Download Plate Tectonics

Plate Tectonics

Plate tectonics is the theory
that Earth's outer layer is made
up of plates, which have
moved throughout Earth's
history. The theory explains
the how and why behind
mountains, volcanoes, and
earthquakes, as well as how,
long ago, similar animals could
have lived at the same time on
what are now widely separated
continents.
Pangea –A Supercontinent


You probably wouldn't recognize the Earth if you could see it 225
million years ago. Back then, all the major continents formed one
giant supercontinent, called Pangaea.
Perhaps initiated by heat building up underneath the vast continent,
Pangaea began to rift, or split apart, around 200 million years ago.
Oceans filled the areas between these new sub-continents. The land
masses continued to move apart, riding on separate plates, until they
reached the positions they currently occupy. These continents are still
on the move today.
A Driving Force

Exactly what drives plate
tectonics is not known. One
theory is that convection
within the Earth's mantle
pushes the plates, in much the
same way that air heated by
your body rises upward and is
deflected sideways when it
reaches the ceiling.
Another Idea
Another theory is that gravity is pulling the older, colder,
and thus heavier ocean floor with more force than the
newer, lighter seafloor.
 Whatever drives the movement, plate tectonic activity
takes place at four types of boundaries: divergent
boundaries, where new crust is formed; convergent
boundaries, where crust is consumed; collisional
boundaries, where two land masses collide; and transform
boundaries, where two plates slide against each other.

The Sea Floor Spread
Divergent Boundaries

The Earth's longest mountain
chain isn't the Andes in South
America, or the Himalayas in
Asia, or even North America's
Rockies. It's an underwater
chain of mountains 47,000
miles long. The chain runs
down the middle of the
Atlantic Ocean (surfacing at
Iceland), around Africa,
through the Indian Ocean,
between Australia and
Antarctica, and north through
the Pacific Ocean.
The Continental Crush
Convergent Boundaries

An ocean floor pushed toward a
land mass will always slide under
the land mass. This is because the
land mass is more buoyant, or
lighter, than the ocean floor. When
two land masses meet, on the other
hand, neither will slide under the
other. Instead, the two crush
together at what is known as a
collisional boundary. They crumple
and fold. Some pieces of land are
thrust over or under other pieces.
The result is a mountain range.
Slipping And Sliding
Transform Boundaries

Transform boundaries
neither create nor consume
crust. Rather, two plates
move against each other,
building up tension, then
releasing the tension in a
sudden and often violent
jerk. This sudden jerk
creates an earthquake.
Evidence for Plate Tectonics
1.
The shapes of many
continents are such that
they look like they are
separated pieces of a jigsaw puzzle. For example,
look in the adjacent map at
the shape of the east coast
of North and South
Americal relative to the
shape of the west coast of
Africa and Europe
2. Many fossil comparisons along the edges of
continents that look like they fit together suggest
species similarities that would only make sense if
the two continents were joined at some point in the
past.
3. There is a large amount of seismic, volcanic, and
geothermal activity along the conjectured plate
boundaries. This is shown clearly below in the figure
labeled "Crustal plate boundaries" where the
epicenters of earthquakes above Richter magnitude
5.0 are plotted for a 10-year period. The concentration
is striking, and indeed this plot serves to define the
plate boundaries extremely well. Here is a clickable
map of current volcanic activity on Earth.
4. There are ridges, such as the Mid-Atlantic Ridge (see
figures above and below) where plates are separating
that are produced by lava welling up from between
the plates as they pull apart. Likewise, there are
mountain ranges being formed where plates are
pushing against each other (e.g., the Himalayas,
which are still growing).
5. Magnetic striping and
polar reversals .
6. Earthquake patterns also
lend evidence to plate
tectonics. Most
earthquakes occur along
convergent plate
boundaries. This is only
possible if plates are
moving and thus
subducting.
7. In the Hawaiian Emperor Chain, they
have found that in the curve of
islands, the largest and least eroded of
the islands is at one end of the string,
and contains active volcanoes. At the
other end, is a small, eroded island,
time-dated to be much older than the
currently volcanic island. Also, on all
the islands is increasingly older
volcanic materials, proving that the
islands were once volcanic
themselves, or actually caused by a
volcano. The strip was created by the
hot spot under the plate creating
volcanoes. As the plate moved by the
hot spot, it made volcanoes, and
islands, and then the island would be
carried away by the plate, and another
one would be created.
Hot Spots
Paleomagnetism
The magnetism of an iron-bearing rock imparted to it by the
Earth’s magnetic field when the rock formed. Literally, early
magnetism; meaning magnetism formed in a past geologic
era
8. Polar Wandering
 Based on paleoclimatic and paleomagnetic evidence, to geologists, the
words 'polar wandering' have two distinctly different meanings. That's
partially because 'pole' can refer to the magnetic pole or to the
geographic pole. So what, if anything, moved in the past? The magnetic
poles? The geographic poles? Both?
1. CONTINENTAL DRIFT - the poles stayed put but the
continents shifted position
2. POLAR WANDERING - the continents stayed put but the poles
shifted position
 In any case, many geologists go no further than saying that it appears
there was some sort of shift in the pole position. But since the magnetic
poles and geographical poles are so close together, maybe it happened
and maybe it didn't.
 Wegener's idea was that the continents did the moving, drifing through
the oceanic crust and pushing up mountain ridges ahead of them as they
moved.