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Transcript of Plate Tectonics
1. Sea Floor Spreading
Sea Floor spreading is one of the key reasons that we know about crusts and the layers of the Earth.
Sea Floor spreading is one of the main pieces of evidence that supports the Theory of Plate Tectonics
At the center of the Atlantic Ocean new crust is being made and pushed outwards towards the continents by hot magma
theory that Earth’s crust and part of the upper mantle are broken into sections called plates.
Earth's Crust & Layers
2. Continental Drift
in order to understand this we need to know about the structure of the Earth...
Ocean floor spreading
hot less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions.
The Earth is divided into four main layers.
is the outer most part of the Earth’s surface.
The Earth’s crust is like the skin of an apple. It is very thin compared to the other three layers.
The crust makes up 1% of the Earth.
The crust of the Earth is broken into many pieces called plates.
I. The Mantle
is the layer below the crust.
is the largest layer of the Earth.
is divided into two regions: the upper and lower sections.
Lithosphere – all of the earth’s crust and part of the upper mantle.
Asthenosphere – plastic like layer below the lithosphere. The ridged plates of the lithosphere “float” on the more plastic layer called the asthenosphere.
II. The Outer Core
The core of the Earth is like a ball of very hot metals.
is made up of iron and is very dense.
III. The inner core
has temperatures and pressures so great that the metals are squeezed together and are not able to move.
is a solid.
Evidence Supporting Plate Tectonic Theory
Evidence Supporting Plate Tectonic Theory
or...How do we know about all of this?
Early explorers were the 1st to notice that the continents seemed to fit together like pieces of a jigsaw puzzle
They wondered if all continents could have been part of one large land mass in the past....
In 1912 Alfred Wegener proposed a hypothesis called:
The hypothesis stated that all landmasses were at one time all part of a large landmass called Pangaea (meaning “all land”).
Wegener called the ocean surrounding this landmass Panthalassa (“all ocean”).
- Wegener’s theory that all continents had once been joined together in a single landmass and have drifted apart since. The continents have moved slowly to their current positions due to convection currents in the mantel.
The continents seemed to fit together
Fossils of an animal called Mesosaurus were found in West Africa and in Eastern South America
Since there is no way Mesosaurus could have swam all the way across the ocean, the continents must have been closer
Rocks match across oceans
Rocks found on the west coast of Africa match rocks found on the east coast of South America
The Appalachian Mountains are the same age and made out of the same materials as the mountains in Scandanavia
Coal Deposits are formed from ancient tropical swamps
Today, coal deposits are found in places that are not tropical like the eastern US, Siberia and Europe
At one time they must have been at tropical latitudes
In 1947 Scientists mapped the ocean floor and discovered a long volcanic mountain range running down the center of all the oceans.
The rocks that made up the ocean floor were all much younger than rocks that made up the continents
Scientists called underwater mountain ranges
Harry Hess: scientist suggested that mid-ocean ridges were actually cracks in the earth’s crust where magma was erupting and making new crust
Robert Dietz: liked Hess’s idea and named this process Sea Floor Spreading
The two pieces of evidence connect!
Sea Floor Spreading pushes the continents away from each other causing continental drift: Wegener was right!
Convection current – unequal distribution of heat in the mantel causes movement in a circular motion.
1. The Convection Cell Model: large convection cells in the earth's partially molten mantle exert a drag on the overlying plates, carrying them along like objects on a giant conveyor belt (Fig. 2).
Where Plates meet there's always trouble....
The place where the edge of one plate meets another is called a Plate Boundary.
There are 3 Types of plate boundaries:
Plate boundary interactions can result in:
Found at Mid-Ocean Ridges
Form where magma beneath the crust pushes up through the crust cracking it open and creating new oceanic crust and new lithosphere
Form rifts along central axis of mid-ocean ridge
Oceans can open anywhere: rift valleys
Ex: the Red Sea is a Rift Valley and will someday become an ocean
Form when any 2 plates run into each other or collide
3 types of Convergent Boundaries (remember there are 2 types of crust)
Continental vs. Continental
The continents crash into each other but are too thick to subduct: Large Mountains form
Ex: The Himalayan Mts. are forming due to a collision between India and Asia
Oceanic vs. Oceanic
Since oceanic crust is thin one slab will override the other and subduction will occur
This usually causes volcanic island arcs to form on the overriding plate
Ex: Indonesia is made of a series of volcanic Islands formed because parts of the Pacific Plate are subducting beneath oceanic crust of the Australian plate
Oceanic vs. Continental
The continental crust is thicker so it overrides the oceanic crust
Volcanoes form on the overriding continental crust
Ex: The Andes Mts. in South America are formed from parts of the Pacific plate (oceanic) subducting underneath South American plate (continental)
Form when 2 plates slide or grind past each other
Also called “faults”
Responsible for some of the worlds most powerful earthquakes
Ex: The San Andreas Fault in California is a Transform Boundary
When two plates collide and one plate dives underneath another
Overriding plate: plate that goes over top of the other plate
Down-going plate: Plate that dives beneath the other plate
Melting of the down-going plate often causes volcanism in the overriding plate
Because one plate gets pushed under another, it is called subduction. This is where volcanoes form!