MadSci Network: Earth Sciences

Re: Direction of Earth's rotation and shape of the continents

Date: Sun Sep 3 13:22:36 2006
Posted By: David McMillan, Post-doc/Fellow, Earth and Space Science, York University
Area of science: Earth Sciences
ID: 1156582571.Es

In short, neither Earth's rotation nor its orbit affect the shapes or motion of the continents. Their shapes are determined by random processes that occur when one larger continent breaks up into two smaller ones at a spreading centre . It is true that rotation affects the path of moving objects as we observe them while we sit on the surface of a rotating Earth. This is due to the Coriolis force , which is important in the atmosphere and for example, results in the rotation of hurricanes and cyclones. The Coriolis force is also plays an esssential role in the generation of the geomagnetic field through motion of liquid iron in Earth's outer core .

Plate tectonics is caused by the motion of thermal convection Earth's solid mantle. The common thread in each of these examples (atmosphere, mantle and outer core) is the motion of a fluid in or on a rotating planet. The main difference for the mantle is the exceptionally slow velocities associated with convection and plate motion; the opening of the atlantic ocean has taken over 100 million years. This means that the effects of rotation, acting through the Coriolis force, are not felt by the tectonic plates as they move slowly about the planet. The converse is, however, not true: the arrangement of the tectonic plates (and ice caps and glaciers) at any given time can have a profound effect on Earth's rotation. As the continents move around, the distribution of mass on Earth's surface is continually changing (even though this change occurs very slowly over a long period of time). A spinning object like a top or a planet will develop a wobble in its rotation if there is a large irregularity in its shape or mass distribution. The concept of mass distribution as it relates to the rotation of a solid object is stated mathematically through the moment of inertia. The continents are currently in a relatively uniform arrangement, having little effect on Earth's rotation. It is thought by some scientists that, in the distant past, the distribution of Earth mass caused a sudden and large change in the rotation axis, which left formerly tropical regions near the poles and vice versa. Earth's orbit causes annual and semi-annual variations in atmospheric pressure due to differences in the heating of the northern and southern hemispheres. This "atmospheric loading", in addition to similar cycles of oceanic loading and groundwater movement, can be seen in measurements of Earth rotation. The atmosphere and oceans are very thin compared to the mantle and heat up much more easily and convect more rapidly, so there are no similar effects on the mantle, its convection system or plate motions. Earth also experiences many different small deviations in its gravity and orbit as it travels around the Sun, the most notable of which is due to the Sun and the Moon, and causes the daily and semi-daily tides. Because these types of deviations occur over relatively short periods like days or years, and again because the periods associated with plate motions are hundreds of millions of years, the mantle's convection system appears stationary and is unaffected. Thus two identical planets that rotate or orbit in opposite directions will not have mirror-image arrangements of continents. And finally, just to be complete, two identical planets are not likely to have similar continents because it is a chaotic system that behaves randomly.

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