| Lecture 14 Notes October 26, 2004 |
Plate Tectonics Lecture by Dr. Greg Balco, these
notes by Prof Bitz
. 
In the 1920s Alfred Wegner pointed out that South America looked like
it would fit nicely next to Africa, so the continents probably had been toghether
at one time - and the notion of continental drift was born. The idea was
bolsterd by evidence of common rocks on either continent that match up with
the geometry on the right. The next 40 years or so, scientists argued vehemently
about the idea. (I'm not that old, but when I first learned about
continental drift, I was told it was probably wrong! Sadly, text books often
have outdated information about exciting science.)
Other 20th century mysteries played into the controversy:
- Mid-ocean ridges were measured with sounding instruments in the 1970s. The picture of linear ridges was surprising
- Coastal trenches were also a surprise
- Meanwhile geologists were wondering why there are mountains
- And it was clear that earthquakes were common in the vicinity of all three of the above
The theory of plate tectonics was solidified by evidence of magnetic stripes on either side of the mid-ocean ridges. It is believed now that ocean crust is formed at these ridges, which are sites of the mantle pushing upward. Oceanic crust is a thin layer on top of a convecting mantle. Earth's mantle is nearly fluid and it is hot, so it convects (the text explains how rocks can be fluid). The ridges lie at the point where upward branches of the convecting cells meet. The mantle upwells at these ridges in the form of molten magma, carrying with it magnetized iron minerals. The magnetic poles of the iron are align with Earth's magnetic field. They are free to do so because the iron is in a fluid. When the magna solidifies into crust, it then freezes the magnetic iron and its polarity into place. Earth's magnetic field reverses polarity every few tens of million years, which is then recorded as stripes of alternating magnetic alignment in the solid crust.
The crust is created and diverging at the mid-ocean ridges so it has to converge (or be taken away) somewhere else. In fact it subducts at the coastal trenches where the spreading ocean crusts meets the more buoyant continental crust. Subducting crust "melt" as it sinks and makes new magma.
The buoyant continental crust keeps continents floating about, so they can bump into one another and make mountain ranges. The magma created at the coastal trenches flows under the continental crusts and makes it way back to the surface at volcanoes.
What does this have to do with climate?
We know that land and ocean have rather different surface albedos, and we know that snow and ice feedbacks on land can only occur if land is cold enough to support ice in the first place (ice feedbacks don't come into play in the tropics). Continents drifting about occasionally bunch together near the poles. When this happens,
giant ice sheets can form. Such giant
ice sheets may actually hold a substantial amount of water and lower sea
level - further altering the total surface albedo. Also giant ice sheets
probably have a profound effect on the atmospheric circulations.We know that land and ocean have rather different surface albedos, and we know that snow and ice feedbacks on land can only occur if land is cold enough to support ice in the first place (ice feedbacks don't come into play in the tropics). Continents drifting about occasionally bunch together near the poles. When this happens,
The geometry of the continents can also influence ocean heat transport. Presently the ocean basins are oriented mostly east-west, except for the Southern Ocean (around Antarctica). If the oceans could circulate around the entire planet in the tropics, there might be much less poleward heat transport in the ocean.
Mountains also strongly influence moisture patterns through upslope winds that produce clouds and rains and downslope winds that are associated with very dry conditions.
Contact the instructor at: atms211@atmos.washington.edu
Last Updated: 10/26/2004