March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 29, 2001: Leg 3; Day 9
Dave Caress writes: Greetings all, and once again special greetings to Mrs. Fletcher's K-1 class and Mrs. Apis' 4th grade class at Spreckels School in Spreckels, CA. It's now Sunday morning, and we are coming close to the end of this leg. We have just two more days (including today) of ROV diving.
Yesterday and today we are focusing on a very interesting geological controversy by investigating and sampling ancient coral and algae reefs on the southern margin of Lanai island. Jim Moore, who is aboard, has found deposits of coral and basalt debris 100 to 200 m (300 to 600 ft) above sea level on Lanai. He has interpreted these outcrops of coral as deposits emplaced by a huge tsunami (wave). All of the Hawaiian volcanoes have suffered major landslides. More than half of Molokai, for instance, has fallen away to the north. Many of these landslides undoubtedly generated large tsunamis that inundated the coasts of neighboring islands and left behind deposits of rock debris torn from the seafloor. Other geologists interpret these deposits as remnants of a coral reef that once grew at that level, 200 m above today's sea level. The problem for both views is the height of the corals above the sea. Large waves are likely, but its hard to imagine a wave capable of washing rocks 200 m vertically up a valley. Sea level rises and falls over time (mostly in response to ice ages), but at no time has sea level been 200 m above the current level. In order for these corals to have formed where they are now located, the island of Lanai must once have been 200 m lower than it is now. Ocean volcanoes generally start subsiding as soon as they are created because they represent a weight on the earth's crust, pushing it down. This flexure, or downwarping of the crust can actually cause nearby islands to move upward if they are located in just the right spot (the deflection takes the form of a stationary wave, with a ring around the volcano where the seafloor moves upward), but this upward movement is never known to be as much as 200 m. So, we have a problem in which both likely solutions require unlikely events. As always in science, the key is obtaining the right observations. We have to focus not on what is more or less likely, but rather on finding out what actually happened.
One approach is to sample and date all of the coral reefs around Lanai. Each reef forms at sea level, and then later drowns if the island subsides (or sea level rises) or dies on land if the island rises (or sea level falls). The bathymetry around Lanai indicates that there may be several former fringing reefs at different depths. If we can sample and date these ancient reefs, we can document the history of vertical motion of Lanai (and learn about the history of all the Hawaiian islands), and perhaps prove which hypothesis is correct. If, for example, we find that one of the drowned reefs has the same age as the corals found on land (two ages: 100,000 years and 200,000 years before present), then the corals in question must actually have been taken from the now-drowned reef and deposited far up on land because there can't have been two sea levels at once. However, if we find the reef ages are consistent with Lanai rising with time, then it is likely that corals up on the island actually grew in place.
For, now, we have to put the ROV in the water and see if these reefs really exist, if they are exposed enough to reveal the rock exposures (sometimes sediments cover the rocks of interest), and if we can get the rock samples we need (old reef structures are often made of rocks that are too hard for the ROV to break off pieces). If we are reasonably successful today, then in a few months chemical and isotopic analyses of the samples will yield reliable ages of the reefs, which in turn just might yield an answer to today's question.
The ROV has just sighted bottom. I'm off to the control room.