|Image from Sinton et al 2014 (see references).|
In fact, our knowledge of the island of Oahu was recently enhanced by an exciting discovery revealed by a research team led by geologists at the University of Hawai'i. Previously the conventional thinking was that Oahu was formed by two shield volcanoes: the Ko'olau volcano and the Wai'anae volcano. However, the recently announced discovery indicates that there is in fact a third shield volcano which makes up part of the island of Oahu. The newly-discovered volcano has been called Ka'ena by its discoverers, and is located approximately 20 kilometers off the coast of Ka'ena point. In this post we'll tell you about the "new" volcano, but first we describe the context with a sort of "primer" as to how the island of Oahu formed in the first place.
Hot Spot Volcanism
The first thing to understand is that our volcanoes are special and differ from other volcanoes you might be familiar with, like Mt. Fuji, Mt. Pinatubo, and Mt. Saint Helen's. This is because they form above a hot spot, which is an upwelling of mantle from deep within the earth. The composition of the magma that flows out of our volcanoes is different from most other volcanoes found on continents or island arcs in that it flows much more freely and has a lower solidification point. Practically speaking this means that the lava(1) can flow further away from the vent, and also that it doesn't plug up the vent, and so instead of a "classic" cinder cone volcano (like Mt. Fuji), our volcanoes grow much bigger and have gently sloping sides. Indeed, our volcanoes are called "shield" volcanoes because they resemble a warrior's shield that has been laid upon the ground.
|Relative size of cinder and shield volcanoes. Graphic from US Geological Survey.|
Another important thing to understand about our volcanoes is that while the hot spot doesn't move, the islands themselves do! This is because the earth's surface is made up of a number of tectonic plates that move around due to convection currents within the mantle of the earth. While a full description of plate tectonics will have to wait until a later post, we can understand that the tectonic plate upon which are islands sit (the Pacific plate) is moving in a northwesterly direction at about the same speed at which your fingernails grow. This is why we have a chain of volcanic islands and not just one big island.
|Diagram from Wikipedia.|
Stages in the Lifecycle of a Hawaiian Island Volcano
Now that we understand the fundamental difference between Hawaiian shield volcanoes and cinder cones, we can understand the various stages in the lifecycle of the Hawaiian Islands. You might have noticed that the Big Island, which is currently still experiencing volcanic activity and is still growing, is far larger than the other islands in the archipelago, and that as you move towards the northwest the islands seem to get smaller and smaller, until finally you find very small atolls of Kure and Midway. This is no coincidence. This section will help you understand why this is the case.
1. Preshield Stage. This first stage happens deep in the depths of the ocean', several kilometers beneath the surface where there intense pressure due to the weight of the water above. The preshield stage is characterized by infrequent, small-volume eruptions which produce pillow lava. As the volcano grows the composition of the lava changes. The volcano erupts more often, and produces more and more lava. When this happens the new undersea volcano enters the second stages. It is very hard to observe the preshield stage due to the extreme depths and the fact that these vents are so hard to locate.
2. Submarine Shield Stage. This stage features continued effusive eruptions of pillow lava deep
beneath the surface of the ocean. The undersea volcano grows into a huge mountain. Here in the Hawaiian islands the Lō'ihi volcano, which is located approximately 1000 meters beneath the surface of the ocean about 35 kilometers southeast of the Big Island, is in the submarine shield stage. Volcanologists expect Lō'ihi to breach the surface and become our newest island within the next 100,000 years.
|Lō'ihi image from Wikipedia.|
3. Explosive Stage. The shield volcano enters this stage when the top of the erupting volcano begins to approach the surface of the ocean and the lava and ocean water mix to produce explosive eruptions. Think about squirting a water gun at a campfire. It sizzles, right? Well, the explosive stage in the volcano's life cycle is based on the same principle, only a bazillion times bigger. Currently there are no volcanoes in Hawaii in this stage, but Hunga Ha'apai island in Tonga is.
4. Subaerial Shield Stage. After enough ash, debris, and other volcanic matter accumulates such that the volcano is no longer erupting into the sea but rather upon its own flanks it is said to have entered the subaerial shield stage. Since the lava is no loner in direct contact with water when it is extruded the eruptions are much calmer. Currently Kilauea on the Big Island is in this stage. During this stage eruption rates increase and the island grows rapidly over a period of approximately half a million years.
|Kilauea is currently in the subaerial shield forming stage. Map from here.|
5. Capping (postshield) stage. For some of the volcanoes, once they get to be high enough the composition of the lava they produce changes. This different lava produces steep peaks on the top of the volcano that look like pointy hats. If you've ever been up top he top of Mauna Kea you've probably noticed the steep landforms; these are the products of the capping stage. However, it seems that not all of the volcanoes experience this stage; Ko'olau and Lana'i are notable exceptions. Eruption rates slow significantly during this stage until the volcano stops erupting all together.
