Both theories are about the movement of Earth's
surface but they differ in when the
theories were first posed and what Earth phenomena
Continental drift was coined in the early
1900's by Alfred Wegener, though he did not
discover this concept. Wegener saw that the
continental landmasses on maps line up like a
jig-saw puzzle, as well as geologic features (like
mountains and valleys), glacial-caused erosion
patterns, and fossil areas line up across the
continents for a once connected single landmass.
Continental drift was based on the idea that
all the modern landmasses were once connected
into a single continent. However this theory
was rejected because there was no known mechanism
for how the continents could have broken apart and
ended up where they exist in the modern time.
Then, as World War II was ending, a new radar
technology revealed the Atlantic sea floor was
spreading away from a mid-ocean ridge in the
center of the ocean bottom. Further studies
showed the mid-ocean ridge is a chain of volcanoes
running down the center of the ocean floor, a
magmatic zone where lava comes up from the mantle,
creating new ocean floor and pushing the crust
apart. Suddenly there was a mechanism for the
movement of the continental masses.
Other studies of spreading at rift valleys on
continents, subduction zones destroying the outer
edges of ocean crust, and the theory of plate
tectonics was born. Plate tectonics theory
became widely accepted starting in the 1970's, and
describes Earth's surface as broken into
numerous plates that move around Earth's
surface, either slamming into another plate,
destroyed under a neighboring plates, or grinding
past another plate. The tectonic plates, or slabs
of Earth's crust sitting on top of mantle rocks,
move as rigid crust on flowing mantle rocks (crust
on the mantle is like a graham cracker on a
marshmallow, both are solids but the mantle rocks
can be deformed plastically like a marshmallow).
That movement can cause magma, gas, and ash to
shoot up and create a volcano and/or an
earthquake when the plates shift and slide.
Earthquakes are caused when two tectonic
plates are colliding. They move because of the
intense heat from the Earth's core. As the
lava cools it shrinks and as it heats up it
expands causing the plates to move.
Thus continental drift describes continental
crust moving around the ocean basins, while
plate tectonics describes how and why Earth's
crust is created, destroyed, and moves.
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