Wednesday 30 March 2011

Many processes sculpt the surface of the Earth. In the 20th century an extremely successful idea arose that can explain much of the large scale features on Earth- such as where mountain chains are, as well as account for the distribution of geologically active places on Earth- places where there are earthquakes and volcanoes. This idea is called Plate Tectonics. The basis idea is that the outermost part of the Earth- the crust - is broken into about a dozen pieces called plates. These plates move around, driven by motions in the next layer down, the mantle. The mantle is so hot that the rock it is composed of is "plastic", and flows much like cold maple syrup (but much more slowly!). At the boundaries between plates, where the plates grind and bump into each other, geological (and human) hell all too frequently happens- earthquakes, tsunami, and volcanoes.

Active regions on Earth Each red dot represents an earthquake or volcanic site. The white arrows show the direction of motion of the plates relative to each other. Note that the plates tend to "pull apart" (diverge) in the middle of the oceans (e.g. middle of Atlantic). Where one plate is diving below another, mountain ranges can form (western edge of South America, for instance).

Earthquakes on Earth Same idea as previous, but with lots more real data! This shows position of over *1/3 of a Million different earthquakes** over 35 years. These earthquakes outline the plate boundaries with amazing sharpness.

The major tectonic plates The Earths outer shell is divided into a dozen or so "plates" that move around. The plate boundaries are outlined by earthquakes and volcanoes, as shown in the previous slides.

Earthquake waves The chief tool to study the Earth's interior are waves from earthquakes. When a large earthquakes happens, the entire Earth rings like a bell struck with a hammer. By measuring how long it takes for various types of waves to pass through the Earth, we can study the interior of the Earth. We have no other way to directly study the Earths interior. The deepest holes we have drilled are only a few miles deep- barely a scratch on the surface of the Earth.

(1) Plate boundaries (2) Plate boundaries - spreading ocean floor to subduction zone (3) Subduction zone, ocean trench and orogeny - west coast of South America There are 3 kinds of plate boundaries- converging (coming together), diverging (pulling apart) and transform (sliding past each other).

At diverging boundaries (in oceans) "new" sea floor is made, which is then "lost" at converging boundary (subduction zone). By the way, "orogeny" has nothing to do with erogenous or orgy- orogeny means the process of mountain formation.

Volcanoes over Earth Volcanoes usually are related to regions of orogeny (mountain building) near subducting plates. Perhaps the most spectacular long mountain chain on Earth are the Andes (shown in previous slide).

Age of sea floor This color- coded map of the ages of sea floor rock shows the spectacular success of the idea of sea floor spreading- you can clearly see the long linear "sources" of the sea floor rocks marked by the youngest rock (coded as red- make sure you look at the color-age bar in lower left). These sources are basically cracks in the Earth's crust through which liquid rock (magma or lava) comes oozing out of the mantle.

As new magma comes out of the cracks, the older rock is pushed away from the crack, causing the age of rocks to increase as you go away from the crack, as seen in this map.

San Andreas fault Probably the most famous plate boundary is the transform fault that passes through the sunny state of California. (Well the first thing ya know old Jeds a millionaire- the kinfolk said Jed move away from here- they said Californ-i-a is the place you otta be- so they loaded up the truck and they moved to Beverly- Hills that is- swimming pools - movie stars. Sorry! Every time I think about California the song from the Beverly Hillbillies comes into my head.). Most of California lies on the North American Plate, as does Oklahoma. However, part of western California lies on the Pacific Plate, which is moving northwards relative to the North American Plate. The plate boundary, where the plates grind past each other, is marked by the San Andreas Fault (SAF). Sudden motion of the plates along the fault (and numerous smaller faults radiating from the SAF) cause the earthquakes which plague the Golden State.

(1) Continental drift (2) Pangea breakup animation The continents are lighter rock (mostly granite- about 2.7 times density of water) that "float" on the denser sea floor (basaltic rock- about 3.3 times density of water). The continents move about slowly - about as fast as your fingernails grow.

In the past, the continental land masses were part of larger structures than they are now. These ancient "supercontinents" are given names of Pangaea, Laurasia, and Gondwana Land. (old bumper sticker: "Reunite Gondwana Land!")

Fossil map evidence of continental breakup Maps of fossils of ancient creatures show that the ranges of some creatures, presumably once contiguous land, are now split onto two continents- just what you would expect from continental breakup and drift.

India raising Himalayas The continuing "slow motion crash" of India into Asia is pushing up the Himalaya Mountains, the highest elevation area on Terra.

Mantle convection driving plates What drives the plate motions? Perhaps there are large motions in the plastic mantle, maybe driven by convective currents driven by heat in the core of the Earth. Mountain chains (such as the Andes on the western edge of South America) can be created when a plate dives below another and pushes up the edge of the overlying plate.

Plate descending into mantle Earthquakes mark the path of a piece of plate as it moves down into the mantle near Fiji Island. We can easily find the position and depth of earthquakes from the pattern of shaking detected by sensitive devices called seismometers.

Simplified internal structure of Earth Using earthquakes to make a type of "CAT scan" of the Earth, we surmise this structure of crust (thin layer of brittle rock), mantle (large volume of "plastic" rock which can flow VERY slowly), outer liquid iron core, and inner solid iron core.

Earth interior: density vs. depth This gives a schematic idea of the way the local density changes with depth as we go deeper and deeper into the Earth.

The sharp discontinuity (jump) in density at about 3000 km depth marks the mantle-core (or rock - iron) interface. The Earth is clearly very well differentiated- almost all the high density iron sank to the center during formation of the Earth.