Transcript Plate tectonics

The Forces That Shape the
Earth & Plate Tectonics
Part 1 Notes
The Forces That Shape the Earth
• Geologist – scientist who studies
forces that make and shape the Earth.
• Constructive forces – forces that
shape the surface by building up
mountains and landmasses.
• Destructive forces – forces that slowly
wear away mountains and other
features on the surface.
Structure of the Earth
Mantle
• The Earth is
made up of 3
main layers:
Outer core
Inner core
– Core
– Mantle
– Crust
Crust
The Crust
• This is where we live!
• The Earth’s crust is made
of:
Continental Crust
Oceanic Crust
- thick (10-70km)
- buoyant (less dense
than oceanic crust)
- mostly old
- thin (~7 km)
- dense (sinks under
continental crust)
- Young & old
Mantle Layers
• Lithosphere: The rigid, stiffer part of the
outer mantle and the crust. The
lithosphere 'floats' on the
asthenosphere, like ice on water.
• Asthenosphere: The thick semi-liquid
part of the outer mantle.
• http://pubs.usgs.gov/gip/dynamic/inside.
html
CORE
– a) Outer core – layer of molten/liquid
metal that surrounds the inner core.
Convection currents here creates Earth’s
magnetic field-giant bar magnet
– b) Inner core – dense solid ball of metal
• Composition: nickel, iron. The core is thought to be
composed of nickel and iron alloy. The outer core is
liquid while the inner core is solid.
• Tremendous pressure, produced by the weight of the
overlying rocks is believed to crowd the atoms tightly
together and prevent the liquid state.
How do we know what the
Earth is made of?
• Geophysical surveys: seismic, gravity, magnetics,
electrical, geodesy
– Acquisition: land, air, sea and satellite
– Geological surveys: fieldwork, boreholes, mines
Rock Cycle
• Igneous Rocks-melting is the process that creates the
magma and lava that can form this kind of rock
• Metamorphic Rocks-formed from any rock undergoing
intense heat & pressure
• Sedimentary Rocks-form from the processes of
weathering, erosion & deposition
Processes:
• Weathering-breaks down the rocks
• Erosion is the movement of the
weathered pieces of rock-sediments
• Deposition is the settling out and layering
of sediments
Sedimentary rocks
• Grand Canyon
• Youngest rocks at the
top
• Rocks and fossils
older as you go down
• Why is that?
Unstable Earth
• The Earths crust is
subjected to huge
forces
• Very large forces can
fracture the rock
• This can be seen as
fault lines in the rock
layers
Tilted and Folded
• Sedimentary rocks
are often found tilted
• They can also be
folded by the huge
forces
• Sometimes the rock
layers can even be
turned upside down
• Part 2 Notes
Key Questions:
What is Plate Tectonics?
What are the different theories
and evidence that support plate
tectonics?
• If you look at a map of the world, you may notice that some of the
continents could fit together like pieces of a puzzle.
Alfred Wegener
• Some people thought the
Earth was shrinking and
cracking
• Some people did not
understand and thought the
continents floated around in
the oceans (continental drift)
• Continental drift –
Wegener’s hypothesis that the
continents slowly move across
the Earth’s surface.
• Wegener could not explain
the force that pulls or
pushes the continents so
his hypothesis was rejected.
Evidence for Continental Drift
Landforms – continents can be pieced together like a jigsaw
puzzle and features like mountain ranges match
Fossils – similar reptile and plant fossils found in places now
separated by oceans
Climate – tropical plant fossils in now polar climates,
continental glacier evidence found in South Africa
Sea Floor Spreading
• Sea-floor spreading –
the process by which
molten material adds new
oceanic crust to the
ocean floor.
• In 1960, Harry Hess
proposed that the ocean
floors move like conveyor
belts carrying the
continents with them.
• mid-ocean ridge – the
undersea mountain chain
where new ocean floor is
produced.
Evidence for Sea-floor Spreading
• New molten material in erupting along the midocean ridge.
• When Earth’s magnetic poles reversed the
occurrence was recorded in the newly formed
rock at the mid-oceanic ridge.
• Using drilling samples scientists have
determined that rocks are youngest in the center
of the mid-ocean ridge and get older the farther
you move from the ridge.
