We had an earthquake in LA today. Five-four. No big deal, but it got my attention, and got me to thinking about what earthquakes have to tell us about the age of the earth.
The modern scientific explanation of what causes earthquakes is that what we think of as solid ground is really just a thin (relatively speaking) layer of frozen rock floating on an ocean of molten magma that makes up the earth's mantle. New crust is formed at mid-ocean ridges, floats along for a while, and is ultimately re-cycled into the mantle in subduction zones. Where the plates of land mass butt up against each other there is friction, and so as the plates move past each other they don't move smoothly, but in little fits and starts that we feel as earthquakes.
Plate tectonic theory used to be considered patently absurd, but is nowadays as well established a scientific theory as you could hope to find. It explains not only earthquakes, but a host of other phenomena including the formation of mountain ranges and why fossils of tropical plants and animals can be found in the arctic. Thanks to modern GPS we can actually measure the motion of the continental plates with mind boggling accuracy.
Which brings me to Hawaii.
The eight major islands of the Hawaiian group (Hawaii, Maui, Molokai, Lanai, Kahoolawe, Oahu, Kauai and Lanai) are actually just the beginning of an enormously long chain of islands, islets, atolls and seamounts extending over 1500 miles of the Pacific ocean in an almost perfectly straight line. How did they get there? Well, on the southeastern extreme of the Hawaiian chain is the Big Island of Hawaii, and on the southeastern coast of the Big Island is Kilauea, he world's most active volcano. Kilauea has been spewing hot lava into the sea more or less continuously for the last twenty five years (and off and on for thousands of years before that). The lava from Kilauea flows down into the sea where it hardens into rock and becomes new land. The current eruption has created almost 600 acres of new land in the last twenty five years.
Hawaii is growing.
The total area of the Big Island is about 2.6 million acres. If Kilauea keeps building new land at the current rate the size of the island will double in about 4300 years. The historic accretion of the island is being chronicled in great detail. You can actually go to Volcano Nation Park and see it happening before your very eyes. They even have signs telling you which lava flows happened when. And for those who don't have time to actually go there, there are handy dandy maps.
What about all the other islands in the Hawaiian chain? None of them have active volcanoes. The closest thing to an active volcano on the other islands is Haleakala on the southeast lobe of the island of Maui, the next island up the chain from the Big Island. Haleakala is a dormant volcano that last erupted in 1790 [UPDATE: turns out that it was probably closer to 1500]. The lava flowed into the sea, creating a new peninsula that is today the southern boundary of La Perouse Bay. But since then no new land has formed on Maui, and the island has begun to erode away.
As you travel northwest along the island chain a striking pattern emerges: all of the islands are made of the same kind of volcanic material, but the further you get from the Big Island the smaller and more eroded the islands become. On the Big Island you can see the lava coming out of the ground. On Maui you can see the lava flow of 1790 [UPDATE: new evidence indicates that the flow is a bit older, dating back to around 1500 or so] -- it looks like a barren river of rock. But on Lanai, Molokai and Oahu there is no fresh lava at all, but there are still recognizable volcanic features, like Diamond Head. By the time you get to Kauai you have to look very closely to find the clues that it was once a volcano. You can still see the crater that used to be the volcano, but it has mostly eroded away.
Out past Kauai the last "real" island is Niihau, which is privately owned and closed to the public. There is no geomorphic evidence of volcanic activity on Niihau, but we know that it was formed by the same volcanic activity that formed the other Hawaiian islands because of the chemistry of the rocks.
Out beyond Niihau the remaining islands are mostly atolls, which is to say, the remains of coral reefs that formed around the islands and remained even after the island proper has completely eroded away.
The remarkable regularity of the Hawaiian chain can be very easily understood if we postulate that the volcano itself is actually a structure in the earth's mantle that stays in one place as the continental plate moves above it. As each new island forms (as the Big Island is currently being formed) the movement of the plate eventually carries it away from the volcanic hot spot and it stops growing and starts to erode. This explains why the islands are all in a line, why the chain only extends in one direction, and why they appear to get older the further they get from the Big Island. Furthermore, if this theory is correct, we might expect to see a little "proto island" being formed under water to the east of the Big Island. And indeed that is exactly what we find. It is called Loihi and it is on track to matriculate into a fully-fledged island in about ten thousand years, give or take.
Now, here's the rub for the young-earth creationists: if the earth is only 6000 years old, how did the Hawaiian islands form? 6000 years is barely enough time to build one island, let alone the dozens and dozens that make up the Hawaiian chain. To build the whole 1500-mile chain in 6000 years by passing the continental plate over the hot spot it would have to be moving at about a quarter of a mile a year, which is faster than it really moves (as measured by GPS) by several orders of magnitude.
The young-earth creationist's answer, of course, is that God simply created the Hawaiian chain in (more or less) its current form. But that just begs the question: why did He put them in a line? Why did he arrange them just so that it would appear that they were built by the processes that we know are operating today?
The evidence for the extreme age of the Hawaiian islands is not buried in obscure fossil strata. You can go to Hawaii today and watch the island grow. You can set up a GPS yourself and measure the rate at which it moves. (It will take you a while -- the actual rate of motion is a few centimeters a year -- but you can do it.) You can go to the Big Island and see what a one-day-old lava flow looks like, and then you can go to Maui and see what a several-hundred-year-old lava flow looks like. You can even still see the cinder cone of the volcano that created it.
And then you can go to Kauai and see what a few million years of erosion can do.