Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: THURSDAY, March 4, 1993 TAG: 9303030138
SECTION: EXTRA PAGE: 1 EDITION: METRO
SOURCE: Joel Achenbach Associated Press
DATELINE: LENGTH: Long
NO NEED TO WORRY ABOUT THE EARTH GETTING COLD INSIDE
Q: Why is the interior of the Earth still hot after 4.6 billion years of letting off steam (and lava)? Why doesn't this thing ever cool down?A: We're not handwringers, not at all, but lately we've been having trouble sleeping because we're worried that the Earth will get so cold inside that all the volcanoes will die out, and no new mountains JOEL ACHENBACH will form, and the continents will erode away until finally they will be as flat as a table and right at sea level and one day there will be a bad high tide and we'll all drown.
Think about it: The planet was formed 4.6 billion years ago (though our watch runs about 5 minutes fast) and ever since then it has been giving off heat in the form of volcanoes and deep sea vents and geysers and so forth. So shouldn't it be cold by now? Why is Earth still bubbling and churning and doing that magma thing?
There are three reasons:
1. There aren't as many volcanoes as you think. "Heat loss by vulcanism is not very important," says Peter Olson, professor of geophysics at Johns Hopkins University.
2. There are radioactive elements that increase the heat of the interior. Your basic decaying lump of uranium or potassium or thorium provides heat through the radiation of atomic particles.
And most importantly:
3. Rock is a good insulator. If you could take the Earth's overall temperature, it would be about 3,200 degrees Fahrenheit, but the rock of the outer crust doesn't conduct much of that heat to the surface.
(Right there you have a snapshot of the three mechanisms of heat transfer: Convection, radiation, conduction. The next time you struggle to remember what convection is, precisely, just think of lava.)
The fact is that the Earth has probably only cooled a few hundred degrees since it formed, says Olson. If the Earth ever did cool down, the mantle, the thick layer of the planet below the crust, would stop convecting, and there'd be no more earthquakes or volcanoes or ocean-floor spreading, no new mountains, no continental drift, and yes, after a couple of hundred million years the land would be eroded away by wind and rain and ocean.
By the way, we always picture the mantle as gray, like your average rock, but in fact it's green. Seriously. "Sort of an olive," says Olson. Our planet, the Earth, is mostly olive-colored. ("How fabulous!" you are probably thinking. "It matches the drapes!")
Q: Why do apes stagger when they walk on two legs? Why can't they stride along smoothly the way humans do?
A: Apes stagger because they have to shift their weight from leg to leg. Humans don't do this, in part because our thigh bones are slanted inward, toward the knee, so that our knees are close together. This makes it easy to stand on one leg and shift to the other - each leg is already nearly underneath our center of gravity.
Moreover, humans have pelvic muscles that apes don't have. You can feel them contracting when you walk. These muscles keep our pelvis level and stable.
Why humans are bipedal to begin with is a major scientific mystery. Most people don't wonder about this because they assume that going from four legs to two legs is simply what happens when you evolve. That's what evolution is: Learning to stand up straight. But the truth is that bipedalism is a quirky trait. To be smart and technological we don't have to walk on two legs; we could be knuckle-walkers, like gorillas or chimpanzees. (Is "The Knuckle-Walk" a dance craze yet? Why not?)
Theories about bipedalism often are inspired by the latest political fashion of cultural obsession, notes Alison Brooks, a professor of anthropology at George Washington University. During the Vietnam War, she says, everyone talked about bipedalism as a mechanism for allowing men to engage in aggressive behavior like hunting prey and throwing spears and whatnot.
This is related to the old theory that we became bipedal so that we could free our hands to use tools. It sounds right! But the fossil record shows clearly that we started walking long before we started using tools.
Bipedalism probably has something to do with the need to carry stuff. Geologic changes in Central East Africa - that mantle was convecting again - caused a shift in climate and a drying of the ecosystem. Jungle became savannah. If you were an ape, you couldn't just hang out in the trees anymore and swing from branch to branch, but rather had to travel over the ground, and carry food long distances. Human bipedalism isn't particularly fast, but it is smooth, and much more efficient, in terms of energy expended, than simian quadrupedalism.
So we probably walked on two feet because it was easier to lug our dinner around.
Incidentally, John Eccles in "Evolution of the Brain" gives a good summary of how a human walks: "Each leg alternately is thrust forward (`the swing leg') while the weight is supported by the `stance' leg. The swing leg is flexed to clear the ground and the step is over as its heel strikes the ground, the body weight moves forward, and the initial stance leg soon becomes the swing leg, beginning with a thrust exerted from the big toe by the flexor hallucis longus contraction."
Yeah, but try to contract your flexor hallucis longus and chew gum at the same time - very difficult! Washington Post Writers Group