The Geology of Hawaii’s Seafloor is Totally Incredible.


The Pacific Plate is gradually shifting northwestward at a rate comparable to that at which your fingernails grow. The sequence of volcanic islands is the result of continuous plate movement over a local volcanic “hot spot,” or plume. For real, that’s what they’re called: Hawaii.

Eight main islands and 124 islets make up the Hawaiian Islands (also known as the Hawaiian archipelago), which stretches northwest from the Big Island of Hawaii for 1,500 miles toward Japan and the Aleutian Islands of Alaska. Altogether, the islands’ surface area amounts to around 6,459 square miles.

Currently, the Big Island is the largest island in the Hawaiian chain (though this might change in the future), followed by Ni’ihau, Kaua’i, Oahu, Molokai, Lanai, Kahoolawe, and Maui in order from west to east.

Hawaii, the youngest island in the group, began as five submerged volcanoes more than a million years ago. Hawaii was formed when five volcanoes erupted repeatedly, building up through the years in a series of thin layers of lava atop one another until the volcanoes’ heads finally broke through the surface of the ocean.

How, therefore, can five volcanoes form a single island? Flows from each eruption likely merged with those from the others, and the five peaks finally fused into a single island.

Because of their location directly over the plate’s “hot spot,” the Kohala Mountains were the first to rise. Rising magma first occurred under Mauna Kea, but when the plate changed, it moved to Hualalai, Mauna Loa, and finally Kilauea.

Furthermore, this procedure has continued uninterrupted ever since! A new seamount, Loi’hi (see second map above and more on Loi’hi below), is now developing off the southeast coast of the Big Island. It might combine with the other islands to form the Big Island’s sixth peak in another 50,000 years or so. Only Kohala’s smoldering ash remains are fully dormant at the present time; nothing else on the island is volcanically active. The Big Island’s other volcanoes aren’t ready just yet, either.

Around half of the Big Island is occupied by the massive Mauna Loa volcano. Still, most people have a difficult time locating it, since its shield form makes it difficult to recognize as a real mountain.

All of Hawaii’s volcanoes are classified as “shield volcanoes” due to their similarity in shape to a shield. Over millions of years, shield volcanoes emerge as molten lava flows up from a hot area in the Earth’s crust and erupts via different vents and rifts on the top, then flows down the gentle slopes into the ocean.

Gordon Joly

The fire goddess Pele resides atop the world’s most active volcano, Kilauea, is located on Mauna Loa’s eastern slopes. Kilauea used to be thought of as a vent of Mauna Loa, However, recent research has revealed that this little sibling is actually rather unique, including its own magma chamber.

About a quarter of the island’s entire land area is occupied by the massive volcano Mauna Kea. During the winter months, Mauna Kea has a snowy crown, which is whence it gets its name (Mauna Kea means “White Mountain”). This makes it much more visible than neighboring Mauna Loa. Here’s something noteworthy about this volcano: it’s the highest point in the Pacific Ocean and the tallest mountain in the world from base to peak, with a total height of almost 33,000 feet above the sea bottom, of which only 13,780 (approximately) feet exist above sea level.

View of Halema’uma’u Crater from the top of Kilauea Volcano in the early evening of January 30th, when lake level was 88 feet below crater floor.

Eruption of the Klauea volcano.

 Photo: USGS

Taken from the observatory atop Mauna Loa, this picture shows the mountain of Mauna Kea. The Hawaiian island that’s the biggest.

Photo: Nula666

The other two volcanic mountains on the Big Island are Kohala, in the northwest, and Hualalai, in Kailua-Kona, in the west.

Kohala, the island’s oldest peak, has more eroded away due to geological processes than any of the island’s other mountains. Amazing sea cliffs apparently resulted from a massive landslide over 200,000 years ago.

Sea cliffs in the background of Waipio Valley, the most notable valley cutting through the Kohala volcano’s face.

Photo: Paul Hirst

As was noted before, the underwater volcano of Lo’ihi is located only 18 miles off the southeastern shore of Hawai’i. Lo’ihi is a seamount that is currently erupting and may be found around 3,178 feet below sea level.

When Lo’ihi finally breaks the surface of the ocean, it will probably combine with Kilauea (which, in theory, will be considerably larger by that time) to form the sixth peak on what is now Hawai’i’s biggest island. However, it won’t occur overnight; at the very least, you’re looking at a 50,000-year time frame. You shouldn’t reserve a stay at the hotel just yet.

Locating Lo’ihi to the pinpoint.

Image credit: Sémhur derivative work: Kmusser (talk)

The Big Island of Hawai’i appears enormous in comparison to the other islands in the chain, even if that sixth peak will have to wait to be added to it. However, in terms of its place in history, it’s not all that much bigger. The island of Maui, home to Haleakala, the world’s biggest dormant volcano, is only across the “Alenuihaha Channel.” The tallest point is 10,023 feet above, and its crater is around 7.5 by 2.5 miles in size. In spite of this, the mountain looks considerably smaller than Big Island since so much of it is submerged in water. However, Haleakala would reach approximately 30,000 feet in elevation if measured from the ocean floor. This huge volcano, responsible for the formation of 75% of Maui Island, is currently inactive but not extinct. The time since the previous eruption is estimated to be in the late 1700s.

Geologists think that Haleakala and the western part of Maui were formerly linked into a single landmass known as Greater Maui, which also included the islands of Lanai, Molokai, and Kahoolawe (Maui Nui). Large sections of the Big Island have vanished in the Pacific as a result of the submergence of that landmass, which caused the volcanic body to move away from the Hawaiian hot point. This process produced the four main islands we see today.

Look down on the Hawaiian Islands from space.

Image source: NASA

The Big Island will meet the same fate as the other islands in the chain. With the hot spot “moving away” and the Pacific Plate carrying the islands piggyback-style to the north-west, the Big Island will eventually succumb to subsidence and erosion and meet the same destiny as Maui Nui. As the water gradually erodes the mountain slopes, it will break up into a series of smaller islands.

The Big Island, however, will keep its name and size for the next several thousand years at least! The island of Hawaii is still expanding as both Mauna Loa and Kilauea continue to erupt and be active.

It is too soon to tell what the islands’ geological future will look like.

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