1. The asthenosphere is the portion of the mantle that can flow most easily, though it is not liquid (toothpaste and Silly Putty are both good analogues). The asthenosphere is also the source of the magma that erupts at spreading ridges to form oceanic crust.
Activity at Plate Boundries
Click on the image to learn more. Click "Begin Animation" to see how it works.
4. New oceanic crust is constantly being generated at mid-ocean ridges (also called spreading centers) as magma from the asthenosphere cools to form basalt. In most places, that magma travels along a network channels to reach the mid-ocean ridge, represented in this animation by the root-like structure.
3. As magma that erupts at mid-ocean ridges cools, magnetic minerals within the magma line up with Earth’s magnetic field. In this animation, the [black] zones represent magma that cooled to basalt while Earth’s magnetic was the same as today, which we call normal polarity. The [white] zones represent magma that cooled to basalt while Earth’s magnetic field was reversed, meaning that the magnetic north pole was in the southern hemisphere. Note that the changes in the oceanic crust color occur when the compass needle flips.
2. Tectonic plates are made up of the lithosphere, which consists of the crust plus the uppermost portion of the mantle. The lithospheric plates can include both oceanic and continental crust with the uppermost mantle, and they move on top of the asthenosphere.
5. Volcanoes occur near plate boundaries where one plate is being subducted beneath the other. The subducted plate does not melt, but mineral- and CO2-rich fluids are released from the subducted plate and cause melting in the overlying plate. Eventually, this magma reaches the surface and erupts to form a volcano.
This compass represents what your compass needle would do over the last 5 million years or so. Over that time period, Earth’s magnetic field has flipped, or reversed, several times. The reversals don’t happen at regular intervals, so the patterns of magnetism on the seafloor can be used like tree-rings to correlate ages of rocks all over the world.