Hawai’i is an amazing place. And not just for volcanologists. This is a world-on-an-island, with (apart from the most accessible eruptions in the world) Mars-sized mountains, pristine beaches, coral reefs, a world class city for shopaholics and night owls, rain forest with world-class mosquitos, desert, archaeology, astronomy, volcanoes, agriculture, flying fish and diving birds. It is the only US state that once had a king or queen, and it gave us the first American president not born on the continent. It is both a paradise and a troubled place, with enviable life style clashing with grinding poverty, unequal opportunity and discrimination. It is paradise for the wealthy, and home for everyone else.
And that is just on the surface. There is much more here that doesn’t meet the eye.
Let’s look. Hawaiʻi is the largest, and southernmost island of Hawaiʻi. It is also the youngest. There are 8 major islands in the state. From southeast to northwest, these are Hawaiʻi, Maui, Kahoʻolawe, Lānaʻi, Molokaʻi, Oʻahu, Kauaʻi and Niʻihau. They vary greatly in size. Hawaiʻi itself is 10,000k km2 while Niʻihau, the ‘forbidden island’, is only 180 km2. Kahoolawe is even smaller, at less than 120 km2. But there are many more deserted islets. There is one fewer than there used to be: East Island (a name that promises too much) was wiped off the map by hurricane Walaka in 2018. Few had ever heard of it.
The unknown Hawaiʻian islands form the long chain to the northwest, trailing the hot spot. Just looking at the map indicates how they came to be. The chain runs from young volcanoes at one end to ancient ones at the other. All but the youngest are very close to sea level. Each volcano formed in a burst of activity, grew to a massive size, faded and died, eroded and sank. At the youngest end, the volcanoes have merged to form one combined island: the main island of Hawaiʻi consists of no fewer than five volcanoes. The islands are pushed up by the hot spot: this is one of the reasons why such a large area has appeared above sea level in a very deep ocean. Once the island migrates off the hot spot, the heat is no longer there and like a cooling souffle, the island begins to sink. Now the island begins to break up into the individual volcanoes as the lower areas between them sink below sea level. Erosion takes hold and the mountains take on a rugged appearance and grow lower. Dramatic events can happen, where an entire face of the island collapses into the sea. Niʻihau was originally a slope of a volcano of which the peak has disappeared into the ocean. We are left with a scattering of islands where originally there was only one.
But erosion must stop once the island reaches sea level: otherwise why is the entire northwestern chain so close to sea level? There are two aspects here. First, once sea level is reached, erosion by rain ends and only wave erosion is left. Wave erosion does not reach far below sea level. Second, coral reefs build on the sinking slopes and reef growth manages to keep up with the initial sinking. The two combined creates the atolls, circular or semicircular reefs often dotted with sandbanks. The hard core of the volcano, the volcanic plug, can stand up to the rain and waves longer than any other part of the volcano. Mixed with the atolls you can get one or more rock islets.
Midway atoll is a good example of the end phase. It has a ring of sand banks and islands, formed by the coral reef. The volcanic rock is 150 meters below: that is how much the coral has build over the 28 million years since this was an active volcano. Why 150 meter? That is much deeper than the ocean waves could disturb the sea bed! The reason lies in the ice age. Even a sub-tropical reef was not safe from that. During the depths of the ice ages the sea level was more than 100 meters below the current level. At the time, wave erosion was able to reduce the volcanic rock. When the ice melted and sea level rose, the coral managed to keep up and return the island to the surface – but not beyond. The highest point is 13 meters above sea level. Will this island survive the current sea level rise? Yes, because only the most pessimistic predictions get more than 10 meters of rise out of our on-going climate catastrophe. And the coral should be able to keep up with the rise in any case. However, if acidification of the ocean were to stop coral growth, Midway will have a problem.
The French frigate shoals show a younger phase in the decline and fall of Hawaiʻi. Like Midway, it is a large atoll. But at the centre is a solitary rock, perhaps 10 million years old, peaking at 35 meters above the sea. It is called La Perouse, and it is the last remnant of the volcanic plug. Soon it will be gone, victim of the sea.
The crescent of the French Frigate atoll contains numerous small islands. They are in continuous flux. Whale-Skate island, for instance, broke into two 30 years ago, and foundered. At high tide it is now submerged. One island here has the discouraging name ‘Disappearing island’. East Island was the second largest of the islands here, and it was lost to the hurricane. But change is a constant here. New islands will appear when the sand again comes together.
North of the French Frigate Shoals is the Gardener Rock, a larger double pinnacle which reaches 50 meters above sea. It too is the plug of an ancient volcano, at 12.3 million years probably a bit older than La Perouse. There is no atoll here. But under water is another wonder. Gardener Rock is the peak of an immense subsea volcano. It is estimated at twice the volume of Mauna Loa. (The volume includes the part of the volcano that is below the ocean floor: it is based on the seismic reflection between sea floor and lava, 9 kilometers deep.)
