The Earth’s Crust and Plate
The Earth’s Crust
This is where we live!
The Earth’s crust is
- thick (10-70km)
- buoyant (less
- mostly old
- thin (~7 km)
- dense (sinks
If you look at a map of the world, you may notice that some of the
continents could fit together like pieces of a puzzle…
• The Earth’s crust is divided into many major
plates which are moved in various
• This plate motion causes them to collide,
pull apart, or scrape against each other.
• Each type of interaction causes a
characteristic set of Earth structures or
• The word, tectonic, refers to the
deformation of the crust as a consequence
of plate interaction.
What are tectonic plates made of?
Plates are made of rigid lithosphere.
The lithosphere is made
up of the crust and the
upper part of the mantle.
Alfred Wegener proposed the Continental Drift Hypothesis in
1912, that stated Earth’s continents were once joined in a
single landmass and gradually moved or drifted apart.
Theory of Plate Tectonics
Fossils, Climate, & Geology
Evidence from the Sea Floor
Three types of plate boundary
Also called spreading ridges
As plates move apart new material is erupted to fill the gap
Iceland has a
Where plates slide
past each other
• 2 plates that are pushing together
• There are three types of convergent
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
Forms mountains, e.g. European Alps, Himalayas
• Oceanic lithosphere
• Oceanic lithosphere
dehydrates as it
• The melt rises
• Deep ocean
trenches & Coastal
• When two oceanic plates
collide, one runs over the other
which causes it to sink into the
mantle forming a subduction
• Island arcs form
• The subducting plate is bent
downward to form a very deep
depression in the ocean floor
called a deep ocean trench.
• The worlds deepest parts of the
ocean are found along trenches.
– E.g. The Mariana Trench is
11 km deep!
“Plates” of lithosphere
are moved around by
the underlying hot
mantle convection cells
Magnetic Reversal• After molten lava emerges from a volcano, it solidifies to a
rock. In most cases it is a black rock known as basalt, which
is faintly magnetic (iron)
• Its magnetization is permanently fixed like tiny compass
needles pointing north & south.
• Instruments can measure the magnetization of basalt.
• Surprisingly, this procedure suggested that times existed
when the magnetization had the opposite direction from
today's. All sorts of explanation were proposed, but in the
end the only one which passed all tests was that in the
distant past, indeed, the magnetic polarity of the Earth was