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Plate Tectonics
Plate Tectonics
Why Continents and Ocean Basins Exist
Topics
• Density Structure of Earth
• Isostasy
• Sea-Floor Spreading
• Mechanical Structure of Earth
• Driving Mechanism of Plate
Tectonics
• Lithospheric Plate Interactions
• Plate Tectonics and the Depth
of the sea floor
Topics
• Density Structure of Earth
• Isostasy
• Sea-Floor Spreading
• Mechanical Structure of Earth
• Driving Mechanism of Plate
Tectonics
• Lithospheric Plate Interactions
• Plate Tectonics and the Depth
of the sea floor
Isostasy
Isostasy
Rocks Can Float On Other Rocks
Distribution of Crustal Elevations
km above
sea level
km below
sea level
8
7
6
5
4
3
2
1
-1
-2
-3
-4
-5
-6
-7
-8
1
1
2
5
21
9
3
5
14
23
16
1
0
5
10
15
20
Percentage of the Earth’s surface
25
30
Distribution of Crustal Elevations
km above
sea level
sea level
km below
sea level
8
7
6
5
4
3
2
1
-1
-2
-3
-4
-5
-6
-7
-8
1
1
2
5
21
9
3
5
14
23
16
1
0
5
10
15
20
Percentage of the Earth’s surface
25
30
Distribution of Crustal Elevations
km above
sea level
sea level
km below
sea level
8
7
6
5
4
3
2
1
-1
-2
-3
-4
-5
-6
-7
-8
1
1
2
5
Avg 840 m
above sea level
21
9
3
5
14
23
16
1
0
5
10
15
20
Percentage of the Earth’s surface
25
30
Distribution of Crustal Elevations
km above
sea level
sea level
km below
sea level
8
7
6
5
4
3
2
1
-1
-2
-3
-4
-5
-6
-7
-8
1
1
2
5
Avg 840 m
above sea level
21
9
3
5
Avg 3800 m
below sea level
14
23
16
1
0
5
10
15
20
Percentage of the Earth’s surface
25
30
Another way to present the
same data.
The average elevation of
continental crust is 840 m
above sea level.
The average depth of oceanic
crust is 3800 m below sea
level.
Crustal Elevations
Two Types of Crust
The two average crustal elevations means that Earth
has two types of crust:
Two Types of Crust
The two average crustal elevations means that Earth
has two types of crust:
1. Continental crust
Two Types of Crust
The two average crustal elevations means that Earth
has two types of crust:
1. Continental crust
2. Oceanic crust
Two Types of Crust
The two average crustal elevations means that Earth
has two types of crust:
1. Continental crust ---------- The high crust
2. Oceanic crust
Two Types of Crust
The two average crustal elevations means that Earth
has two types of crust:
1. Continental crust ---------- The high crust
2. Oceanic crust
---------- The low crust
Continental crust
Avg 840 m
above SL
Avg 3800 m
below SL
Avg 840 m
above SL
Continental crust
Oceanic crust
Why two elevations?
Why two elevations?
• Each type of crust forms by a different
process
Why two elevations?
• Each type of crust forms by a different
process
• But why two different elevations?
Why two elevations?
• Each type of crust forms by a different
process
• But why two different elevations?
• Geologists explain the different elevations
using the concept of isostasy
Definition
Definition
Isostasy - a condition of equilibrium, similar to
buoyancy, in which the crust floats on the mantle
Definition
Isostasy - a condition of equilibrium, similar to
buoyancy, in which the crust floats on the mantle
The theory of isostasy was developed to explain
the deep roots of mountains
Mountains have deep roots
Roots
Isostasy - the crust floats on the mantle
Isostasy - the crust floats on the mantle
• Isostasy explains why mountains have roots
Isostasy - the crust floats on the mantle
• Isostasy explains why mountains have roots
• Isostasy implies two concepts:
Isostasy - the crust floats on the mantle
• Isostasy explains why mountains have roots
• Isostasy implies two concepts:
1. The crust is buoyant
Isostasy - the crust floats on the mantle
• Isostasy explains why mountains have roots
• Isostasy implies two concepts:
1. The crust is buoyant
2. The mantle behaves like a fluid
Crust is buoyant
Ship moves up and down in
response to changes in mass
Add containers and the ship sinks
Add containers and the ship sinks
Remove containers and the ship rises
Remove containers and the ship rises
Crust is Buoyant
Crust is Buoyant
• As the mountain grows, the crust sinks into the mantle
Crust is Buoyant
• As the mountain grows, the crust sinks into the mantle
• Forming the roots of the mountain
The Mantle Behaves like a Fluid
The Mantle Behaves like a Fluid
• As the crust sinks into the mantle
The Mantle Behaves like a Fluid
• As the crust sinks into the mantle
• The mantle flows out of the way
The Mantle Behaves like a Fluid
• As the crust sinks into the mantle
• The mantle flows out of the way
Crust is Buoyant
Crust is Buoyant
• As the mountain erodes, the crust rises
Mantle Behaves Like a Fluid
Mantle Behaves Like a Fluid
• As the crust rises, the mantle flows back
Question
Question
Why are the plains surrounding the mountains subsiding?
Question
Question
Why does the mantle flow when stressed
but the crust does not?
Question
Why does the mantle flow when stressed
but the crust does not?
The mantle flows because it is very hot
Temperature increases
with increasing depth
The mantle flows because it is very hot
The mantle flows because it is very hot
The mantle flows because it is very hot
The mantle flows when
stressed because it is
very hot compared to
the crust
The mantle is hotter and
weaker than the crust
Earth’s Interior Heat Source
Earth’s Interior Heat Source
• The primary heat source for Earth’s interior
is radioactivity
238U 235U 87Rb
232Th
40K
Why does continental crust float
higher than oceanic crust?
Continental crust floats higher than oceanic crust
for two reasons:
Why does continental crust float
higher than oceanic crust?
Continental crust floats higher than oceanic crust
for two reasons:
1. Continental crust is thicker
Why does continental crust float
higher than oceanic crust?
Continental crust floats higher than oceanic crust
for two reasons:
1. Continental crust is thicker
2. Continental crust is less dense
For example, both icebergs
have the same density
But one iceberg floats higher
because it is thicker
Thicker
But one iceberg floats higher
because it is thicker
Thicker
Thinner
But one iceberg floats higher
because it is thicker
One reason that continental crust floats higher
than oceanic crust is because it is thicker
Thickness of the crust in kilometers
Another reason that continental crust floats higher
than oceanic crust is because it is less dense
Question
Question
Why is continental crust less dense
than oceanic crust?
Question
Why is continental crust less dense
than oceanic crust?
Crustal Composition
Continental crust is less dense than oceanic
crust because it has a different composition:
Crustal Composition
Continental crust is less dense than oceanic
crust because it has a different composition:
• Continental crust predominantly is Al silicate
Crustal Composition
Continental crust is less dense than oceanic
crust because it has a different composition:
• Continental crust predominantly is Al silicate
• Oceanic crust predominantly is Fe, Mg silicate
A
A
Fe, Mg silicate
A
Fe, Mg silicate
Al silicate
Sea-Floor Spreading
Next
Sea-Floor Spreading