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.

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.