Transcript 8.4 Plate Movement and Continental Growth

8.4 Plate Movement and
Continental Growth
Objectives:
At the end of this lesson we should be able to:
1.
Explain how Earth’s land masses have changed
positions over the past 200 million years.
2.
We understand and explain the roles of plate
tectonics, igneous activity, and deposition in
the formation of continental land masses.
Key Ideas:


The movement of tectonic plates have
caused Earth’s continents to change their
positions on the globe over time.
New material has been added to the
continents as a result of plate tectonics.
Reconstructing the Past

There is a vast body of scientific
evidence showing that the continents
have changed dramatically along the
geological time.
Some of the evidence is presented below:
Evidence of Change over Time
Distribution of old mountain ranges show
that in the geological past two older plates
collided to form these mountains. Such
examples are Ural Mountains, which “welds”
Europe with Asia, Appalachian Mountains in
SE of the United States, The Great Dividing
Range in Eastern Australia, etc.
Evidence of Change over Time
-
Ages of rocks which form the ocean
basins show that in the proximity of the mid
ocean ridge there are young rocks, which
formed recently from the magma which was
spewed through the rift. As we move further,
towards the seduction zones, we find older and
older oceanic crust. For example, the oldest
oceanic crust is about 200 million years old.
Evidence of Change over Time

Fossils offer some of the most reliable
evidence about movement of continents.
Imagine this: on the top of Himalaya
Mountains scientist have found fossils of
aquatic organisms, which means that
layers of rocks which are now on
mountain tops were once on the bottom
of the sea.
Evidence of Change over Time
Evidence of Change over Time

The presence of coal in now cold areas,
without vegetation, show clearly that
these areas had once a warm and humid
climate, which means that they migrated
in cold areas over time.
Coals were found in Antarctica, and
Greenland, to mention only a couple.
Pangaea

Scientific evidence shows that all
continents were once united in a super
continent which they called Pangaea.
Breakup of Pangaea
First, Pangaea broke into two landmasses:
In the past 200 million years the continents moved
continuously until they reached today’s
configuration.
Plate Tectonics and Continental
Growth


In millions of years the continents changed not
only their location but also shape, and size!
Every continent has a core of the old Pangaea.
This old crust is called craton.
The North part of Canada and the neighboring
islands are part of an old, old piece of crust
called the Canadian Shield. Here we find some
of the oldest rocks on Earth. They are over two
billion years old!
Continental Growth


The ancestors of most continents were smaller.
Processes associated with plate movements
have added rock materials to the margins of the
ancient continent cores.
Sources of growth material are:
-deep-sea sediments
-igneous rocks
-river sediments
-terranes
Deep Sea Sediments
The brown layer in the image are sediments which
accumulate in the planetary ocean
Deep-Sea Sediments

A thick layer of sediments accumulates on the
bottom of the planetary ocean. These
sediments consist of dead organisms, debris
carried from the surface of the continents by
major rivers, or loose material carried by air
currents from continents (dust particles, volcanic
ashes, etc.) This material will be scraped off at
subduction zones, and added to the edges of
continents as new mountain ranges.
Igneous Rocks

Volcanoes bring to the surface huge
amounts of new material which is
deposited on the continents .
River Sediments
River Sediments

All rivers carry debris that is eroded along
their course. This material is deposited
along the banks, if the slope of the terrain
is very small or horizontal. Some rivers
form a delta at their mouth. A good
example is Mississippi River, which builds
new land rapidly, filling the Gulf of Mexico.
New Land: Mississippi Delta formed by
accumulation of sediments carried by the river
Terranes
A terrane is a large block of a lithosphere plate that has
been moved, often over distances of thousands of
kilometers and attached to the edge of a continent.
 Terranes are:
-bounded on all sides by major faults
-the rocks and fossils do not match with the neighboring
land masses
-the magnetism of the rocks is different than the
surrounding land masses.
All these show that a terrane formed elsewhere and was
attached to another land mass.
