What on Earth is Plate Tectonics?
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 Into the Earth
The story of plate tectonics really starts deep within the Earth, so lets
take a look inside first. Although the Earth appears to be made up of
solid rock to us surface-dwelling humans, it’s actually made up of three
distinct layers: the crust, mantle, and core. Each layer
has its own unique properties and chemical composition. |
Crust
The crust is the thin, solid, outermost layer of the Earth. The crust
is thinnest beneath the oceans, averaging only 5 kilometers thick, and
thickest beneath large mountain ranges. Continental crust (the crust
that makes up the continents, of course!) is much more variable in thickness
but averages about 30-35 km. Beneath large mountain ranges, such as
the Himalayas or the Sierra Nevada, the crust reaches a thickness of
up to 100 km. |
Mantle
The layer below the crust is the mantle.
The mantle has more iron and magnesium than the crust, making it more
dense. The uppermost part of the mantle is solid and, along with the
crust, forms the lithosphere. The rocky lithosphere
is brittle and can fracture. This is the zone where earthquakes occur.
It’s the lithosphere that breaks into the thick, moving slabs of rock
that geologist’s call tectonic plates.
 As we descend into the Earth temperature
rises and we reach part of the mantle that is partially molten, the
asthenosphere. As rock heats up, it becomes pliable
or ‘plastic’. Rock here is hot enough to fold, stretch, compress, and
flow very slowly without fracturing. Think about the behavior of Silly
Putty® and you have the general idea. The plates, made up of the relatively
light, rigid rock of the lithosphere actually ‘float’ on the more dense,
flowing asthenosphere! |
Core
At the center of the Earth lies the super-dense core. With a diameter
of 3486 kilometers, the core is larger than the planet Mars! The core
of the Earth is made up of two distinct layers: a liquid outer layer
and a solid inner core. Unlike the Earth’s outer layers with rocky compositions,
the core is made up of metallic iron-nickel alloy. It’s hard to imagine,
but the core is about 5 times as dense as the rock we walk on at the
surface! |
