Transcript Layers of the and Plate Tectonics Power Point

Plate Tectonics
CHAPTER 17
Layers of the Earth
A. Crust- solid, 5-70 km thick
Moho
Two Types of Crust:
Oceanic- ocean floor, more dense then
because of more iron
Continental-dry land (mostly silicates
which are lighter)
B. Moho- boundary between
mantle and crust
Lithosphere- upper part of the mantle (solid),
and the crust
Asthenosphere- softer layer under
lithosphere
C. Mantle-(lower) melted, “plastic”
magma
D. Outer Core- liquid metal
E Inner Core- solid metal (iron and
nickel)
Moho is short for the Mohorovicic Discontinuity and is the boundary between the crust and the mantle.
Continental Drift:
ALFRED WEGNER-
(A GERMAN METEOROLOGIST)
•
IN 1912 HE PROPOSED THE CONTINENTS
WERE ONCE CONNECTED.
•
HE NAMED IT PANGAEA (GREEK FOR ALL
EARTH)
•
HOWEVER, STATED ONLY THE CONTINENTS
MOVED.
•
HE BASED THIS OFF THE FIT OF THE
CONTINENTS AS WELL AS ROCK, FOSSIL,
CLIMATIC AND FOSSIL EVIDENCE.
•
THIS BECAME THE THEORY OF
CONTINENTAL DRIFT.
• Pangaea collided
225myaand broke
up 65 mya.
• The northern half of
Pangaea was
referred to as
Laurasia and the
southern portion is
known as
Gondwanaland.
Wegner’s Evidence in the
Rocks

Wegner believed that the
rocks on either side of the
Atlantic should be the
same.

After examining mountain
ranges, he found that the
Appalachian Mountains
were similar as mountains
in Greenland and Europe.

These ranges were also all
formed 200mya, during
the time of his proposed
Pangaea.
Wegner’s Climatic
Evidence:

Wegner used his background in
Meteorology to support his theory. He
stated the fossils of the plant
Glossopteris (a deciduous forest
shrub) could not have thrived on
continents such as Antarctica, South
America and India if those currents
were at their current locations.

Therefore, since fossils of the plant
was found on all of these continent,
the continents MUST have been closer
to the equator for this plant to thrive.
Fossils of
Glossopteris
Wegner’s Fossil Evidence:
 Wegner studied fossils
from all over the world
and simply stated that
land-dwelling animals
such as the
Cynognathus and
Lystrasaurus could NOT
have swum across vast
oceans.
 He also noted that the
Mesosaurus was a
freshwater reptile and
would not have
favored salt water to
swim across an ocean.
Wegner’s Glacier
Deposits as evidence:

Glacial deposits were discovered by
Wegner in parts of Africa, Australia
and South America.

The deposits were located in regions
that are far too warm to have
glaciers today.

The glacial deposits were dated to
be present around 290 million years
ago, which, puts them during the
time of Pangaea.

These glacial deposits were as thick
as the deposits that are currently over
Antarctica and therefore took
hundreds of thousands to millions of
years to be produced.
Wegner’s Coal Deposits as
Evidence:

Tropical Coal Deposits were
discovered in regions that
are now too cold to carry
this type of plants. Places
like; northern North America
and northern and central
Europe. Therefore, these
places had to be along the
equator to form these
deposits.

Coal deposits, made
accumulation of tropical
swamp plants, were also
found on Antarctica.
From Continental Drift TO Plate Tectonics!
In the early 1900’s Wegner’s theory was rejected
by the Scientific community.
Continental Drift had two major flaws:
A. Wegener could provide a satisfactory force
that would be powerful enough to move the
earth’s crust such great distances.
B. Wegner could not state HOW the continents
were moving.
TWO Devices changed our perception of the
ocean floor:
A. Magnetometer- a device that can be
attached to ships that can detect and record
the magnetic field given off by rocks on the
ocean floor.
B. Sonar- showed us a picture of the topography
and elevation of the ocean floor (which was
always believed to by flat) so we could see
that there are mountains, trenches and even
volcanoes on the ocean floor.
Ocean Floor Mapping Changes the world!
Ocean
Discoveries:
SONAR AND MAGNETOMETER REVEALED:

A MID OCEAN RIDGE IS THE LARGEST MOUNTAIN
RANGE ON THE PLANET. FORMED ALONG A
DIVERGENT BOUNDARY AND IS ALMOST 50,000
MILES LONG AND PLACES ARE UP TO 1.5 MILES
ABOVE SEA LEVEL!

SCIENTISTS ALSO FOUND TRENCHES THAT WERE
MILES DEEP. THE MARIANA'S TRENCH, CREATED
FROM A SUBDUCTING PLATE, IS 6 TIMES DEEPER
THAN THE GRAND CANYON.

SCIENTISTS ALSO COLLECTED ROCK AND
SEDIMENT FROM THE OCEAN FLOOR WHICH
REVEALED TWO THINGS:
1.
ROCKS WERE YOUNGER ALONG THE RIDGES
(CENTER) AND OLDER ALONG TRENCHES
(WHERE OCEANIC CRUST HITS CONTINENTAL
CRUST).
2.
THE OCEAN CRUST WAS MUCH THINNER THAN
CONTINENTAL CRUST (OCEANIC= 100-200M,
WHILE CONTINENTAL IS 20,000M THICK!)
Paleomagnetism

THE EARTH’S MAGNETIC FIELD IS CREATED
BY THE FLOW OF MOLTEN IRON IN THE
EARTH’S OUTER, LIQUID CORE.

