Trying to Understand the Earth
I've had a range of reactions and feelings about last week's devastating earthquake and tsunami in the Indian Ocean. Working as I do in the newsroom of a newspaper I observed the editors wrestling with how to handle it. They tried to address the magnitude of the disaster, the unreal and growing numbers of dead, with a single headline: "60,000," which immediately became meaningless, as did comparing it with previous calamities. They showed us close-ups of stricken survivors, and piles of anonymous bodies to help us understand it.
For me, though, these images only go so far. Put up against competing images of soldiers in hummers wearing night-vision glasses and shooting aimlessly into a darkened building, I don't know which is more horrible. But one involves people being cruel to other people, the other is our planet shuddering, blissfully unaware of the people living on its surface.
I want to understand how the earth did this thing. Living as I do in another earthquake-prone area, I want to think about how something like this might happen here. Because it will, of course, eventually.
I'm in love with the earth. I love looking at it, from right here at earth-level, touching it with my hands and feet, feeling it in all its moods.
I like to get up on high places - tall buildings, hills, mountain peaks, even airplanes - and look down at the world, to see for myself how the land unfolds. A small patch of snow on one side of a ridge melts into a rivulet, which joins others and eventually forms a river that pours its contents into the ocean. Another snow patch a few inches away on the other side of the ridge forms a drainage that ends in a basin. They both take their time, through deluge and drought, to carve the earth, working to bring everything down to sea level.
If you go high enough and look down, the forests cease being a collection of trees and become a single organism, like a mold growing over an orange. Signs of humanity all but disappear, and where they are distinguishable, it seems more like an infection of unseen germs.
Earth from Apollo 17, the last trip to the moon by humans, 1972. All life on this planet is supported by the thin and fragile skin of air that supports the clouds.
The earth itself, though, is in constant flux. A very thin crust of rock floats on an ocean of molten lava. It's about 4,000 miles from the earth's center to the bottom of the crust. The crust itself is only about 30 miles thick and four-fifths of its surface is covered with liquid water.
Above the rock and water is the layer of gas we can breath, about 10 miles thick. Above that is another 20 miles of thin air that's just substantial enough to support aircraft.
The Earth spins on its axis, allowing its surface to be alternately heated by the sun and cooled by the cold of space. It is also tilted on its rotational axis so that as it makes its way around the sun, the northern and southern parts of its surface are heated and cooled at different rates. The orbit itself is not circular, so that at different points Earth is closer to the sun.
All these factors keep the Earth's surface moving, alive. By an incredible balance of heat, Earths's water is constantly changing states. From solid as ice and snow it melts into liquid and seeks a low spot to collect. It evaporates into a gas and rises into the atmosphere, where it condenses to a liquid and falls back to earth.
The part of Earth's surface affected by the Indian Ocean earthquake and tsunami.
Because the surface is alive and moving, the crust is broken up into large chunks, or plates, that float independently on the molten mantle. These plates bump up against each other and the edge of one is sometimes pulled beneath its neighbor, forming trenches as deep as seven miles beneath the oceans. The neighboring crust is forced upward, forming mountains that may rise as high as five miles.
Imperceptible to us humans, these trenches and mountains are built over millions of years, at the rate of only a few inches or feet a year. The grinding of plate against plate is not always smooth. Friction can stall it for years or decades, building up pressure until the pent-up force overcomes the friction and both plates move with a sudden jolt.
The main 9.0 earthquake was followed by many powerful aftershocks, all located along the Sunda Trench, a subduction zone between two major plates. As the India plate dove below the Burma plate, the Burma plate was forced upward, creating a tsunami that spread over the Indian Ocean. Banda Aceh and Lhoknga, towns on the northern tip of Sumatra, were first reduced to rubble by the earthquake, and then washed away by the ensuing tidal waves.
When this happens on the surface of a continent, earthquakes occur. The ground along the place of slippage is jarred, cracks form and some of the life clinging to it is damaged. But soon the rains come and in time the earth is healed.
When such an event occurs at sea there's an added dynamic. The piece of crust being lifted up displaces the water that covers it. Suddenly the water itself is lifted, and it immediately tries to make itself level, running down over its own mountain in a great rush. When the wave it forms rushes over deep water, it can move very fast - up to 500 miles an hour - but the wave itself is very shallow. Someone in a boat on open water may not even notice it.
As the wave passes over shallower water it is slowed, but the wave height grows. When the wave approaches land, its lower reaches slow to a stop, backing up the advancing water behind it. It has nowhere to go but up, and as it finally reaches the shore, it can rach great heights.
The Sumatra coast, with Lhoknga at top, before and after the tsunami. The tsunami that passed over this area may have been 50 feet high.
Close up of Lhoknga. The white rectangle is the mosque, the only building left standing after the tsunami.
For more information and satellite photos of the tsunami area, and general information about our fabulous planet, Earth Observatory News can't be beat. http://earthobservatory.nasa.gov/Newsroom/
For a terrific explanation of the physics of tsunamis, including animations showing how they are created and travel, try http://www.geophys.washington.edu/tsunami/general/physics/physics.html
5:47:12 PM 
|
