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Plate Tectonics Lesson

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Layers of the Earth

Continental Drift + Sea-Floor Spreading + Paleomagnetism = Plate Tectonics

Earth’s lithosphere is broken up into plates. Beneath this the hot, weak asthenosphere allows for plates to move. Plates are in motion and continually changing in shape and size. They move very slowly; about 5 cm/yr. The movement is NOT constant but more of stop and go (earthquakes!).

Layers of the Earth


(image source)

There are two types of crust: Continental and Oceanic

Continental Crust

Average density: 2.7 g/cc
Average composition: Granodiorite.
There are many different types of rocks on the continents. So, in order to determine the average composition of continental crust, all of the rock densities are combined and averaged. Thus, the average composition of the continental crust is a rock called granodiorite and the average density is 2.77 g/cc.
Continental crust is thicker & lighter than oceanic crust.

Oceanic Crust

Average density: 3.0 g/cc
Average composition: Basalt.
Unlike continental crust, oceanic crust is composed entirely of a rock called basalt. It is also thinner & denser

Lithosphere

Includes the crust and upper mantle

Density: varies, ~ 4.4 g/cc
Composition: varies, mostly peridotite

The lithosphere bends elastically when loaded, while the asthenosphere flows plastically when loaded. This concept called isostacy, or the balancing of pressures. People tend to think of rocks as being hard and breakable. However, rocks can bend quite a bit before they will break. When a load, like an ice sheet for example, presses down on the crust, the crustal rocks will warp downward. As they do so they push aside the rocks in the asthenosphere. These rocks are ductile, meaning they can flow. So these rocks flow away from where the load is depressing the crust. Floating solids displace water equal to their mass. So the amount of asthenosphere displaced is equal to the volume of the item pushing down on the crust.



In the images below, continental crust is represented by cork and basaltic crust by pine, which is more dense. Because cork is very light it will "float" higher than the block of wood with pine. This produces a gap between the top of the continental crust and the top of the oceanic crust. On the Earth's surface the oceanic crust is depressed, and forms the basins in which the oceans reside.


Continental Lith. ~150 km thick

Granitic crust ~35-40 km thick

  • Lighter (less dense)
  • More buoyant (floats higher)

Oceanic Lith. ~ 7 - 133 km thick

Basaltic crust ~7 - 10 km thick.

  • Heavier (more dense)
  • Less buoyant – sinks lower

Mohorovičić discontinuity

The boundary between the lithosphere and the mantle is called the Mohorovičić discontinuity boundary. This boundary is marked by a change in velocity of the P waves. In the crust, the waves travel at a velocity of 6.7–7.2 km/s (similar to that of basalt); below this they travel at a rate of 7.6–8.6 km/s (similar to that of peridotite or dunite). So the Moho layer marks a change in composition from one layer to the next.


(image source)


Mantle:

The mantle comprises 80% of the earth and is broken into two layers: the Asthenosphere and the Mesosphere.

The Asthenosphere is a ductile, highly viscous & mechanically weak layer located just below the lithosphere.

Temperature: 500 to 900 °C (932 to 1,652 °F)
Density: 3.4 – 4.4 g/cc
Composition: Peridotite, Dunite & Eclogite

The Transition Zone

All minerals form at specific temperatures and pressures. Minerals become unstable due to an increase in T & P the deeper one goes into the Earth, and new minerals form in an effort to become stable. These changes in mineralogy may influence mantle convection. It has been recently discovered that these minerals contain a lot of water in their structure – more than all of the oceans combined!

Mesosphere

The asthenosphere gradually transitions into a rigid, solid layer called the Mesosphere.

Temperature: +4,000 °C (7,230 °F)
Density: 4.4 – 5.6 g/cc
Composition: Uncertain, but likely similar to the asthenosphere and is relatively seismically homogeneous

The Core

The core of our planet is larger than Mars and is composed mostly of iron with a bit of nickle. It is subdivided into to two parts: the outer and inner core.

Outer Core

Temperature: 4,300 K (4,030 °C; 7,280 °F) to 6,000 K (5,730 °C; 10,340 °F)
Density: 9.9 – 12.2 g/cc
Composition: Liquid Iron & Nickel

Inner Core

Temperature: +5,700 K (5,430 °C; 9,800 °F)
Density: 12.8 – 13.1 g/cc
Composition: Solid (?) Iron & Nickel

The relationship between the outer and inner cores are responsible for Earth’s magnetic field

Computer models: suggest that reversal likely when magnetic field lines don’t match up

Mathematical models: suggest the opposite

Other factors: Rapid changes in the churning of the liquid of the outer core can weaken the Earth’s magnetic field.

Reversal rate varies wildly. During the last 10 – 20 million years the average rate was 5 times/million years. However, during the Cretaceous Normal Synchron (84 – 125 million years ago) it only flipped once or twice. Keep in mind, though, that the oldest bit of oceanic crust is only 200 million years old, so this may be an anomaly.

Flips take thousands of years to complete. Or so we think, as no human has experienced one. So, how do we know this? Most migrating animals use the Earth's magnetic field to navigate. With each new generation, whatever is going on with the magnetic field gets imprinted on the animal. Since there are no mass extinction events associated with flips, and no evidence of confused animals going in the wrong direction, it is safe to assume that flips just don't happen overnight. When was the last flip? 780,000 years ago.




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