MAGNETIC REVERSAL OCCURS WHEN
THE FLOW OF THE OUTER CORE
CHANGES, GENERALLY EVERY 10-15,000
YEARS.(IS CURRENTLY CHANGING)

SINCE OCEAN CRUST IS MOSTLY BASALT
(WHICH HAS A HIGH IRON CONTENT) THE
MAGNETIC REVERSAL IS SHOWN EASILY.


NORMAL AND REVERSE POLARITY FORM
AS STRIPES ALONG RIDGES AND
SCIENTISTS FOUND THAT AGES OF ROCKS
ARE DIRECTLY RELATED TO THE
PALEOMAGNETISM.
THEREFORE, NEWER ROCKS FORM ALONG
RIDGES AND OLDER ROCKS ARE FOUND
ALONG THE CONTINENTS, SUPPORTING
PLATE MOVEMENT.
Younger newly
formed rock
Older, subducting rock
Seafloor Spreading
A.
SEAFLOOR SPREADING IS THE THEORY
THAT EXPLAINS HOW NEW OCEAN
CRUST IS FORMED AT OCEAN RIDGES
AND DESTROYED AT DEEP-SEA
TRENCHES.
•
DURING SEAFLOOR SPREADING, HOTTER
LESS DENSE MAGMA, IS FORCED
UPWARD TOWARDS THE CENTER OF THE
OCEAN RIDGE. (CONVECTION
CURRENTS)
•
THEN, THIS MAGMA COOLS WITHIN THE
GAP OF THE RIDGE TO CREATE NEW
OCEAN FLOOR.
•
OLDER OCEAN FLOOR IS THEN PUSHED
OUTWARD, SUBDUCTS UNDER THE LIGHTER
CONTINENTAL CRUST AND MELTS INTO THE
MANTLE.
Types of Plate Boundaries:
Tectonic Plates are huge pieces of crust and
rigid upper mantle that fit together at their
edges to cover Earth’s surface.
A.
B.
DIVERGENT-WHEN TWO PLATE BOUNDARIES
ARE SEPARATING AND MOVING AWAY
FROM EACH OTHER.
•
Mid ocean Ridges
•
Rift Valleys
CONVERGENT- WHEN TWO PLATE
BOUNDARIES ARE COLLIDING OR COMING
TOGETHER.
•
C.
Divergent
Convergent
Creates mountain ranges, land volcanoes
or island arc (oceanic volcanoes)
TRANSFORM- WHEN TWO PLATE
BOUNDARIES SLIDE PAST EACH IN TWO
DIFFERENT DIRECTIONS.
•
Can cause large earthquakes as the plates
slowly slide past each other
Transform
Details of types of plate boundaries:
Tectonic Plates of the World
Mt. Saint HelensO-C Convergent
San Andreas
Fault (transform)
The Ring of FireCreated by the
subducting Pacific
Plate.
Mid-Ocean
Ridge and
Iceland being
separated by a
divergent
boundary
Himalayan Mts.
Created from C-C
Convergence
Divergent Boundaries
DIVERGENT BOUNDARIES:
A.
MOST DIVERGENT BOUNDARIES ARE
FOUND ALONG THE SEAFLOOR IN RIFT
VALLEYS.
B.
THE MID OCEAN RIDGE APPEARS TO BE A
CONTINUOUS MOUNTAIN CHAIN ON THE
OCEAN FLOOR.
A.
Example is the Mid Atlantic Oceanic
Ridge
RIFT VALLEY- IS WHEN CONTINENTAL CRUST
SEPARATES, THEN THE STRETCHED CRUST FORMS
A LONG NARROW DEPRESSION.
•
Example is the African Rift Valley
African Rift
Valley
Convergent Boundaries
Convergence- when two plates collide, the
denser plate will be pulled by gravity and
subduct.
Since oceanic crust is basaltic (mostly iron) and
continental crust is mostly silica and less dense,
oceanic crust subducts under the lighter
continental crust.
A.
B.
C.
OCEANIC-OCEANIC•
Subduction creates a trench
•
Creates island arc (volcanic island)
OCEANIC-CONTINENTAL•
When two oceanic plates converge, one is denser
•
The denser subduct under the other oceanic plate
creating volcanic islands
CONTINENTAL-CONTINENTAL•
Continental crust cannot subduct due to its low
density
•
Therefore, when two continental pieces of crust
converge, they crumble and uplift creating
mountains.
Examples of
Convergent
Boundaries
A. Continental-Oceanic:
Examples: Chilean Andes
Mountains and Mariana
Trench, Mount Saint
Helens
B. Oceanic-Oceanic:
Examples: Fiji, Japan
(NOT Hawaii)
C. Continental-Continental:
Examples: Himalayas,
Appalachian
Transform Plate
Boundary
TRANSFORM BOUNDARY WERE NAMED FOR
THE WAY EARTH’S CRUST TRANSFORMS .
•
CRUST IS ONLY DEFORMED OR
FRACTURED SOMEWHAT ALONG
TRANSFORM BOUNDARIES.
•
HOWEVER, TRANSFORM BOUNDARIES
CAN CAUSE LARGE EARTHQUAKES.
•
Example: San Andreas Fault
Causes for plate motion:

Convection currents- are created in the mantle from heat rising heated magma along the core
and cooled magma that surfaces closer to the surface.
Motion of the Earth’s Plates
“
Convection
currents within
the earth's
mantle PUSH our
tectonic plates.
”