Plate Tectonics
– A Reminder of Earth’s Structure
– Two physical units of Earth that are
important too!
– How the Earth Works
• Plate Tectonics
• Plate Boundaries
Chemical Layers of the Earth
• Crust - low
density rock
• Mantle - high
density rock
• Core - metal
(iron & nickel)
– Divisions based
on chemical
composition
Physical Layers of the Earth
Outer layers divided based on how easily the rock flows.
The Two Types of Crust
The Lithosphere
Relatively rigid, nonflowable layer that lies between
the surface and depth of ≈ 100 km.
Continental: “ granitic”, low density, 15 –70 km thick.
Oceanic: “basaltic”, slightly higher density, 5-12 km thick.
The Asthenosphere
Relatively weak, flowable layer that lies below a depth of
≈ 100 km.
1
How The Earth Works:
The Basic Premise of Plate Tectonics
Identifying Plate Boundaries
The Major Plates
The Earth’s lithosphere is divided into plates that move relative to
one another and the underlying asthenosphere.
As a plate moves, its interior area remains largely intact and
rigid, but rock along the plate’s boundaries undergoes
deformation (cracking, sliding, bending, stretching, and
squashing) as it grinds against its neighbor causing earthquakes,
volcanism, and mountain building events.
As plates move, so do the continents that are part of the plates,
resulting in continental drift. Because of plate tectonics, the
map of Earth’s surface constantly changes over time.
Types of Plate Boundaries
•
Based simply on relative motion of the
plates on either side of boundary.
Most plates contain both continents and oceans.
Divergent Plate Boundaries
How do the two plates move?
Seafloor Spreading Occurs at
Divergent Boundaries
• New ocean crust is formed!
1. Divergent Plate Boundary
2. Convergent Plate Boundary
3. Transform Plate Boundary
2
Divergent Boundaries:
Mid-Ocean Ridges
Iceland
Rifting continents can form new
ocean basins!
Tension causes
crust to stretch
and crack apart
Sub-aerial “spatter
cones” along the
Mid-Atlantic Ridge
Rift Valley
People for scale
Continued divergence leads to:
1) Widening
of rift valley
2) Formation
of mid-ocean
ridge!
A Rifting
Continent
The Great Rift
Valley of Eastern
Africa.
Convergent Plate Boundaries
How do the plates move?
Convergence
leads to
destruction of
ocean crust!
Volcanism occurs
in the rift valley.
3
Features of a Subduction Zone.
Features of a Subduction Zone.
Volcanic Arc
Trench
Where is the nearest volcanic arc?
From Subduction to Collision.
1_23
Mt. Redoubt
volcano
Transform Plate Boundaries
How do the plates move?
Anchorage
NORTH AMERICAN PLATE
Bering
Sea
JUAN DE FUCA PLATE
Aleutian
Islands
Ocean-to-ocean
subduction
(a)
Where is the nearest volcanic arc?
Mt. Saint
Helens volcano
PACIFIC
PLATE
Cascade Range
GORDA
PLATE
Ocean-to-continent
subduction
(b)
• The death of an ocean basin and the
formation of collisional mountains.
4
Fracture Zones on the Ocean Floor
Offset the mid-ocean ridges
Fracture Zones are Transform
Boundaries
A Transform Boundary on Land
mid atlantic ridge.kmz
san andreas sf bay.kmz
http://quake.wr.usgs
.gov/research/defor
mation/gps/
san andreas sf bay.kmz
mid atlantic ridge.kmz
A Transform
Boundary
What Drives Plate Motion?
What Drives Plate Motion?
• An oversimplified view: Heat transfer by
convection of mantle ROCK creates a
frictional drag on the plates.
5
How Fast Do Plates Move?
Summary
The Earth has gaseous, liquid, and solid layers.
The solid Earth can be divided into 3 layers based on
chemical composition.
The lithosphere and asthenosphere are layers based on
their physical properties – one flows, one doesn’t.
Lithospheric plates move around on the underlying
asthenosphere and interact in 3 basic ways: divergence,
convergence, and by translational motion (transform
boundary where plates slide by each other).
1 to 15 cm/yr!!
Most “geology” happens at or near plate boundaries
where they interact.
6