Download The plate tectonic revolution part II.

The Plate Tectonics
Revolution
A paradigm shift
in the Earth
Sciences
Topics:
 Review of last class
 Why isn’t the Earth expanding?
 Plate boundaries, earthquakes and volcanoes
 What causes plate tectonics?
 How fast do plates move?
 Earth evolution: the supercontinent cycle
 The Snowball Earth hypothesis
Alfred Wegener
Key evidence for continental
drift:
 coastline fit
 fossil record
 similarity of rock
assemblages on different
continents
Development of Plate Tectonic Theory
Harry Hess
Key evidence for “seafloor
spreading”
 Young age of oceans relative
to continents
 Mid-ocean ridges
Development of Plate Tectonic Theory
The Vine-Matthews-Morley Hypothesis
Oceanic Magnetic Stripes =
Seafloor Spreading +
Magnetic Field Reversals
Key evidence:
 Discovery of magnetic
field reversals
 Magnetic mapping of
oceans
Development of Plate Tectonic Theory
J. Tuzo Wilson
1. Hotspot hypothesis
• Linear volcanic island/seamount
chains
2. Transform faults
• Earthquake activity on oceanic
fracture systems
3. Wilson Cycle
• Geology of mountain belts
Development of Plate Tectonic Theory
Why isn’t the Earth expanding?
Theory of Plate Tectonics
1. Earth’s surface is divided
into rigid lithospheric plates
2. New oceanic lithosphere is
created at mid-ocean ridges
3. Oceanic lithosphere is
destroyed at deep-sea
trenches (subduction zones)
4. This process largely explains
the global distribution of
earthquakes and volcanoes
Development of Plate Tectonic Theory
Tectonic plates
Types of plate boundaries
Types of plate boundaries
Characteristic Faulting
Example: Iceland
Type of Volcanic Activity
Fissure eruptions at
mid-oceanic ridges
Types of plate boundaries
Continent-Continent
Ocean-Ocean
Ocean-Continent
(Phillipines)
(Chile)
(Tibet)
Characteristic Faulting
Type of Volcanic Activity
Stratovolcanoes
Types of plate boundaries
Example: California
Characteristic Faulting
San Andreas Fault
What causes plate tectonics?
 Heat from deep
within the Earth
causes mantle
convection
 Decompression
melting produces midocean spreading
 Plate motion is
assisted by slab pull
How fast do plates move?
Fastest:
East Pacific Rise (15 cm/year)
Slowest:
Arctic Ridge (2.5 cm/year)
How do plates change with time?
California has
changed from
a convergent
boundary to a
transform
boundary in
the last 30
million years
Mount Everest
India has been colliding with
Asia for the past 50 million
years
The plate-tectonic rewind button:
Age of oceanic crust
Earth evolution:
The supercontinent cycle
 About 225 million
years ago, a single
supercontinent
existed: Pangea (all
lands)
 The ancient world
ocean is called
Panthalessa (all seas)
Earth evolution:
The supercontinent cycle
 About 200 million
years ago, Pangea split
into Laurasia and
Gondwanaland
 The ocean created
by the split is called the
Tethys Sea. This is now
the Mediterranean Sea
Earth evolution:
The supercontinent cycle
 About 180 million
years ago, the Atlantic
ocean began to form
 Continued spreading
has produced the
current configuration of
continents
The supercontinent cycle
 The formation and
breakup of supercontinents seems to
happen on a ~ 500
million year cycle
 The previous
supercontinent (~ 700
million years ago) is
called Rhodinia
 Another
supercontinent will most
likely form in Earth’s
distant future
The Snowball Earth hypothesis
 Some scientists believe
that 600-700 million years
ago, Earth experienced a
series of global ice ages
 This occurred at a time
when Rhodinia was at a high
latitude
1. Ice caps
extend toward
equator
2. Snowball
Earth: H20 and
carbon cycles
shut off
3. Supergreenhouse effect
from volcanoes
4. Greenhouse
Earth
The Snowball Earth hypothesis
 These climate shocks
appear to have triggered
the evolution of
multicellular animal life
 Snowball Earth is
challenging long-held
assumptions regarding the
limits of global change.