Hawaii and the future

The submersed, volcanically inactive islands of the northwest part of the volcanic archipelago of volcanic islands of which the state of Hawai'i is a part, points to the future of the present Hawaiian islands.

As the Pacific Plate continues its cm-by-cm journey to the northwest, the Big Island of Hawai'i will move off the hot spot, its volcanic activity will cease, and erosion will eventually reduce the island to a submersed shadow of its former self.

This rather bleak view of Hawai'i’s future is tempered with the knowledge that even as erosion claims the older islands, and eventually the big island, even now the next island in the chain is growing on the seafloor off the southeast coast of Hawaii. Loihi is the name already given to this next piece of real estate in the Hawaiian chain.

Image of Loihi from here.

Hawaii and hot spots

The Hawai'i archipelago is the product of the interplay of plate tectonics—the shifting of the Earth’s outer, rigid crust—with a plume of molten material welling up from the Earth’s mantle, a hotspot.

The Hawaiian islands formed sequentially as the Pacific plate moved to the northwest over this hotspot. The upwelling magma erupted as basaltic flows on the ocean floor that eventually built up thousands of feet to breach the surface and form an island.

As the Pacific plate continued its movement to the northwest, the new island moved off the hot spot, and volcanic activity ceased. Once volcanic activity ceased, no new rock was added to the island, and surface processes, the action of wind and waves, became the dominant processes in shaping the island, and the quieted volcanic island began to succumb to erosion, eventually disappearing beneath the waves.

Photo: the southern coast of the Big Island takes a pounding from the waves.

More Aloha

The islands that comprise the state of Hawai’i are part of an extensive archipelago of volcanic islands—most of them well below sea level and most of them dormant---that stretches from the Big Island of Hawaii north and west to the Aleution Islands, another volcanic archipelago that stretches westward from Alaska to Asia.

Radiometric dates of basalts show that Hawaii’s islands grow older to the northwest; the big Island, the southernmost island in the chain, is the youngest (and indeed is still growing through active volcanic flows).

The Big Island is also the largest of the islands, thus it’s name, and the older islands are smaller.

The Big Island is the only currently volcanically active island; the older islands are not.

These three different lines of observation led geologists to formulate the hot spot theory.

Illustration from here.

First continents

Although we tend to think of continents as being permanent features on Earth, in fact the earliest Earth was devoid of continents.

Basaltic ocean crust formed first, as the early Earth cooled from its original molten state. Continental crust is formed through tectonic plate collisions, which involves the subduction of one plate, recycling the plate back into the Earth’s mantle, causing melting of lighter minerals and migration of this new, granitic magma upward where it solidifies, forming new continental crust.

Continental crust is less dense than ocean crust, and in a plate tectonic collision the lighter continental crust is not subducted but remains at the earth’s surface.

Continents grow by accretion of granitic crust through eons of plate collisions.

This process of continental growth and accretion continues today; much of the state of California is a mosaic of microplates that have been accreted to the west coast of North America.

Illustration from here.

Earth's first crust

As Earth cooled from its molten origin, the earliest crust probably consisted of iron and magnesium-rich minerals, minerals that can crystallize at high temperatures. This assemblage of minerals forms the igneous rock basalt.

Once the Earth had cooled sufficiently for this initial crust to form, shifting plates of basaltic crust collided, one plate sinking back into the Earth’s mantle, causing the less dense minerals in the upper mantle such as quartz and feldspar, to melt, and forming a lighter, less-dense magma that erupted at the Earth’s surface in a chain of volcanoes, or island arc.

This is the same sort of tectonic activity that formed the Aleutian Islands and the islands of Japan, so we can picture the first landforms on Earth as chains of volcanic islands.

Continents would form later, as tectonic plates carrying these island arcs collided.

Illustration of a very young Earth from here.

More on early Earth from National Geographic.