Plate tectonics developed from Alfred Wegener's theory of continental drift in the early 20th century. Wegener noticed coastlines of Africa and South America fit together and proposed all continents were once joined in a supercontinent called Pangaea. Later, scientists proposed mantle convection and sea floor spreading caused by convection currents in the mantle to explain plate movement. Today, plate tectonics theory describes the lithosphere broken into rigid plates that move across Earth's surface due to mantle convection and other forces, forming boundaries and landscape features.

5. • Plate tectonics grew out of a theory that was
first developed in the early 20th century by
the meteorologist Alfred Wegener.
• In 1912, Wegener noticed that the coastlines
of the east coast of South America and the
west coast of Africa seemed to fit together like
a jigsaw puzzle.

7. • Wegener proposed an idea that all of the continents had at
one time been connected in a single supercontinent
called Pangaea.
• Wegener believed that the continents gradually began to
drift apart around 300 million years ago - this was his theory
that became known as continental drift.
• In 1929 Arthur Holmes, a British geologist, introduced a
theory of thermal convection to explain the movement of
the Earth's continents.
• By the 1960s Holmes' idea began to gain more credibility as
scientists increased their understanding of the ocean floor
via mapping, discovered its mid-ocean ridges and learned
more about its age.
• In 1961 and 1962 scientists proposed the process of sea
floor spreading caused by mantle convection to explain the
movement of the Earth's continents and plate tectonics.

8. • » BIOLOGY → same
fossils in different
parts of the world.
• » GEOLOGY → same
rocks in different
parts of the world.
• » CLIMATE → fossils
of plants that live in
tropical conditions
in Antarctica.
• » Paleomagnetism and
the reversal of the
earth's magnetic field
(1950's)
• » Sea floor spreading -
dating of the sea floor
rocks (1960's)
• » Earthquake Patterns
• » Ocean Drilling
• » Hot Spots

9. • Plate tectonics is the theory that the outer rigid layer of the earth (the lithosphere) is divided into a couple of dozen
"plates" that move around across the earth's surface relative to each other, like slabs of ice on a lake.
• .In the middle of the drawing that is the CONTINENTAL CRATON (STABLE CONTINENT).
• The line under the craton, ; that is the lower boundary of the plate.
• Everything above that line is the plate.
• All similar lines in the cross section mark the bottom of the plates.
• Everything above that line is lithosphere, the rigid, brittle shell of the earth.
• Below is ASTHENOSPHERE, the hot, plastic interior of the earth.
• Within the asthenosphere are convection cells, slowly turning over hot, plastic rock.

10. • Scientists today have a better understanding of the make-up of the Earth's tectonic plates, the
driving forces of their movement, and the ways in which they interact with one another.
• A tectonic plate itself is defined as a rigid segment of the Earth's lithosphere that moves separately
from those surrounding it.
• There are seven major plates (North America, South America, Eurasia, Africa, Indo-Australian,
Pacific and Antarctica) as well as many smaller, microplates such as the Juan de Fuca plate near the
United States' state of Washington.
• There are three main driving forces for the movement of the Earth's tectonic plates. They are
mantle convection, gravity and the Earth's rotation.
• Mantle convection is the most widely studied method of tectonic plate movement and it is very
similar to the theory developed by Holmes in 1929.
• There are large convection currents of molten material in the Earth's upper mantle. As these
currents transmit energy to the Earth's asthenosphere (the fluid portion of the Earth's lower mantle
below the lithosphere) new lithospheric material is pushed up toward the Earth's crust.
• Gravity is a secondary driving force for the movement of the Earth's tectonic plates. At mid-ocean
ridges the elevation is higher than the surrounding ocean floor.
• As the Earth's tectonic plates move they interact in a number of different ways and they form
different types of plate boundaries.
• No matter the type of boundary though, the movement of the Earth's tectonic plates is essential in
the formation of the various landscape features we see across the globe today.

13. • Layers
a. Continental and
Oceanic Crust
b. Mantle
c. Inner and Outer Core
• Lithosphere and
Asthenosphere; Nature
of Lithosphere (rigid),
Asthenosphere, Low
Velocity Zone
• Discontinuities; Inner
Core, Outer Core,
Mantle, Continental
Crust, Oceanic Crust
• Crust → the upper layer
which is solid (Continental
and Oceanic Crust)
• Lithosphere → the crust
plus the upper part of the
mantle (solid).
• Mantle
» Upper part is solid and
the part of lithosphere.
» Below this is the
Asthenosphere which is
partly molten and can
flow.
» Rest of mantle is
liquid.
• Outer Core: Liquid.
• Inner Core: Solid.

15. AsthenosphereLithosphere
Outer layers of the Earth. Plates are
made up of crust and rigid upper
mantle.

17. • Oceanic Crust that lies under the ocean, is
mainly basalt and 5-7km thick.
• Continental Crust 20-50km thick and
composed mainly of granite; it makes up the
large land masses- the continents
• Continental Crust is low density and “floats”
on top of the mantle whereas oceanic crust
is much higher density.