Sea Floor Spreading & Pole
Reversal Evidence Website
http://oceanexplorer.noaa.gov/
edu/learning/player/lesson02/l
2la2.htm
World Plates-Part 3
Plate Tectonics-Part 3
Theory of Plate Tectonics
•In 1965, J. Tuzo Wilson proposed that the
lithosphere is broken into separate sections called
plates.
•Plate – A section of the lithosphere that slowly
moves over the asthenosphere carrying pieces of
continental and oceanic crust.
•Plate tectonics - The theory that pieces of
Earth’s lithosphere are in constant motion, driven
by convection currents in the mantle. This
explains the formation, movement and subduction
of Earth’s plates.
Convection Currents
and the Mantle
Remember, heat is transferred through radiation, conduction,
and convection.
Radiation – the transfer of heat by electromagnetic waves.
Sunlight warms surface of Earth.
Convection Currents
and the Mantle
Conduction – heat transfer by direct contact of
particles of matter.
-Burning your hand when touching a stove
Convection Currents and the Mantle
Convection – heat transfer by the movement of heated fluid (liquid
or gas). This is what makes tectonic plates move.
-Convection oven or heating soup
-Wind in the atmosphere
-Heat from the Earth’s core and the mantle itself causes
convection currents in the mantle
Plate Tectonics
• The Earth’s crust is divided into 12 major
plates which are moved in various directions.
There are many minor plates as well.
• The plates are made of the thin, rigid
lithosphere with the continental and oceanic
crust attached to it.
• This plate motion causes them to collide, pull
apart, or scrape against each other.
Plate Tectonics
• Each type of interaction causes a
characteristic set of Earth structures or
“tectonic” features.
• The word, tectonic, refers to the
deformation of the crust as a
consequence of plate interaction.
• “Plates” of lithosphere are moved
around by the underlying hot mantle
convection cells
Rates of Movement
Plates move at amazingly slow rates, from one to
ten centimeters a year. Fingernail growth speed!
Plate Movement
• “Plates” of lithosphere are moved around by
the underlying hot mantle convection cells
Part 4: Plate BoundariesKey Questions:
What are the three types of plate boundaries?
What kind of movement occurs at the different
tectonic plate boundaries?
What crustal features result from the movement
at these plate boundaries?
Three types of plate boundary
• Divergent
• Convergent
• Transform
Divergent Boundaries
mid-ocean ridge & rift valleys
• Spreading ridges~mid-ocean ridge on ocean
floor
• As plates move apart new material is erupted to fill
the gap; volcanic activity
Iceland: An example of continental rifting
• Iceland has a divergent
plate boundary running
through its middle
Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
Convergent Boundaries
• There are three types of convergent
plate boundaries
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
Continent-Continent Collision
• Forms folded mountains, e.g. European Alps,
Himalayas
Himalayas
Continent-Oceanic Crust Collision
Subduction
Occurs
• Oceanic lithosphere
subducts underneath the
continental lithosphere
• Oceanic lithosphere heats
and melts forming magma
• The magma rises forming
volcanic mountains.
• Ex. The Andes (S. America)
Ocean-Ocean Plate Collision
• When two oceanic plates collide, the older, denser
oceanic plate sinks under the newer, less dense plate
into the mantle forming a __________________
zone.
• The subducting plate is bent downward to form a very
deep depression in the ocean floor called a
________________.
• The worlds deepest parts of the ocean are found
along trenches.
– E.g. The Marianas Trench is 11 km deep!
Transform Boundaries
• Where plates slide past each other
• Events & features that result at this
boundary type: Faults & Earthquakes
Above: View of the San Andreas
transform fault
Volcanoes and Plate
Tectonics…
…what’s the connection?
Pacific Ring of Fire
Volcanism &
earthquakes
are mostly
focused at
plate
margins
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.
Earthquakes and Plate
Tectonics…
…what’s the connection?
• As with volcanoes, earthquakes are not
randomly distributed over the globe
Figure showing
the distribution of
earthquakes
around the globe
At the boundaries between plates, friction
causes them to stick together. When built up
energy causes them to break, earthquakes
occur. Earthquakes occur in subduction zones
from the displacement of magma.
Plate Tectonics Summary
• The Earth is made up of 3 main layers (core,
mantle, crust)
• On the surface of the Earth are tectonic
plates that slowly move around the globe
• Plates are made of crust and upper mantle
(lithosphere)
• There are 2 types of plate
• There are 3 types of plate boundaries
• Volcanoes and Earthquakes are closely
linked to the margins of the tectonic plates