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Earth ~200 million years ago
The Continental Drift Hypothesis
• Proposed by Alfred Wegener in 1915.
• Supercontinent Pangaea started to break
up about 230 million years ago
• Continents "drifted" to their present
positions
• Continents "plowed" through the ocean
crust.
Continental Drift
• This idea was not widely excepted until the
1960’s. It was at this time that different kinds of
data indicated that the continents have moved.
• Continental drift turns out to be just one aspect of
the broader theory known as plate tectonics
• Tectonics is the study of large scale movement
and deformation of the Earth’s outer layers
Continental Drift: Evidence
• Geographic fit of South America and Africa
• Fossils match across oceans
• Rock types and structures match across
oceans
• Ancient glacial features
Continental
Drift:
Evidence
Tight fit of
the continents,
especially
using
continental
shelves.
Fossil Evidence
• Leaves of a fossil plant Glossopteris were found in
southern Africa, Australia, South America, India,
and even Antarctica.
• Certain dinosaur and other vertebrate fossils were
found only over limited areas of several different
continents.
• The question was asked: how did a given life form
develop, identically and simultaneously, on
different continents now widely distributed
around the globe.
Continental Drift:
Fossil Evidence
Climatic Evidence
• Many factors determine a region’s climate, but a
dominant one is latitude.
• In general equatorial regions tend to be the
warmest, polar regions tend to be the coldest,
with more moderate temperatures in between.
• Sedimentary rocks formed at Earth’s surface,
often preserve evidence of the climate conditions
under which they were formed.
• Fossil remains of plants known to thrive in moist
heat imply a tropical climate, and some
sandstones indicate desert conditions
Question: Did the climate change of
did the continent move, or both
Continental
Drift:
Evidence
Correlation of
mountains
with nearly
identical
rocks and
structures
Continental
Drift:
Evidence
Glacial features
of the same
age
restore to a
tight polar
distribution.
The Rise of Plate Tectonics
• U.S. Navy mapped seafloor with echo sounding
(sonar) to find and hide submarines. Generalized
maps showed:
• oceanic ridges—submerged mountain ranges
• fracture zones—cracks perpendicular to ridges
• trenches—narrow, deep gashes
• abyssal plains—vast flat areas
• seamounts—drowned undersea islands
Evidence for
Plate Tectonics
In the 1960’s scientists
started to map seafloor
magnetism. The
research showed that
has a striped pattern
completely unlike
Mason on
& Raff,
patterns
land.
1961
Black: normal polarity
White: reversed polarity
Evidence for
Plate Tectonics
When reviewing the
data the scientists
noted that the
patterns are
SYMMETRICAL across
oceanic ridges.
The dates
show the youngest
rocks at the ridge (red color).
Meanwhile back on the military
front
• U.S. military developed new, advanced
seismometers to monitor Soviet nuclear tests.
• By the late 1950s, seismometers were
recording 24 hrs/day, 365 days/year
• All of this recording led to a detailed idea of
what the ocean floor really looked like
Bands of seismicity—chiefly at trenches and oceanic
ridges
What did the data and research
show?
• After years of data collecting and analysis it was
determined that the surface of the Earth is broken
into 7 major plates.
• 1. Indian-Australian
• 2. Pacific
• 3.North American
• 4.South American
• 5.Antarctic
• 6.African
• 7.Eurasian
The seven major plates in
Earth’s crust
The Theory of Plate Tectonics
• Earth’s outer shell is broken into thin, curved
plates that move laterally atop a weaker
underlying layer
• Most earthquakes and volcanic eruptions
happen at plate boundaries.
• Three types of relative motions between
plates:
• Divergent, Convergent, and Transform
Divergent
Convergent
Transform
Important to remember and
understand
• The Earth is not completely solid from the
surface to the center of the core
• There is a “plastic”, locally partly molten zone
that lies relatively close to the surface, where
a thin, solid, rigid shell of rocks floats on a
weaker, semisolid layer below
The lithosphere
• This is the Earth’s outermost solid layer
• The name reflects the rigid quality of the
lithosphere.
• It varies in thickness from place to place but
the thinnest is under the oceans where it
extends to a depth of about 50 kilometers (30
miles).
• It is thicker under the continents where it can
be as thick as 100 kilometers (60 miles)
The asthenosphere
• This layer has “plastic” quality to it, which
allows it to flow slowly as contrasted with the
more rigid lithosphere.
• This layer lies within the upper mantle and
extends to an average depth of 500
kilometers (300 miles).
• This zone is actually not all molten, only a
small part is magma, but with the immense
pressure the rocks behave like fluid but
remain solid
Locating Plate Boundaries
• Locating the boundaries consists of two main
ways:
• Earthquakes
• Volcanic eruptions
Polar Wandering
• It is the migration over the surface of the Earth of
the magnetic poles of the Earth through geological
time. It was long recognized that the directions of
magnetization of many rocks do not correspond to
the present direction of the geomagnetic field at
their sites
• On the time scale of polar wandering, polarity
reversals of the geomagnetic field are relatively
frequent
• What this shows is the directions continents have
drifted
How fast are they moving?
• By looking at Hawaii and the its previous path and
looking at the amount of time it took to cover that
distance we can get a speed.
• 2,700 kilometers
• 25 million years
• =
• 11 centimeters per year
• **However looking at many determinations from
all over the world, movement has been determined
at 2 to 3 centimeters per year
Why do the plates move?
Slab pull: Denser, colder plate sinks at
subduction zone, pulls rest of plate behind it.
Ridge push: At spreading ridges, the plates
push material ahead of the spreading
Mantle convection: Hotter mantle material rises
beneath divergent boundaries, cooler material
sinks at subduction zones.
Convection cells
Evidence for
• From seismic and other geophysical evidence and
laboratory experiments, scientists agree with the
theory that the plate-driving force is the slow
movement of hot, softened mantle that lies below
the rigid plates
• Below the lithospheric plates, at some depth the
mantle is partially molten and can flow, albeit
slowly, in response to steady forces applied for long
millions of years.
Mechanics of
• The mobile rock beneath the rigid plates is believed
to be moving in a circular manner somewhat like a
pot of thick soup when heated to boiling.
• The heated soup rises to the surface, spreads and
begins to cool, and then sinks back to the bottom
of the pot where it is reheated and rises again.
• This cycle is repeated over and over to generate
what scientists call a convection cell or convective
flow.
How does convection
work? No one knows—
but they aren’t afraid to
propose models!
Whole-mantle convection
Two mantle convection cells
Complex convection
Divergent
boundaries
also can rip
apart (“rift”)
continents
All of these oceans were caused by
rifting of the Earth’s crust
• The Atlantic Ocean
• Red Sea
• East Africa Rift
It is presumed that Pangea was
ripped apart by the same process
This is how the Rocky Mountains and the
Himalayas were formed
Hotspots, such as the one under Hawaii,
have validated plate tectonic theory.