The Hawaiʻian chain ends a little northwest of Midway atoll, at a place where the volcanoes are 43 million years old. Here begins the Emperor chain, a chain of seamounts extending north all the way to the Aleutian trench in which they disappear. The bend between the two chains is caused by a change in the movement of the Pacific plate. Plates are pulled in by subduction zones, and 45 million years ago there was a change in subduction around the Pacific. Exactly what happened is still being discussed. The problem is that the direction of the chain is not purely caused by the plate movement. The hot spot itself has also moved southward, but we don’t know by how much. If you are interested in the history of the Pacific, read all about it here.
The Emperor chain is named after emperors of Japan. Unlike the younger volcanoes of the Hawaiʻian chain, these are well under water. Take as an example Nintoku seamount. It is 56 million years old and is a typical subsea volcano with a conical slope, but with a flat top (such a top is called a guyot). From magnetic profiles we know that this volcano formed about 27 degrees north. But Hawaii is at 19 degrees north. This is one of the pieces of evidence that suggests that the hot spot has not been stationary, but has moved southward at some point during those 56 million years. It may have moved east or west as well: we have no data on that.
The flat top of Nintoku shows the effect of wave erosion: it spend a long time at sea level. But it is there no longer. Even though the volcano is 5 km tall, the sea here is deeper than that. The top is 1200 meters below sea level. There is some old coral on top, and some sedimentary rock. But clearly, this island did not manage to maintain its sea level position. And this is true for the entire Emperor chain. What happened?
Two things happened. First, the drift of the Pacific plate took the volcano northward – and it was never as far south as Hawaiʻi is now. Initially this did not matter: the climate was much warmer in those days than it is now. But soon, the old mountain found itself in waters too cool to support coral. And once the coral stopped growing, the mountain disappeared under water.
That explains why it became submerged. But why did that not stop at depths where wave erosion ceased? Here, the Pacific plate took over. For oceans too sink. As the ocean floor moves ages, it cools, and is it cools, it becomes denser. This causes it to sink a little deeper into the lithosphere. In fact this is what eventually causes the oceanic plate to begin subduction. So while the old volcano drifts with the plate, it finds its feet below it sink. This is why it ended up more than a kilometer below the sea, still a proud volcano but no longer meeting the eye.
Let’s go back to Hawaiʻi. Because there is more here too than meets they eye. There is a dramatic landscape below the sea.
The sea floor map shows the landscape. You probably want to see in full resolution: if so, click here. If this wasn’t hidden deep below water, it would be a world wonder all by itself. There is spectacular but invisible scenery. The underwater plateaus around the island have sharp edges, remnants of old sea shores. Beyond, the ‘land’ falls down steeply into a 5-km deep abyss, three times deeper than the (not so) Grand Canyon. There is the deep trough surrounding the southern islands, where the weight of the volcanoes has pushed down the lithosphere.
Very obvious are the immense debris flows where the sides of volcanoes have collapsed. The hummocky ‘terrain’ on the sea floor is a dead give-away. Even the smaller islands have these: for instance the North Kauaʻi Slide is larger than the island it left behind. The collapse happened at a time this island hosted a huge volcano; it is now a shadow of itself but the slide is still there.
The Nuʻuanu Slide extends almost 200 kilometers and is 200 meters thick on average. It even managed to travel through the trough and up the other side, gaining 300 meters in elevation. This slide took down half the volcano! A big part of the volcano came down as a single block, and is now known as the Tuscaloosa Seamount, 30 by 15 kilometers wide and almost 2 kilometers high. This single block contains almost 1000 km3! The rest of the 1-million-years old slide may contain several times as much. Fifteen major slides have been identified. There may have been a few more: slides that happen to go southward tend to get buried by the next volcano to develop. From the numbers, we can expect several such events per million years. To put that in context: it means that we have a chance of up to 1 in a thousand to see such an event during our life time. Don’t bother buying insurance: if this happens the insurance company will be bankrupt. The Alkali slides are thought to be around 100,000 years old, and these may be the youngest. They are relatively small, at a combined volume of ‘only’ around 500 km3.
Where would it happen? Looking at the sea map, the slides are most obvious around the northern islands. This is also where the volcanoes look like big bits are missing. Maui does not have them, and it’s volcanoes still look like volcanoes should. So one may guess that here is where the next such event may happen. It is part of the normal demise here. Maui and Hawaiʻi both do have smaller slides on their west side and perhaps this is normal behaviour for younger Hawaiʻian volcanoes. The Hilina slump on the south side of Hawaiʻi seems to come not from a fast land slide but from slower creep, with a bit of movement once a century after every major earthquake. It lacks the hummocks that characterize the real collapse slides.
(One may wonder about tsunamis. If a 5000 km3 slide tumbles into the ocean, the water displacement could in theory generate a tsunami tens of meters high on the American Pacific coast. That is the tsunami height on the coast: the run-up height can be several times higher. However, it is likely that such slides happen fairly slowly and not in a single block, and that would reduce any tsunami by quite a lot. However, when this happens do not be on the Hawaiʻian islands themselves. The Alaki slides have been associated with a major tsunamic deposit on Lanaʻi with run-up height over 300 meters!)
