Earth rotation and revolution, variations thereof, other events, and 2012
MacNfries said:
Speaking of the earth rotation ... has anyone read about the theory that the earth changes its polar positions every 25-35 million years, reversing its rotation, etc, and that we're in that period of time when it is expected to happen again? Supposedly it takes the earth about 10,000 years, once it changes its poles, to start rotating at the correct speed again. In the meantime, many of the mammals on earth die ... including humans. It's an interesting read ... falls in line with this 2012 thing. —Mac
The earth does not reverse its direction of rotation — meaning, at some time in the future the sun will not "rise" in the west instead of in the east. You can take that as a fact. It's required by conservation of angular momentum.
The earth's magnetic field does, however, reverse itself periodically — meaning, the magnetic north pole becomes the magnetic south pole, then later, vice versa. These are called "magnetic reversals." The evidence for it is in the form of magnetized "stripes" in the rocks of the ocean floors, parallel to the mid-ocean ridges. As liquid magma emerges from (say) the mid-Atlantic ridge and cools to form the solid basalt of the ocean floor, thus causing the Atlantic sea floor to spread perpendicular to the ridge, the magnetic minerals in the cooling basalt align themselves parallel to the earth's magnetic field. As you move out from the ridge (in an oceanographic vessel) west or east towing a magnetometer, you discover magnetic "stripes" on the sea floor oriented first north, then south, then north, then south, etc. These magnetic stripes are what resulted in recognition of sea-floor spreading (first published in 1961). The time periods required for reversals of the earth's magnetic field are "long" relative to a human lifetime (but I can't cite a number.... I'd have to look it up).
There are variations in the earth's rotation and in the shape of the earth's orbit caused by the fact that the earth is not subject only to the gravitational field of the sun. Rather, as the earth revolves around the sun it is simultaneously attracted by the gravitational fields of all the other planets, which are all changing position with respect to each other as they too revolve around the sun. This causes:
—precession of the earth's axis, like the "wobble" of a top as it spins. At present the north star is Polaris; in the future it won't be; in the more distant future (more than 10 thousand years from now), Polaris will again be the north star.
—slow periodic change of the angle of tilt of the earth's axis (the angle of tilt is what causes the seasons) by roughly 1 to 2 degrees, over longer time periods of 10's of thousands of years.
—gradual periodic change in the eccentricity of the earth's orbit, which is slightly elliptical (not a perfect circle). That is, the shape of the earth's orbit varies from a "thinner" ellipse to a "slightly fatter" ellipse then back to a "slightly thinner" ellipse, etc., over a time period of about one-hundred thousand (100,000) years.
These slow, steady, periodic changes in the earth's orbit, which combine to form irregular changes (they're rather complicated to compute), act as the pacemaker of the ice ages. These are not changes capable of causing sudden massive extinctions in the biosphere.
There have, however, been catastrophic events in the geologic past that have caused massive extinctions in the biosphere. (The earth, viewed over geologic time, is a rather violent and risky place to live — but fortunately, there are long time periods between the most violent events.) The most catastrophic events appear to be large asteroid impacts, like the one — or several — thought to have caused the extinction of the dinosaurs. In doing so, it caused the end of the Cretaceous and beginning of the Tertiary geologic time periods. One huge crater, now buried beneath sediments and covered by jungle, is on the Yukatan(sp?) Peninsula, Mexico. Another huge impact crater, dated to the same time, has been discovered recently in India. Those impacts caused the extinction of (I think) over 90% of the species living on earth at that time.
The second most-catastrophic events that can occur on our planet appear to be volcanic super-eruptions; i.e., those with a volcanic eruptive index of 7 (VEI 7). Yellowstone National Park, in the U.S., has been the scene of several of these in the geologic past.... all those scenic geysers and a huge crater are the smoking guns, one might say. The most recent super-eruption was Toba, in Indonesia — it occurred about 70,000 years ago. At that time, we homo sapiens had been around for quite a while. Prior to Toba, the human population appears to have been roughly similar to the number of people now living in the Detroit area (i.e., on the order of a few million). Toba and the multi-year volcanic winter that followed, which was then followed by an ice-age maximum, reduced the human population to about the number that could fit into a Detroit baseball stadium (on the order of 50,000). For us homo sapiens, it was a *very* close call.... we very nearly did not make it. Throughout the biosphere, many other species were ****** into extinction at that time. (The "History Channel" ran an interesting program on Toba earlier in 2009.)
NASA has a program to track asteroids that may conceivably hit the earth sometime in the future; there are a huge number of them in the asteroid belt but no candidates have been identified for earth impacts anytime soon, it appears. Nothing is on the radar, so to speak, for 2012 or the considerably more distant future. This is good.
Volcano observatories maintained by the U.S. Geological Survey, in cooperation with universities and other agencies in the U.S. and other countries, monitor pending eruptions. No super-volcanoes are on the seismometers, so to speak, for 2012 or anytime in the considerably more distant future. This too is good.