To understand the origins of the islands, you need to know a little bit about modern geology. Scientists today consider the earth's crust to be made up of a series of tectonic plates that float on an inner sphere of molten rock. These plates actually move, albeit very slowly, and their movement explains how the continents moved apart and became separate, millions of years ago. Plate movement is a tremendously powerful force. At places like the San Andreas Fault, the movement of tectonic plates can cause earthquakes. Whole new mountain ranges, like the Himalayas, can be created when plates collide.

The Hawaiian Islands are located on top of the Pacific Plate, which moves at the rate of about 3.5 inches a year. Over millions of years, the plate has been drifting northwest over a “hot spot” in the inner sphere of molten rock. The magma that has spewn forth from this spot is responsible for building this chain of islands. As the Pacific Plate continues its drift, the hot spot spews magma in different places on the plate, creating a long chain of different islands. The oldest is Kure Atoll, in the northwest. The chain continues all the way to the islands of Niihau and Kauai, the oldest Hawaiian islands, and on to Big Island in the south, which is currently located over the hot spot and is presently volcanically active.

Mountains Beneath the Sea

The Hawaiian Islands are a form of land mass known as a “shield volcano.” As opposed to the kind of volcano that looks like a steep-sided cone, a shield volcano emerges gradually and spreads out. This process has given the islands their characteristic profile of a shield with a bulging center. Shield volcano formation starts on the seafloor, and each island grows over a period of hundreds of thousands of years of eruptive activity. The Hawaiian Islands, then, are really the tops of subterranean mountains that were once active volcanoes!

On the Big Island, there are two mountains, Mauna Loa and Mauna Kea, that reach heights more than 13,000 feet (13,796 feet and 13,677 feet respectively) and on Maui, the crater of Haleakala reaches more than 10,000 feet. When it's measured from its base on the seafloor, some say that Mauna Loa is the biggest mountain on earth!

The Lava Keeps Coming!

Today, the eruptive process is quite visible on the Big Island, where the Kilauea Volcano has been actively spewing lava over the last several years. In fact, the Kilauea Volcano is arguably the most active on earth. Another volcano on the island, Mauna Loa, has erupted about three dozen times in the last 150 years or so. Hawaii is the only state in the United States that is physically growing. Lava from the Kilauea Volcano continues to flow into the ocean on the Big Island's southern shore, thus slowly but surely adding to the island's landmass. Since 1983, the volcano has added more than 500 acres to the island's area.

It's possible for visitors to view flowing lava from the Big Island's Kilauea Volcano. Air tours make it easy to approach the active volcano from a safe distance. To learn what's currently happening, visit the U.S. Geological Service Volcano Observatory site at http://hvo.wr.usgs.gov and the National Park Service's visitors' information site for Hawaii Volcanoes National Park at www.nps.gov. You'll also find more information about the Hawaii Volcanoes National Park in Chapter 13.

The Aging Process

Following their eruptive and island-building phases, some volcanoes become dormant with the possibility of future eruptions, while others become virtually extinct. In either case, nature takes its course in the form of erosion and weathering, as wind, rain, and waves conspire to sculpt and wear down the landscape. The older islands, such as Kauai, provide dramatic evidence of nature's relentless processes with their massive canyons, towering cliffs with waterfalls, and craggy, mountainous terrain. Plants, too, contribute to the ongoing changes as their prying roots assist in splitting rocks and building up the soil.

Coral atolls, which can be found throughout much of the Pacific, form around eroded volcanic formations that have been worn down to sea level. The edges of these diminished features provide a great home for coral growth, and the atolls form with their classic horseshoe configuration with an interior lagoon.