7. Renewed volcanism (rejuvenated) stage. The seventh stage is one of the most fascinating and least understood stages. It is clear that at some point long after (in some cases millions of years) the shield volcano stops erupting, new lava flows and hydromagmatic eruptions occur, which seem to be much more violent but much smaller than the shield volcano eruptions. Virtually all of the craters on Oahu are examples of this stage, as are Kūpikipiki'ō (Black Point) and the Sugarloaf flow out of Mānoa valley. There are a number of hypotheses to explain renewed volcanism and a full discussion is beyond the scope of this blog post. This stage seems to continue for a long period of time, and some geologists date some of the volcanic activity over on the Waimanalo side as recently as 4,000 years ago, so it seems to me that Oahu is probably still in this stage!
8. Atoll stage. After the rejuvenated period ends, the islands continue to erode and erode for millions
|Kure atoll image from here. The Hawaiian name for Kure|
atoll is Moku Pāpapa.
9. Guyot (seamount stage). The last stage begins when the islands are pulled by the tectonic drifting of the Pacific plate out of water warm enough to support coral. When this happens the reefs stop growing and eventually sink back into the sea, and so all that is left is a high spot on the ocean floor, known as a "seamount" or a "guyot". Since these are beneath the surface of the water you can't see these, but they are very clear from sonar imagery. There is a long chain of seamounts extending to the north all the way up to the Aleutian trench, where the seamounts are being subducted back into the earth's bowels by the slow grind of tectonic forces. This is where the Hawaiian islands finally pass into oblivion. We have no idea how many islands there have been in the past, but we do know that the oldest seamount is approximately 81 million years old. The seamounts are called the "Emperor Seamounts" and are named for Japanese emperors.
|The Hawaiian Islands and Emperor Seamounts. Note the "bend" in the line of volcanoes, which indicates a shift|
in the motion of the tectonic plates. "My" means "million years" and indicates the age of the volcano.
The Ka'ena Volcano
Dr. John Sinton of the Department of Geology at the University of Hawai'i conducted the research that confirmed the existence of the Ka'ena volcano. Based on maps of the sea floor, scientists had long suspected that there was third volcano, but until Dr. Sinton's research, no one could say for sure. The way that Dr. Sinton and his team made the distinction was by collecting rock samples from beneath the ocean's surface off the coast of Ka'ena point using remotely-operated undersea vehicles (robots). They compared the chemical composition of these rocks with other rocks taken from the Wai'anae volcano. Geologists are able to look at the chemical building blocks of rocks, the actual minerals that make up the rock, for clues as to where the rock came from and how it was formed. Each rock has its own chemical fingerprint. This analysis confirmed that the lava that produced each of the rocks is chemically distinct.
According to Dr. Sinton's research, the Ka'ena volcano erupted before the Wai'anae and Ko'olau volcanoes, approximately 5 million years ago. Thus the Ka'ena volcano had to grow from the bottom of the sea, and eventually reached a height of about 3,000 feet above sea level. The Wai'anae volcano was eventually much higher, but it didn't have to grow from the very bottom of the sea and was instead able to use the Ka'ena volcano as a sort of stepping stone. The discovery is really important because it addresses some questions geologists and geographers had about the hot-spot explanation for the Hawaiian volcanoes. Most of the volcanoes are between 20 and 40 kilometers apart, but there is a gap of about 90 kilometers between the Wai'anae volcano on Oahu and the volcano that formed the island of Kaua'i. Before the recent discovery no one could figure out why there was such a big gap between these two volcanoes. Now we understand that there isn't really that big a gap since the Ka'ena volcano is approximately 20-30 kilometers off the coast of Oahu, which is very consistent with the spacing between all of the other volcanoes.
So from all of this we can see what makes geography such an exciting field. It's always changing and constantly being updated. Even as we write this blog post research teams from UH and elsewhere are investigated the Ka'ena volcano cite to learn more about the genesis of the islands we call home. Next time your out at Ka'ena Point, you can gaze out across the sea and think about how Oahu must have looked 3-4 million years ago!
(1) We use the term magma to refer to molten (melted, liquid) rock when it is beneath the earth's surface. The term lava is used when it has been extruded onto the earth's surface.
References and For Further Reading
Sinton, John M., Deborah E. Eason, Mary Tardona, Douglas Pyle, Iris van der Zander, Herve Guillou, David A. Clague and John J. Mahoney. 2014. Ka'eana Volcano--A Precursor Volcano of the Island of O'ahu, Hawai'i. Geological Society of America Bulletin.
UH News story on Dr. Sinton's research: