Camouflage in salt-and-pepper sediment, or how to hide on tropical beaches with nearby volcanoes

Here are three photos that demonstrate how well some animals ‘know’ the composition and grain size of the sediment of their homeland. In all three cases, the salt-and-pepper sand of the background is the result of dark-colored grains of volcanic origin mixing with light-colored fragments of corals and seashells.

Ghost crab on Big Island, Hawaii

Ghost crabs are common on many beaches around the world; below is another one from Costa Rica. Note how the different grain size and sediment color are perfectly captured by the crabs.

Ghost crab near Playa Conchal, Guanacaste, Costa Rica

The third example takes us back to Hawaii, under water, where I managed to get this shot of a flounder while snorkeling in Kona. The only reason I saw this guy was that I got really close and part of the seafloor, which later turned out to be the flatfish, unexpectedly took off.

Flowery flounder (Bothus mancus), Kahalu'u Beach Park, Big Island, Hawaii

Snorkeling and geology in Kealakekua Bay, Big Island, Hawaii

For a long time, I didn’t think it was worth spending more than an hour on a beach, even the most beautiful ones, unless there were some nice cliffs nearby showing some interesting geology. My views in this regard have changed dramatically about three years ago, when I spent a week on The Big Island of Hawaii, and the hotel where we were staying offered free rental of snorkeling gear. I put on the mask and the fins, trying to remember how this was supposed to work (I did a bit of snorkeling in Baja California many years before that), and put my face into the not-too-interesting-looking waters in the front of the hotel.

Kealakekua Bay in Google Earth, with some explanations added

I was in for a surprise. The water was far from crystal clear, but I could still see fantastic coral creations lined up along the bay and lots of fish of so many colors and patterns that it felt unreal. Until then I thought that this kind of scenery was hard to see unless you were a filmmaker working for Discovery Channel or a marine biologist specializing in tropical biodiversity. The next day I spotted a couple of green turtles frolicking in the water, clearly not bothered by the nearby snorkelers, and I already knew that I needed to look into the possibility of buying a simple underwater camera.

Lots of coral, mostly belonging to the genera Lobites (lobe coral) and Pocillopora (cauliflower coral)

Three years later I went back to the Big Island with more excitement about tropical beaches, plus bigger plans and a bit more knowledge about snorkeling. After going through a few well-known snorkeling sites on the west coast, like Kahalu’u Beach in Kona and Two Step near Pu’uhonua o Honaunau park, we got on a nice boat (run by a company called Fair Wind – strongly recommended!) and did some snorkeling in Kealakekua Bay.

Visibility in Kealakekua Bay is usually very good

Old wrinkles of pahoehoe lava getting encrusted by algae and corals and chewed up by sea urchins

Kealakekua Bay is difficult to reach; there is no road and no parking lot nearby. You either have to hike in, paddle through the bay in a kayak, or take a boat. I have heard before that this was the best snorkeling spot in Hawai’i, but I think that is an understatement. Unlike all the other spots we tried during the last few years in Hawaii (and that includes several beaches on Kauai and Hanauma Bay on Oahu, the presidential snorkeling site), the water at Kealakekua Bay was calm and very clear, with fantastic visibility.

Heads of cauliflower coral, with yellow tangs for scale

I will not attempt to describe this whole new world; instead I will let the photographs speak for themselves (as always, more photos at Smugmug). Even better, if you go to the Big Island, make sure that you visit this place with some snorkeling gear.

Yellow tangs (Zebrasoma flavescens) often congregate in large schools and it is difficult to stop taking pictures of them

When I was at Kealakekua Bay, I didn’t know much about the local geology. The big cliff bordering the bay toward the northwest, called Pali Kapu o Keoua (see image above), shows a number of layered lava flows that belong to the western flank of Mauna Loa; and I suspected that this must have been a large fault scarp, but that was the end of my geological insight. A couple of hours worth of research after I got home revealed that Pali Kapu o Keoua was a fault indeed: it is called the Kealakekua Fault and it has been mapped, along with the associated submarine geomorphological features, in the 1970s and 1980s by U.S. Geological Survey geoscientists.  It turns out that one of the shipboard scientists and key contributors to these studies was Bill Normark (see also a post about Bill at Clastic Detritus). While in California in the late 1990s, I was lucky to get to know Bill and have some truly inspiring discussions with him about turbidites, geology, and wine, so this was a doubly valuable little discovery to me.

