Notes: Plate Tectonics


Theory of Plate Tectonics: The Earth’s crust is divided into pieces and those pieces are in constant motion.

            1. The continents look like puzzle pieces that could be put together.

2. Identical fossils of extinct herds and identical rock outcrops are found on both sides of the ocean.

3. Earthquakes and volcanic activity almost all seem to happen along specific lines in the earths crust.

4. Magnetic Reversals in the Earths Crust: Mafic igneous rock as it cools has a high iron content and the iron atoms will align themselves with the earth’s magnetic field as it cools. Spreading out from the ridges of the ocean are mirrored patterns of the earth’s magnetic field reversals.







Earth’s Crust:

Lithosphere: Crust plus the top cooler part of the upper mantle. The top part of the lithosphere is brittle the bottom part is fairly rigid but bends and flexes under immense force.


Asthenosphere: Between the lithosphere and the mantle the asthenosphere is a thin liquid layer that allows plates to flow on it.


Oceanic Crust: underneath the sea floor, is thin, and made of high density rocks (basalt and gabbro)


Continental Crust: under the continents, is thicker and less dense allowing it to float higher on the mantle, and made of low density rocks like granite.



Theories for why Plates Move:

Convection: The idea that head rises and cool sinks. So the deeper rocks in the mantle are rising cooling and then falling. And while they do this they create a force that pulls the plates around.

Doesn’t work because of all the random ways the continents seem to be moving.

Gravitational Sliding: (ridge-push force) The mid oceanic ridges are raised portions of the crust. As new crust forms at the top they begin to slide downward and push the plate in front of them.

If this theory is correct it would mean that sea floor spreading causes plate tectonics.

Slab-Pull force: A denser plate will fall under (subduct) a less dense plate. A sinking piece of oceanic crust will work like an anchor and pull the rest of the plate along with it.


Speed of a Tectonic Plate:


1-15 cm per year

            global positioning system (GPS) is now used to track the motion of the plates.


Plate Boundaries:


Earthquake Belts: Plate boundaries are located by the mapping of earthquake foci (the spot where the earthquake originates.

Plate Interiors: regions away from the plate boundaries, remain relatively earthquake free.

Divergent Plate Boundaries: Where two plates are moving apart

            Sea Floor Spreading: where two oceanic plates move apart

mid-oceanic ridge: where the divergent boundary has risen about 2 km above the ocean floor in long straight lines.

new ocean floor is created right at the boundary between the two separating plates.

As the plates spread apart a gap does not form, instead hot asthenosphere rises (magma) to fill the gap. Cools and forms new oceanic crust.

As the new crust moves away it will cool more and become more dense and sink lower. You can actually tell the age of the ocean floor by how deep it is, the older it is the denser it is the deeper it sinks.

symmetrical: the eastern side of a ridge will look just like the western side since they formed at the same time.

fracture zones: often a ridge will slide perpendicular to it direction (jog left or job right). This is where earthquakes have occurred along the ridge.

            Convergent Plate Boundaries: Where two plates are colliding head on.

Oceanic vs. Oceanic: The older denser plate will subduct under the younger lighter plate.

Subduction: oceanic lithosphere, once it has aged at least 10 million years, is denser than the asthenosphere so it sinks back into the mantle.

Deep Ocean Trench: Created where the older plate subducts under the younger one.

Earthquakes: Created where the two plates are rubbing together.

Volcanoes: Created over the spot where the subducting plate melts.




Oceanic vs. Continental: The oceanic plate subducts under the Continental plate.

                        Deep Ocean Trench: off the edge of the continent.


Volcanoes: For on the continent over the place where the oceanic plate begins to melt.


Continental vs. Continental:  Two continental crusts crash together. Since neither can subduct they grind up on top of each other forming mountains.




            Transform Boundary:



Where two plates are rubbing sideways against each other.