Download Oceanic lithosphere

The Structure of the Earth
and Plate Tectonics
The History of Plate Tectonics
• Alfred Wegener is considered
by most scientists to be the
pioneer of the continental drift
theory.
• Wegener proposed that the
continents were once
connected and shared common
species (Pangaea)
• Later these continents drifted
apart carrying their inhabitants
with them.
The History of Plate Tectonics
• Wegener
proposed that
all of today’s
land masses
were once
connected
Pangaea (200 mya)
The History of Plate Tectonics
The History of Plate Tectonics
Evidence
• Using the fossil
record, you can
piece the
continents
together like a
puzzle
Distribution of fossils
The History of Plate Tectonics
Evidence
• Wegener showed
that the same three
layers occur at
several locations on
the earth
• The middle layer is
composed of
sandstone, shale, and
coal beds
Harry Hess and J Wilson propose a mechanism to
explain continental movement in the early 60’s.
• In 1969 the Glomar Challenger drilled a series of
holes in the Mid-Atlantic ridge. What they found
changed geology and oceanography.
• Since the ocean floor is lower in the lithosphere,
scientists expected to find older rocks at those depths.
-Continental geology’s law of Superposition states that oldest
rocks are laid down first and should be found horizontally
lowest in a bed unless uplifted.
• They found no rock older than
3 billion years and most were
younger. How could the ocean
floor be younger than the
continents riding on it?
The Deep Sea Drilling project
showed that rocks became
older as they moved away
from the mid ocean ridge.
Oceanic geology showed rock
layers are created vertically.
Plate Tectonics
• The Earth’s crust is divided into 7-8 major plates
which are moved in various directions.
– The word, tectonic, refers to the deformation of the crust
as a consequence of plate interaction.
• 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 “tectonic” features.
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.
What lies beneath the tectonic plates?
• Below the
lithosphere
(which makes
up the tectonic
plates) is the
asthenosphere.
Plate Movement
• “Plates” of lithosphere are moved around by
the underlying hot mantle convection cells
What happens at tectonic
plate boundaries?
Three types of plate boundary
• Divergent
• Convergent
• Transform
Divergent Boundaries
• Spreading ridges
– As plates move apart new material is erupted to
fill the gap.
1. The picture to the left
is of Iceland where
the Mid-oceanic ridge
comes to the surface
for us to see.
2. The geographic feature
created from divergent
plates is a ridge
system
Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
Convergent Boundaries
• There are three styles of convergent plate
boundaries
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
Continent-Continent Collision
• Forms mountains, e.g. European Alps, Himalayas
Continent-Oceanic Crust Collision
• Called SUBDUCTION
Subduction
• Oceanic lithosphere
subducts underneath the
continental lithosphere
• Oceanic lithosphere heats
and dehydrates as it
subsides
• The melt rises, forming
volcanoes
• E.g. The Andes
Ocean-Ocean Plate Collision
• When two oceanic plates collide, one runs over the
other which causes it to sink into the mantle forming
a subduction zone.
• The subducting plate is bent downward to form a very
deep depression in the ocean floor called a trench.
• The worlds deepest parts of the ocean are found
along trenches.
– E.g. The Mariana Trench is 11 km deep!
Transform Boundaries
• Where plates slide past each other
Above: View of the San Andreas
transform fault
1. San Andres Fault in
California where the
boundary of two plates
come to the surface!
Volcanoes and Plate Tectonics…
…what’s the connection?
Volcanoes are formed by:
- Subduction - Rifting - Hotspots
Pacific Ring of Fire
Hotspot
volcanoes
What are Hotspot Volcanoes?
• Hot mantle plumes breaching the
surface in the middle of a tectonic plate
The Hawaiian island chain are
examples of hotspot volcanoes.
Photo: Tom Pfeiffer / www.volcanodiscovery.com
The tectonic plate moves over a fixed hotspot
forming a chain of volcanoes.
The volcanoes get younger from one end to the other.
The Mechanisms Supporting Continental Drift
Volcanic
Islands
Mid-Ocean
Ridges
Volcanic
Mountain
Ranges