So what is the origin of the Kealakekua Fault? The Hawaiian Islands are far away from any tectonic plate boundaries, so there is not a lot of opportunity here for inverse or strike-slip faults to develop. However, the Hawaiian volcanoes are humongous mountains and their underwater slopes are extremely steep by submarine slope standards: gradients of 15-10˚ are common. [This is in contrast by the way with the relatively gentle slopes of 3-8˚ the subaerial flanks of the volcanoes, a difference that – it just occurred to me – has to do something with the different thermal conductivities of water and air. Water is ~24 times more efficient at cooling lavas, or anything for that matter, than air, so once a volcano sticks its head out of the water, basaltic lava flows are pretty efficient at carrying volcanic material far away from the crater, thus building gently sloping shield volcanoes. The same flows are promptly solidified and stopped by the cool ocean waters as soon as they reach the coast.] Slopes that are this steep are also unstable; the underwater parts of these volcanoes tend to fail from time to time and large volumes of rock rapidly move to deeper waters as giant submarine landslides. Seafloor mapping around the islands revealed that the underwater topography is far from smooth; instead, in many places it consists of huge slide and slump blocks.

Topographic map of the Big Island. Note the location of Kealakekua Fault and the rugged seafloor to the southwest of it, marking the area affected by slides and slumps. This is a map based on higher-resolution bathymetric data collected during a collaborative effort led by JAMSTEC (Japan Marine Science and Technology Center). Source: U.S. Geological Survey Geologic Investigations Series I-2809 

Kealakekua Fault is probably part of the head scarp of one such giant landslide, called the Alika landslide. This explains the steep slopes in the bay itself: after a narrow wave-cut platform, a spectacular wall covered with coral – the continuation of the cliff that you can see onshore – dives into the deep blue of the ocean as you float away from the shore. In contrast with submarine landslides that involve well stratified sediments failing along bedding surfaces and forming relatively thin but extensive slide deposits, the Hawaiian failures affect thick stacks of poorly layered volcanic rock and, as a result, both their volumes and morphologic relief are larger (see the paper by Lipman et al, 1988). The entire volume of the Alika slide is estimated to be 1500-2000 cubic kilometers. That is about a hundred times larger than all the sediment carried by the world’s rivers to the ocean in one year! The slides have moved at highway speeds and generated tsunamis. There is evidence on Lanai island for a wave that carried marine debris to 325 meters above sea level; this tsunami was likely put in motion by the Alika landslide*.

You don’t want to be snorkeling in Kealakekua Bay when something like that happens. And it will happen again, it is a matter of (geological) time. Giant underwater landslides are part of the normal life of these mid-ocean, hotspot-related volcanoes.

Reference
Lipman, P., Normark, W., Moore, J., Wilson, J., Gutmacher, C., 1988, The giant submarine Alika debris slide, Mauna Loa, Hawaii. Journal of Geophysical Research, vol. 93, p. 4279-4299.

*tsunamis generated by landslides is a whole new exciting subject that we have no time now to dive or snorkel into.

Earth, water, wind, and fire: ‘Lava viewing’ in Hawaii

Our Christmas gift to ourselves was a little trip to the Big Island of Hawaii, something we were thinking (dreaming) about for a long time. There are many great posts about the Hawaiian volcanoes on the geoblogosphere (see for example the ones here and here); I will try to add a few notes and pictures without being too repetitive (and will try to seem less ignorant in volcanic and hard-rock matters than I actually am).

Probably the most memorable experience we had was the lava viewing at Kalapana. This is where ‘officially’ you can get relatively close to the place where the lava from Pu`u `Ō`ō enters the ocean. The USGS has a nice website with updates on what’s going on. I was so anxious to see this place that we had to go there on our first day in Hawaii, that is, on December 22. You have to drive all the way to the end of road 130; there are some big ‘No trespassing’ signs at one point, but everybody seems to ignore them, and there is an official parking lot at the end of the road, way beyond the ‘no trespassing’ signs. It is best to get there 30-60 minutes before sunset, and to stay until it’s completely dark, to see the potential show both in daylight and in nighttime darkness. Unfortunately, on December 22 we didn’t see much, apart from a beautiful sunset and a few small puffs of steam:

Sunset at the Kalapana viewing site on December 22, 2008

That was a bit of a disappointment, but I knew I had to give it another try. After talking to a ranger from Volcanoes National Park, we drove back to Kalapana five days later. This time, the show was definitely on. More than that, it was spectacular. A huge column of steam formed where the active lava tube spills the lava into the sea, and repeated explosions painted red the lower part of the column. From time to time, several tornado-like funnels formed and connected the steam cloud to the ocean.

Steam cloud with mini-tornadoes on December 27, 2008; lava-viewing boat on the left for scale

As the sun goes down, the explosions become more colorful and more obvious

S-shaped funnel between the steamy sky and cool hot ocean

This was such a uniquely beautiful scene. I wish we went there more than two times, because the whole spectacle changes as a function of the activity of lava flow, weather conditions, the direction and nature of lighting.

I have also learned that it is not easy to take good photographs of fast-moving and rapidly changing distant things in the dark. Here is the proof: