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The Big Ideas:
• Characteristics of tectonic plates
• How convection in the mantle drives plate motion
• 3 types of plate boundaries and typical features at
each boundary
In 1910 Alfred Wegner, a German
meteorologist, begins to wonder…
Is there a relationship between all of the continents?
Perhaps all the continents pieces used to be
Continental drift = slow movement over Earth’s surface
Scientists thought Wegner was crazy
Two unanswered questions were the main reasons
that the hypothesis of continental drift was
• what forces could move continents?
• how continents could move without shattering?
A. Outer part of Earth consists of thin and rigid
pieces called plates, and these plates move
slowly and continually.
B. 4 main pieces of Evidence:
1. shape of continents = fit together like a
2. Similarity of fern fossils found on India,
Australia and South Africa
3. Similarity of reptile fossils found in
South America and Africa
4. Paleomagnetism – where igneous rocks
that contain iron record Earth’s magnetic
direction at the time the rocks cooled.
Fern fossil evidence
Paleomagnetism evidence
C. Plates: 6 major plates and multiple
smaller plates
- Each plate ~ 100 km thick
- Moving 1 – 20 cm/year.
- Average speed is
2 cm/year
D. Plate tectonics theory explains the
location of mountain ranges, volcanoes,
and earthquakes.
A tectonic plate is like the hard shell on a boiled egg that has been
cracked into pieces or plates.
What do you notice?
1. Core – Earth’s internal
heat source. 2 sub-layers:
a. Inner core: solid iron
and nickel
b. Outer core: liquid iron
c. Heat in the core caused
from remnant heat from
Earth’s formation &
radioactive decay of
2. Mantle – made of plastic-like
magma, 2 sub-layers:
a. Asthenosphere – lower mantle,
made of iron and magnesium. Is
semi-solid an can flow like silly
b. Lithosphere – upper mantle,
rigid and stiff. Does not flow!
c. Asthenosphere and Lithosphere
separated by a change in density
and rock composition. The
boundary between is called the
Moho discontinuity.
3. Crust – outer skin of
the earth. 2 types of crust:
a. Oceanic – thinner,
denser, basalt rock
b. Continental –
thicker, less dense,
granite rock.
B. Earth’s plates are made of either oceanic
or continental crust and the lithosphere.
These tectonic plates “float” on top of the
plastic-like asthenosphere.
But how do the plates move? What causes plate motion?
A. Answer: convection currents in the
B. Convection occurs when a liquid or gas is
heated, becomes less dense and rises.
When it cools, it gets more dense and
sinks, and the process repeats.
C. This cycle of heating and cooling drives
plate motion.
Draw this simple convection current
Convection in the asthenosphere
• So, now we know why plates move…but what
happens at the plate boundaries?
Plate Boundary
2 plates collide into
each other. Mountains
or subduction zones
2 plates move away
from each other.
Magma rises to
2 plates slide past
each other.
A. SUBDUCTION: Oceanic plate (denser) forced under
continental (lighter) plate
1. At the subduction zone a deep sea trench is formed where the
plate is being forced downwards under the continental plate.
2. Subduction causes rocks to melt, and magma rises to surface
to form volcanoes!
3. Examples: Cascades in US, Andes Mountains in South
B. Mountains can be formed when 2 continental plates collide
into each other.
1. Because they both have the same density, neither one
subducts under the other. They crumple together.
2. Classic examples are the Himalayas, which are still going up!
The Himalayas are still growing today at a rate of 1 - 2 cm a year!
Himalayas – 29,029 ft
C. When 2 plate move away
from each other, magma rises
and new crust is formed.
1. This is happening along the
mid-Atlantic ridge, which
passes through Iceland.
2. If continental crust pulls apart
from continental crust then
the same process occurs. As
magma erupts to the surface
to fill the gap, a volcano is
Mid-Atlantic ridge in Iceland
D. When plates move past
one another earthquakes
1. Plates are rigid so it is
difficult for the plates to
slide past each other.
2. The most famous
conservative plate
margin is the San
Andreas Fault on the
western coast of North
San Andreas fault in CA