Earth's crust moves by plate tectonics.
Earth's crust can be continental (the
ground humans live on) or oceanic (the rock
that forms at the bottom of the ocean at mid-ocean
The crust is resting on top of thick slabs of
rock, called tectonic plates. These plates
are always moving and interacting with other
plates; this process is called plate
Tectonic plates can move because the rocks
below the plates have the ability to flow even
though they are still solid and not liquid. You
can think of this movement in the same way taffy
can flow, even though it is still a solid.
Depending on how the plates are moving, the crust
is either being formed, destroyed, smeared against
itself, or squeezed together into thicker crust.
These motions explain all of Earth's surface
geology from where oceans and mountains form, why
and where earthquakes and volcanoes exist, and so
There are three forces that drive plate
(1) Ridge push. Ridge push happens at
mid-ocean ridges, at the center of the ocean
bottoms, due to gravitational forces acting on the
spreading ridges. Newly formed oceanic
crust is less dense than old oceanic crust that
has cooled and become covered in sediment.
Therefore the buoyancy of the new crust can enable
more magma to rise and push the ridges apart.
(2) Slab pull. Slab pull happens at the
edges of ocean basins where oceanic crust is being
destroyed under the edges of continents due to
gravitational forces in the subduction zones.
Subduction zones are where the oceanic
crust meets the continental crust. Because oceanic
is thinner and denser than continental crust, the
oceanic crust is pushed under the continental
crust where is sinks below Earth's crust. As the
oceanic crust is pushed under, gravity can pull
the crust faster or slower depending on its
density. The faster the slab is pulled down,
the faster the ridge will be pulled apart, the
faster new crust is created at the ridge.
These are the main drivers behind how Earth's
crust moves around. However, plate movements are
not that simple. Sometimes ocean crust runs into
ocean crust, continental crust runs into
continental crust, the plates never collide but
grind past each other, or the plates tear apart.
These other settings complicate plate motion and
explain how we know Earth's continents not only
were once all connected as a supercontinent, but
have actually gone through several cycles of
smashing together and ripping apart. The
reason gravity can push and pull slabs is due to
the final driving force:
(3) convection in the mantle. Mantle
convection is largely heat driven and explains how
small perturbations in the mantle can change how
the plates are moving relative to other plates and
where that movement happens (for example, why
continents rip apart to eventually form new ocean
basins or why the Himalayan Mountains
continue to rise higher and higher ).
You may have seen convection happen when
boiling water. The hot water touching the bottom
of the pot rises to the top, while the cooler
water from the top of the pot flows in to replace
the rising hot water. While boiling, the water is
convecting around in the pot.