptions have been nonexplosive or only mildly explosive. In rare instances, however, groundwater mixes with magma, resulting in steam explosions. In 1790, such an eruption killed about 80 people when a band of native warriors and their family members became engulfed in hot gases and burning cinders spewed from Kilauea.
Moving Islands
Recorded history over the past 200 years indicates that only the two southeasternmost islands, Hawaii and Maui, have been active. This puzzling situation moved scientists to study the rock history of the island chain further. Trapped within the lava are minute amounts of a radioactive form of potassium and its decay product, argon, which they could measure in the laboratory to estimate rock ages. Such investigation revealed a systematic, northwestward aging of the entire Hawaiian Archipelago over many millions of years.
Since Hawaiian eruptions have been occurring more in the southeastern part of the island chain, does this mean that the source of magma beneath them has been moving too? In actuality, geologists have determined that the magma source, which they call a hot spot, is stationary. Instead, the floor of the Pacific Ocean has been moving over the hot spot, carrying the island volcanoes away from the hot spot like piles of rocks on a moving conveyor belt. This same movement grinds the Pacific seafloor against neighboring continental masses and other parts of the seafloor, causing many of the great earthquakes that occur along the Pacific Rim. If you live in Hawaii, your home has crept about three inches [7.5 cm] to the northwest since last year!
Scientists suggest that other hot spots like the one beneath Hawaii account for many of the volcanoes around the world, both on land and in the sea. Most of these hot spots also show evidence of migrating eruptions, which means that the earth’s surface has likely been shifting where you live as well.
Formation of New Islands . . .
Since hundreds of thousands of years have been required to build the large volcanoes on the Big Island, we might expect that the island has been moving away from the hot spot during this time. New volcanoes and islands should then develop above the hot spot as it encounters unaffected seafloor. Has a potential successor to the volcanoes of the Big Island already manifested itself?
Indeed it has. A volcanically active underwater mountain, Loihi, is growing south of the island of Hawaii. Don’t expect it to emerge from the sea soon, however. It still needs to rise another 3,000 feet [900 m], which may take tens of thousands of years.
. . . And Destruction of Old Islands
The stout volcanic shields and rugged lava flows making up the Hawaiian Islands appear deceptively secure from resubmergence into the ocean. But small islets and drowned seamounts northwest of Hawaii tell a different story. The sands and coral reefs of Midway and Kure islands, for instance, are built on large volcanic mountains whose tops are now hundreds of feet below sea level. Why do volcanic islands disappear?
The islands gradually give way to steady erosion brought about by stream runoff, wave action, and other forces. The islands also sink under their own weight as they depress the ocean floor. Steep cliffs along the sides of some islands indicate another process by which volcanic islands decay—landslides. Sonar images of the undersea flanks of the islands reveal huge landslides that stretch for tens of miles out onto the seafloor.
Hot Spot in Action
On the island of Hawaii, visitors to the Hawaii Volcanoes National Park can see for themselves the ever-changing landscapes produced by hot-spot volcanic activity. Scientists of the Hawaiian Volcano Observatory, located on the rim of Kilauea crater, monitor ongoing and threatening eruptions. Their studies have led to greater insight into how volcanoes work and how the surface of the earth has been shifting. With awe, we can appreciate that mighty geologic forces have formed and shaped the Hawaiian Archipelago—this magnificent chain of islands in the middle of the Pacific Ocean.
[Map on page 25]
(For fully formatted text, see publication)
Hawaiian Islands
Niihau
Kauai
Oahu
Molokai
Lanai
Maui
Kahoolawe
Hawaii
[Credit Line]
Mountain High Maps® Copyright © 1997 Digital Wisdom, Inc.
[Picture on page 24]
A line of fire fountains on the east rift of Kilauea
[Picture on page 24, 25]
An eruption on Kilauea
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Volcanoes: Dept. of Interior, National Park Service
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A lava river on Mauna Loa
[Picture on page 26]
A curtain of fire on Mauna Loa
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Top left and bottom right: Dept. of Interior, National Park Service
[Picture on page 26]
A fire fountain eruption on Kilauea
[Credit Line]
U.S. Geological Survey
[Picture on page 26]
A lava lake on Kilauea
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Top left and bottom right: Dept. of Interior, National Park Service