So what has caused the continents to drift ??
What do you think?
Recall:
Theory of Plate Tectonics…
Plate Tectonics refers to the movements of plates (large segments of continents and ocean) around the Earth’s surface.
These land masses are seen as a giant jigsaw puzzle, and any one mass can’t move without creating gaps between or overlaps with other pieces
They move on the Lithospheric plates on the Liquid Mantle and move along by convection currents in the magma
Convection currents in the asthenosphere of the mantle.
How many of these plates are there?
There are 7 major plates and 5 minor plates – 12 plates total.
It is on the edges of the plates – near the borders of plates where all the action is! (earthquakes, volcanoes, mountain building!)
These edges of the plates are called PLATE BOUNDARIES
Mountains in red, volcanoes and Earthquakes in yellow…
Did you know…These tectonic plates move
approximately 1 inch per year due to CONTINENTAL DRIFT!!!
So in 100 million years, there will be 1250 miles of movement!!
Ring of Fire
Why is the Ring of Fire so Active?
The ring of Fire is a Ring around the Pacific ocean (on the edges of the Pacific tectonic plate) where there are numerous volcanoes and Earthquakes.
Why???...There are 7 other plates that meet the edges of the Pacific plate there– so there is lots of jostling, smashing, and moving where all the edges meet.
Plate Boundaries… At the edges of the plates is where
all the action occurs because of all the smashing that takes place there.
When Plates move – there are only 3 choices they have…
1) crash directly into each other 2) separate from each other 3)slide past/along one another
3 Types of Plate Boundaries
1) DIVERGENT boundaries 2 plates move away from each other
opening a gap called a spreading center
Sometimes, molten rock rises to the Earth’s surface in the gap (weakness in the crust where magma rises up = Hot spot
Hawaiian Islands
Divergent plates are like…
As molten rock comes up through the gap created by the diverging plates, it pushes the old crust aside causing it to buckle. (you get new crust)
This process is called RIDGE PUSH
Divergent…This is the symbol on a map for divergent boundaries
RIDGE PUSH
Divergent plates under water…
When oceanic tectonic plates diverge, pull part, under water, the spreading center is called a spreading ridge or ocean ridge.
Because the magma rises up through the gap and pushes the old crust aside, new sea floor is created
This is called SEA FLOOR SPREADING
The Mid Atlantic Ridge is formed by Diverging Plates
It is an area in the middle of the Atlantic ocean that is forming an ocean ridge
Sea floor spreading
Divergent plates over land…
When tectonic plates diverge, pull apart, over land, the spreading center is called a rift valley.
This is less common than spreading over water
Here’s what a rift valley actually looks like
Diverging plates in eastern Africa are slowly breaking Africa into pieces
2) CONVERGENT Boundaries When 2 plates move toward each-
other When they collide, they push on
each other and one plate dives under the other. The more dense (heavier) plate goes under.
The process of one plate diving under the other is called SUBDUCTION.
Places where you find subduction are called Subduction zones.
Convergent Plates are like…
Convergent Plates…
When Subduction occurs – the edge of the plate subducts deep into the mantle and starts to melt creating magma. That magma can make it’s way up through cracks in the crust and form volcanoes.
When the plate subducts, it “pulls” the rest of the plate with it - This is called Slab pull (the whole slab is pulled under the other slab)
This slab pull helps keep the tectonic plates in motion.
Subduction, Ridge push, and Slab pull
Oceanic-Continental Plate Convergence… When a dense (heavier) oceanic plate collides
with a less-dense (lighter) continental plate, the oceanic plate subducts.
This creates a deep underwater valley called a trench.
As the subducting plate moves deeper, it starts to melt and magma can work it’s way to the surface forming cone-shaped Volcanoes
A chain of volcanoes = Volcanic belt This force of collision between the 2 plates
creates mountain ranges as the continental rock crumbles and folds
Symbol for converging plates
Oceanic - Oceanic Plate Convergence…Subduction occurs when 2 oceanic
plates converge/collide.The denser plate subducts under
the other, a deep ocean trench is formed, and the subducting plate sinks deep into the mantle
This interaction may produce a long chain of volcanic islands known as a Volcanic island arc ( ie. Japan islands)
Continental - Continental Plate Convergence…
When continental plates collide, subduction DOES NOT occur because the plates are the same density.
Instead, the plates hit head on and their edges crumple and fold forming great Mountain ranges ( ie. The Himalayas)
The Himalayas are the highest and youngest mountain range and are increasing in height several cm a year due to continuous convergence of the plates!
Think you can label the letters with the correct term?
3) Transformation boundaries 2 plates slide past one another Because they are sliding
horizontally past each other – no mountains or volcanoes are formed
Faults (breaks in rock layers due to movement on either side) forms.
San Andreas Fault
San Andreas Fault in California
Transformation Plates are like the Titanic and the Ice-berg…
So… 3 Types of Plate Boundaries
DIVERGENT
CONVERGENT
TRANSFORM
When you are really good.. You can understand this whole diagram…
