PLATE TECTONICS
Plate tectonics states that the Earth's
crust and upper mantle are broken into
sections called plates.
1)These plates move around the mantle.
2)Plates are composed of the crust and a
part of the upper mantle, these two parts
together are called the lithosphere.
3)The layer below the lithosphere is the
asthenosphere.
4)The lithosphere floats or moves around
on the asthenosphere.
The parts of the earth that are directly involved in plate tectonics
are:
a) The lithosphere (solid plates)
b) The asthenosphere (plastic, moving )
The lithosphere floats on top of
the asthenosphere and is pushed
along like a boat on a river.
MAJOR IDEAS IN PLATE TECTONICS
Idea #1: Continental Drift
1) Alfred Wegener first proposes Continental Drift in his book published in 1915.
2) Suggests that 200 million years ago there existed one large supercontinent which he
called Pangaea (All Land)
3) This was not really a new idea, but Wegener offered several lines of evidence in support
of his proposal:
Evidence A: Fit of the
Continents
-Wegener noted the similarity
in the coastlines of North and
South America and Europe and
Africa.
-Today the fit is done at the
continental shelf and it is
nearly a perfect match.
Evidence B: Fossil Similarities – eg Mesosaurus, Lystrosaurus, Glossopteris
-Reptile similar to modern alligator which lived in shallow waters of South America
and Africa.
Evidence C: Rock Similarities
i)-Rocks of same age juxtaposed
across ocean basins.
ii): Termination of mountain chains.
Continental Drift Idea was rejected by
North American geologists because
Wegener couldn't come up with a
mechanism for continental drift. Suggested
tidal forces, but physicists showed this to
be impossible. Wegener dies in 1930 and
his idea dies with him.
Idea #2: Magnetism and Paleomagnetism
1) Earth behaves like a bar magnet with a magnetic north and south.
a) At poles a compass needle dips vertically
(Downward at the north pole, upward at the south pole and horizontal at the equator. )
b) Magnetic poles do not correspond with geographic poles.
c) Magnetic pole moves as much as 25 km per year.
2) Cause of Earth's Magnetism
•First thought to be the result of a permanently
magnetized core. However, it has been shown that
when any substance is heated above 500 degrees C it
loses its permanent magnetism.
•Earth is a Dynamo - Outer core is a fluid consisting
largely of iron, so it is an excellent conductor.
Electromagnetic currents are generated and
amplified by motion within the liquid caused by
convection. Rotation of the Earth unifies the random
convective movements generating the magnetic field.
Side Note:
Variation is termed the
magnetic declination. It is 16
degrees east in California.
However, it has been found that
even though the magnetic and
geographic poles do not
correspond today when the
location of the magnetic north
pole is averaged over a 5,000
year period it does correspond
with geographic north.
3) In the 1950's scientists discover how to measure paleomagnetism
(=magnetism frozen in the rock at the time it formed).
a) With this knowledge scientists could tell the direction and latitude of geomagnetic
pole at the time the rock formed.
b) Found that by 500 MY ago magnetic north was located near Hawaii.
Side Note: North
American geologists
attempted similar studies
largely to disprove the
Europeans. They found
that 500 my ago North
American rocks showed
the magnetic north pole
to be in the East Pacific,
3000 miles to the west of
the European magnetic
north at that time.
Fred Vines supports Hess with his explanation of symmetrical magnetic stripes on
either side of the Atlantic Mid-ocean ridge
Geomagnetic reversal
From Wikipedia, the free encyclopedia
A geomagnetic reversal is a change in Earth's magnetic field such that
the positions of magnetic north and magnetic south are interchanged.
The Earth's field has alternated between periods of normal polarity, in
which the direction of the field was the same as the present direction,
and reverse polarity, in which the field was the opposite. These periods
are called chrons. The time spans of chrons are randomly distributed with
most being between 0.1 and 1 million years[citation needed] with an average of
450,000 years. Most reversals are estimated to take between 1,000 and
10,000 years. The latest one, the Brunhes–Matuyama reversal, occurred
780,000 years ago. A brief complete reversal, known as the Laschamp
event, occurred only 41,000 years ago during the last glacial period. That
reversal lasted only about 440 years with the actual change of polarity
lasting around 250 years. During this change the strength of the magnetic
field dropped to 5% of its present strength.[1] Brief disruptions that do not
result in reversal are called geomagnetic excursions.
SEA FLOOR SPREADING
1) Surveys of the ocean basins
revealed a system of ridges
and trenches with high heat
flow over the ridges.
2) H. Hess (1962) rushes to
print with the idea of Sea
Floor Spreading.
a) Postulates convection
cells beneath ocean
basins to drive the
spreading.
Types of Plate Boundaries
1) Divergent = plates
spreading apart at the
boundaries
-Characterized by ocean
ridges and sea floor
spreading
2. Convergent = plates coming together
-Characterized by trenches and island arcs
-ocean plates are denser than continental plates
SO…
a)ocean plates go below continental plates when they meet
(called subduction)
b)When two continental or two ocean plates meet, they thrust
up into mountain ranges
Examples:
Ocean - Ocean (Japanese Islands)
Ocean - Continental (Cascade Mountains)
Continent - Continent (Himalayas)
3. Transform= plates moving laterally
along the boundary
-Characterized by fault lines
and earthquakes (eg San Andreas Fault)
Convergent
Convergent
Divergent
Divergent Boundary
Convergent Boundary
Additional Evidence in Support of Plate Tectonics
a) Distribution of earthquakes along plate margins
b) Location of earthquake foci along steeply-dipping subduction zones
c) Age dating sediments on either side of the ridge indicates the sediments get progressively older
away from the mid-ocean ridge axis
d) Thickness of sediments also increases away from ridge
Driving Force Behind Plate
Tectonics
1.Convection Cells
2.Hot Spots- thermal plume rises from the mantle.
-can occur in the middle of a plate
(eg., Hawaii)
Hot Spots