The Hawaiian Archipelago, part of the Hawaiian Island-Emperor Seamount chain, is the most isolated island group earth, and is comprised entirely of volcanic islands and their fringing reefs. The archipelago was formed as one of the Earth’s great tectonic plates, the Pacific Plate, moved steadily northwest over a stationary plume of molten material welling up from the Earth’s mantle, called a “hot spot”.
Creating a chain of volcanic islands that, today, stretches from the Aleutian Islands to the Big Island of Hawaii; this hotspot gave rise to at least 129 separate volcanoes in the Hawaii-Emperor chain over the past eighty-six million years. There are ninteen islands and atolls, and dozens of separate islets, seamounts, reefs, and shoals in the Hawaiian Island portion of this chain, stretching from Kure Atoll to Hawaii Island.
Hawai’i is the youngest island in the Archipelago and first began forming about a million years ago as volcanic vents opened above the mantle “hot spot” on the ocean bottom and molten lava began pouring onto the sea floor. Over the centuries, the making of the Big Island as we know it today eventually entailed the growth and conjoining of six separate volcanoes, building all the way up from the seafloor, some 18,000 feet below the ocean’s surface. These volcanoes, from northwest to southeast, are named Mahukona, Kohala, Mauna Kea, Hualalai, Mauna Loa and Kilauea, and become younger as one moves north to south.
Mahukona Volcano, just off the Big Island’s northwest coast, was the first volcano to start forming. Now submerged beneath the surface of the ocean because it is sinking into the Earth’s crust under its own vast weight, Mahukona is no longer visible. As the Pacific Plate slowly continued moving northwestward over the hotspot, the location of the rising magma moved relatively southeastward, and through time the rest of the Big Island volcanoes formed along that path.
After Mahukona, Kohala Volcano, the precursor to today’s Kohala Mountain, erupted next. As Kohala Volcano emerged from the sea and joined with Mahukona, a much larger Big Island began forming. With continued movement of the Pacific Plate, the center of volcanism migrated on to Mauna Kea and Hualalai, the middle-aged volcanoes, and finally on to Mauna Loa and Kilauea, which are the youngest volcanoes on the island. Over the geologically short time of several hundred thousand years, these volcanoes erupted thousands of thin flows which spread over, and built upon, older flows; each volcano growing until it finally emerged from the sea. As time went on, lava flows from one volcano began to overlap flows from other, nearby volcanoes and eventually the peaks coalesced into a single island, the Big Island.
In geologically recent times, a new volcano, Lo’ihi, began forming about eighteen miles off the southeast coast of the Big Island. In time, Loihi may join its mass with that of Kilauea, again changing the size and shape of the Big Island. It is estimated that Lo’ihi, whose summit lies approximately 3,178 feet below the surface of the ocean today, will begin to protrude above the surface in about 10,000 years.
Currently, the remnants of Mahukona and Kohala volcanoes are completely extinct; Hualalai and Mauna Kea are considered dormant and Mauna Loa, Kilauea and Loihi are still very active. This means that eruptions of Mahukona and Kohala are not at all likely. Eruptions of Mauna Kea and Hualalai are probable at some time in the future, though the major phase of mountain building is over for these volcanoes.
Movement of the Pacific Plate has moved both Hualalai and Mauna Kea off the hot spot so only remnants of liquid magma reside beneath them. These late-stage, mature Hawaiian volcanoes experience violent, explosive eruptions, which are spectacular but comparatively small, volume-wise. Late-stage Hawaiian volcanic eruptions are characterized by violent, crater-forming explosions, tephra-cone building and ash ejecting events.
