HOW-waii? Geology’s Influence in Shaping America’s Youngest State

In the midst of a New England winter you would be hard-pressed to find anyone that hasn’t fantasized about exchanging the frigid temperatures of March for the tropical weather of Hawaii. The Aloha State is known for being a vacation hot spot around this time of the year, but did you know that it’s also a famous geologic “hot spot” as well?

Hawaii is geologically unique for a number of reasons, some of which include its isolated location in the middle of the Pacific Ocean, the fact that it’s the only U.S. state made up entirely of islands, and that along with it being the last state to join the U.S., it’s also the youngest state in terms of its geologic age. Considering there are so many ways in which Hawaii is different from its 49 compatriots, you might be surprised to learn that some of the many characteristics that make it different can be attributed to one geologic phenomenon: hotspot volcanism.

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From mountain ranges to deep-sea ocean trenches, and even to the shape of the continents, many of Earth’s major geologic features are the product of plate tectonics. The idea of plate tectonics is that the Earth’s crust is segmented into chunks (plates) instead of being one homogenous layer of rock. While a majority of volcanoes form along the boundaries of two plates where decompression and dehydration melting can occur, some volcanoes form in the middle of a plate due to the presence of a hotspot. Hotspots are areas of anomalously high temperatures that originate deep inside the Earth’s mantle and extend all the way to the Earth’s crust.

It isn’t quite clear how or why hotspots form, but what we do know about them is that due to high temperatures at these hotspots they produce a lot of molten rock called magma. Magma is less dense than solid rock. When rock melts, the magma rises up through the mantle and crust towards the Earth’s surface. When magma breaks the Earth’s surface, it’s called lava, and the opening through which the lava comes out is called a volcano.

As lava is released from the volcano, environmental conditions like cold water or air cause the lava to lose heat and solidify into hard rock. Over time, as more and more lava is released and cooled, the rock that it produces will build up and form a volcanic island, just like one of the many islands found in Hawaii’s island chain. The process of forming volcanic islands will continue as long as the hotspot exists, so brand new land is continuously being produced. Most of the land masses found on Earth have been around for billions of years, but the youngest Hawaiian island is only about 300,000 years old. Because of hotspot volcanism, the Hawaiian islands are among the youngest land masses on Earth.

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Hotspot volcanism explains how a single Hawaiian island forms, but understanding how one island is made isn’t enough to understand how the entire state formed. After all, Hawaii is made up of multiple islands, not just one. Luckily, geology can explain this phenomenon as well.

Volcanic cone Pu'u 'O'o on Kilauea, Hawaii

Unlike tectonic plates that are in constant motion, hotspots are stationary. They form deep within the mantle (some scientists speculate they can form as deep as the core-mantle boundary) where they are unaffected by the movement of tectonic plates. This means that when a volcanic island forms over a stationary hotspot, it will eventually be pushed away due to the movement of a plate. In its place, new crust will pass over the hotspot and the process of forming a volcanic island will begin again.

Over time, as islands are produced by the hotspot and subsequently dragged away by plate motion, a long chain of islands called an archipelago develops. Archipelagos are fairly common features on Earth. Some of the most common include Japan (the entire country is one big archipelago!), the Aleutian islands of Alaska, and the Antilles archipelago. However, few archipelagos are formed via hotspot volcanism like Hawaii is.

The orientation of the Hawaiian archipelago in relation to its hotspot is also interesting because it can reveal information about how the tectonic plate it sits on is moving. If you look at the shape of Hawaii, you’ll notice that the islands form an almost straight line. This is because the tectonic plate that Hawaii’s hotspot is located under has been moving in the same northwestern direction since the formation of the oldest islands, about 5 million years ago. However, what we find above the sea surface doesn’t give us the full story of Hawaii’s islands, or of the tectonic plate they sit on.

Seafloor mapping reveals that the Hawaiian islands are actually a part of a 3,600 mile-long chain of sunken volcanic islands (guyots, if you want to get fancy with your terminology) called the Emperor Seamount Chain.The chain consists of roughly 125 extinct volcanoes, beginning at the presently-growing Lo’ihi seamount just southeast of Hawaii and extending all the way to the Aleutian Trench off of the Alaskan archipelago.

The chain that exists today is about 70 million years old where it ends at the Aleutian Trench. It’s likely that the hotspot has been producing islands for more than 70 million years, but due to plate tectonics it’s hard to be sure. The Aleutian Trench marks the meeting point of two plates where the Pacific Plate begins sinking beneath the North American Plate, so any islands that could have existed more than 70 million years ago are definitely not in existence today.

What’s interesting is that at a certain point, the seamount chain alters its orientation from a northwestern direction to an almost entirely northern direction. This indicates that currently, the Pacific tectonic plate is sinking at the northwestern edge of the plate, but it wasn’t always. Before about 43 million years ago (the age of the kink in the seamount chain) the plate was actually sinking at the northern end. For now, nobody knows exactly what caused this change, but one of the wonderful things about science is that there’s always a mystery to be solved. Centuries ago, nobody knew how volcanoes formed, what plate tectonics were, or how some islands in the middle of an ocean came to be. Centuries from now, one can only imagine what new questions there are to be answered.

- Haley Royer, Guest Writer