Subject: The shapes of the continents

While looking at a globe with the south pole up, I started observing, much to my surprise, that there seemed to be an unusually high majority of land masses that were large at the north end and more "pointy" at the south end. Thus, North America, South America, Africa, India, Malaysia, even parts of Europe and Scandanavia seem to "drip" south- ward. I would have expected an equal distribution of such "pointy-ness" north and south. I can't really say this is statistical in any sense. It's just a general sense I get when looking at a globe. Is this just chance, or is there some kind of explanation?

    Your observation regarding the "pointy-ness" of land masses in the southern hemisphere (with the points facing south) is a good one. Although we undestand the sequence of events that led to this situation, there is some debate as to whether they occured by chance or by a design.
    To understand the present disposition of land masses on the Earth, one must understand that the continents and ocean crust sit on plates know as litho- sphere that move about the surface of the Earth at rates of a few mm to maybe 150 mm per year. This continual drift of lithospheric plates causes land masses to break up and collide continually. The Himalya mountains were caused by just such a collison and the East African rift valley is believed to represent the beginings of a continental breakup.
    Because the Earth is a sphere, the plates move accross the surface in arcuate pathes, defined by an angular velocity and a pole of rotation. The farther one is on a plate from its rotation pole, the faster one would appear to be moving relative to a fixed reference frame.
    This brings us to your question. Some 165 million yrs ago, the continental land masses were centered about present-day Africa and Europe. They began breaking up in such a way that the southern parts of present-day S. America and Africa were far away from the pole of rotation, causing the Atlantic Ocean to unzip from the bottom up, so to speak. About 30 million years later, a second continental mass in the southern hemisphere broke up, causing present-day India to move northward, Australia to move westward, and Antarctica to move south. Because of the way this old land mass (know as Gondwanaland) broke up, it caused India to rip off with a point to the south.
    So, now the question boils down to a chicken-egg issue. Do pre-existing weaknesses in the lithosphere cause the rotation poles to be established in certain places, implying that for some reason the lithosphere was already weak in a just the right zones to cause the pointy-ness? Or were the rotation poles which explain the pointy-ness established as a passive response to large scale convection currents in the Earth's mantle, oblivious to what land masses lay above them? or were the poles established randomly, causing the continents to rend passively along the path of least resistence and the mantle to set up a new pattern of convection in response to the breakup? Justifications for each of these scenarios have been proposed, but are only testable by detailed study of the structure of the lithosphere, how it has changed with time, and how and where the lithospheric plates move about the surface of the earth over time.
    The final answer is not yet in hand, as it requires piecing together a history that has been obscured by time and erosion. My tendency is to believe that a combination of choices one and two have been operating, but its only an informed guess.

Dr. Ken Rubin
Asst. Professor of Geology and Geophysics
Univ. of Hawaii

Return to the Ask-An-Earth-Scientist © page