The Hawaiian Island chain is 3,700 miles long and 70 million years old. Kilauea is the youngest of the subaerial volcanoes on the Island of Hawaii and lies at the southeast end of the Hawaiian chain. The current eruption of Kilauea began in 1983 located on the East Rift Zone, and stands over 4,000 feet tall.
Hawaii itself consists of five connected volcanic mountains that were built by a lava plume rising from the mantle. Kilauea, the world’s largest active volcano, is still rumbling because the island has yet to move completely off the hot spot. The farther the other islands in the chain are from Hawaii, the greater their age. About 150 miles to the northwest is Oahu, which burst out of the sea about 3.5 million years ago. Midway, one of the oldest islands in the chain, was formed between 15 and 25 million years ago.
About 2,000 miles from Hawaii, the chain abruptly veers and extends north as a line of submerged volcanoes known as the Emperor Seamounts. This suggests that the Pacific Plate changed course about 40 million years ago. Where the chain’s long march ends the volcanoes are more than 70 million years old. And, not surprisingly, off the southwestern coast of the island of Hawaii, beneath the ocean surface, Loihi, the next Hawaiian Island, is forming as the Pacific Plate continues its journey over this hot spot.
The aging of the islands varies with the distance from the current hot spot, where distance along the chain is approximated as distance away from Kilauea volcano (the youngest above-sea-level Hawaiian volcano). In fact, even beyond Kure the Hawaiian chain continues as a series of now-submerged former islands known collectively as the Emperor seamounts. The two primary volcanoes that make up Oahu (where Honolulu is) have not erupted for well over a million years!
The age trend of the volcanoes is thought to be due to the way in which the islands are built on the moving sea floor of the North Pacific Ocean: the Pacific Ocean is mostly floored by a single tectonic plate (known as the "Pacific Plate") that is moving over the layer in the Earth known as the Asthenosphere. This movement takes it to the northwest compared to the layers below it at a rate of 5 to 10 cm/yr (the rate depends on where you are on it).
As the plate moves over a fixed spot deeper in the Earth where magma (molten lava) forms, a new volcano can punch through this plate and create an island. As the plate moves away, the volcano stops erupting and a new one is formed in its place. With time, the volcanoes keep drifting westward and getting older relative to the one active volcano that is over the hot spot. As they age, the crust upon which they sit cools and subsides. This, combined with erosion of the islands once active volcanism stops, leads to a shrinking of the islands with age and their eventual submergence below the ocean surface.
The distinctive linear shape of the Hawaiian Island-Emperor Seamounts chain results from the Pacific Plate moving over a deep, stationary hotspot in the mantle, located beneath the present-day position of the Island of Hawaii. Heat from this hotspot produced a persistent source of magma by partly melting the overriding Pacific Plate. The magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust to erupt onto the seafloor, forming an active seamount.
Over time, countless eruptions cause the seamount to grow until it finally emerges above sea level to form an island volcano. Continuing plate movement eventually carries the island beyond the hotspot, cutting it off from the magma source, and volcanism ceases. As one island volcano becomes extinct, another develops over the hotspot, and the cycle is repeated. This process of volcano growth and death, over many millions of years, has left a long trail of volcanic islands and seamounts across the Pacific Ocean floor. To the west of the Hawaiian Islands, a line of sea mounts extends for thousands of miles growing progressively smaller from the effects of millions of years of continual underwater erosion.
Over the past 70 million years, the combined processes of magma formation, volcano eruption and growth, and continued movement of the Pacific Plate over the stationary Hawaiian "hot-spot" have left a long trail of volcanoes across the Pacific Ocean floor. The Hawaiian Ridge-Emperor Seamounts chain extends from the "Big Island" of Hawaii to the Aleutian Trench off Alaska.
According to hotspot theory, the volcanoes of the Hawaiian chain should get progressively older and become more eroded the farther they travel beyond the hotspot. And this is the case. The oldest volcanic rocks on Kauai, the northwestern most inhabited Hawaiian island, are about 5.5 million years old and are deeply eroded. By comparison, on the "Big Island" of Hawaii – southeastern most in the chain and presumably still positioned over the hotspot -- the oldest exposed rocks are less than 0.7 million years old, and new volcanic rock is continually being formed.
The possibility that the Hawaiian Islands become younger to the southeast was suspected by the ancient Hawaiians, long before any scientific studies were done. During their voyages, sea-faring Hawaiians noticed the differences in erosion, soil formation, and vegetation and recognized that the islands to the northwest (Niihau and Kauai) were older than those to the southeast (Maui and Hawaii).
This idea was handed down from generation to generation in the legends of Pele, the fiery Goddess of Volcanoes. Pele originally lived on Kauai. When her older sister Namakaokahai, the Goddess of the Sea, attacked her, Pele fled to the Island of Oahu. When she was forced by Namakaokahai to flee again, Pele moved southeast to Maui and finally to Hawaii, where she now lives in the Halemaumau Crater at the summit of Kilauea Volcano.
The mythical flight of Pele from Kauai to Hawaii, which alludes to the eternal struggle between the growth of volcanic islands from eruptions and their later erosion by ocean waves, is consistent with geologic evidence obtained centuries later that clearly shows the islands becoming younger from northwest to southeast.