Not all hummocks come from land slides. The rifts that extend from the main volcanoes continue under water. Eruptions on the subsea parts cause pillow lavas, and these also cause a rugged terrain. This can clearly be seen on the map, for instance on the Puna ridge. But these ridges themselves can have land slides, and the hummocks on their sides may be due either to lava or the slides – pick your choice. The most recent documented underwater eruption (apart from the next volcano to be discussed) was in 1877, in two places on the west slopes of Mauna Loa. There is some holocene volcanism in the Arch field, well south of Hawaiʻi: this appears to be caused by the buckling of the crust as it begins to be depressed by the weight of Mauna Loa, where the faults let some magma rise up. But this is minor activity that is not directly fed by the hot spot. The North Arch field, 300 km north of Oahu is much more significant, with many small cones and large lava flows. The cause is not well understood, but it appears to be fed by evolved magma from the islands. Once the islands cease to be volcanically active, it appears that the remnant magma finds a way around and surfaces far away and deep below. But the total volume is small compared to those of normal Hawaiʻian volcanoes.
The next volcano to be discussed in Hawaiʻi’s youngest. Loʻihi is located on the sea bed south of the main island. It is highly active with eruptions probably ever few decades. But it hasn’t made the surface yet: the summit is almost a kilometer down. Still, that makes it a big volcano which would be an eye catcher on land anywhere: it is between 3 and 4 kilometers above the sea bed where it began. The volcano is believed to be about 400,000 years old. It is not young by any standard but at this rate it won’t surface for another 100,000 years.
The missing volcano
An interesting aside is that if the current Pahala quakes indicate the centre of the hot spot (a big if), it is equidistant to the three most active volcanoes here. Loʻihi is a north-south ridge, pointing roughly at Pahala. Perhaps this is what it is: a deep Pahala rift reaching the sea bed here at an angle.
It is often assumed that Loʻihi will be the next volcano to develop. But is it? If you look at the island and measure the distances between the volcanoes, it is a fairly consistent 30 km: this is about the distance between the summits of Kilauea and Mauna Loa, Mauna Kea and Mauna Loa, and Hualalai and Mauna Loa. But Mauna Loa to Loʻihi is twice that. In fact if you continue the regular spacing of the other volcanoes, there is a clear gap where a new volcano would fit. And it is close to Pahala. This is illustrated in the map. If a new volcano were to form, you would expect it to be here, not in the sea but on land.
An ancient heritage
But Loʻihi is far from the only seamount in the area. Looking at the map, the place is littered with them. Close to Loʻihi are the Apuʻupuʻu, Hohonu and Green seamounts. Further west are the Dana, Charnell, Day and Palmer seamount. They have summits between 2 and 4 km depth, so well below Loʻihi, but in volume they are competitive. And if height is important, McCall’s seamount reaches less than 1 km from the surface and Pensacola seamount gets to 600 meters. These mounts extend south and west of Hawaiʻi. What is going on? Why are there so many more volcanoes below the sea than above? They are big beasts, up to 4 km above the sea floor. If the hot spot is creating that many other volcanoes here, how big is that hot spot?
It turns out, this has nothing to do with the hot spot. As the Pacific floor moved past the hot spot, it carried with it another volcanic field. It is an old one: these volcanoes formed in the Cretaceous, around 75 million years ago. They can be distinguished from the current volcanism by their rugged appearance.
The Pacific sea floor has several seamount chains, similar to Hawaiʻi. Each formed from a long-lived volcanic region. The seamounts around Hawaiʻi are different: it did not form a chain but a field. This was a different type of event. Was it a collision between two plates? Or (perhaps more likely) a phase of extension? Both can cause volcanic fields. Little research has been done. But all around Hawaiʻi are the massive remnants of its ancient volcanism.
But if so many of these mounts are around the island, why none on Hawaiʻi itself? Perhaps many cones are buried underneath. We always assume that deep under Hawaiʻi the lava flows lie on top of a smooth old crust. Perhaps it isn’t smooth at all. Perhaps the island is kept in place by these peaks sticking up and anchoring the lava above.
The sea mounts peter out to the north. There is little evidence of them north of the big island. Perhaps there is something about the big island. People have asked why the volcanism there occurs at a higher rate than it did when previous islands formed. Does the hot spot wax and wane? Perhaps it is not the spot. Perhaps the oceanic crust underneath the island of Hawaiʻi is different.
Do Mauna Loa, Kilauea and Loʻihi have some memory to those old times? Jurassic Park was filmed in part on Hawaiʻi. As commonly known, it was a complete misnomer: the dinosaurs in the movie were not Jurassic but Cretaceous. In a way, Hawaiʻi still is a cretaceous park. It has an undersea park of cretaceous volcanoes, dwarfing those dinosaurs. There is more here than meets the eye.
Albert, September 2020