earth science3
ademola
Sept14_PlateTectonics_lecture_f20.pptThe Earth’s Dynamic Processes
Chapter 21
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Questions we have encountered
Why do we have two types of igneous rock? (moon only has
one type for example)
Why does sometimes rock get heated, but not melt, but
instead turn into metamorphic rock?
Why do continents look like puzzle pieces?
Why is there sedimentary rock at the top of the Grand Canyon-
hundreds of miles from an ocean and thousands of feet high?
Why are there long mountain ridges at the bottom of the ocean?
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And not only at the bottom of the ocean
all over earth! (and no where else in the solar system)
Ex. Skyline Dr. in Shenandoah Park VA drops off on both sides
Map of Earth’s elevation (relief)- brown is high
In general, the highest mountains are part of long narrow
chains- why?
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Questions we haven’t encountered, but we could ask:
Why was it such a big deal when VA and MD had a moderate
earthquake in 2011, but California has them all the time?
Why is the highest mountain in the world, Mt. Everest, not
a volcano? It would be easier to understand if we saw
all this lava coming out, building it up. There is no lava
on Mt. Everest so………. how did it get so high?
Why, if you go hiking in Scotland, Ireland or Norway, do the
rocks look like New Hampshire or Maine?
The upper section of the Earth is
divided into sections called plates.
These plates move and change size
over time.
Plate boundaries are defined by seismic and volcanic activity.
(what’s the difference?.......hang on…….)
Some Historical Perspective…
Continental Drift
- Land masses were once joined into a supercontinent known as Pangaea which split into the present day continents
- 1st major evidence is the fit of the coasts (with continental shelves) of Africa and South America
- Hypothesis most fully developed by Alfred Wegener in 1915
He was ridiculed because he didn’t have a good explanation
This was a hypothesis
How to test?
First, same fossils on three different continents. How did
The animals get there? Must’ve walked
People didn’t buy it
Counter hypothesis:
There used to be a land bridge spanning the Atlantic.
An attractive idea because you don’t have to invent some weird
way for continents to float around like ice cubes in water
Argument continues
Wegener provides evidence that S. America, Africa, India and Antarctica
were covered by ice sheets (glaciers) at the same time. How can this be
If they were in there current positions, near the tropics for Africa and India
Argument continues
Might work if continents were huddled together near present day Antarctica
Land bridge hypothesis gets a test
Prediction: If correct, we should see evidence of this
at ocean floor
If not, problem for this hypothesis
Marie Tharp gets hired as
a draftswoman in 1952 to
sketch out thousands of
soundings (sonar- bounce
sound waves) of the Atlantic
ocean bottom put them
together on a map
Land bridge hypothesis gets a test
and fails
When I showed what I found to Bruce, he groaned and said, “It cannot be. It looks too
much like continental drift.” At the time, believing in the theory of continental drift was
almost a form of scientific heresy. (note: she used the wrong word, it was a hypothesis)
Almost everyone in the United States thought continental drift was impossible.
Bruce initially dismissed my interpretation of the profiles as “girl talk.”
P.S. Her map was what Google Earth first used to show ocean bottom
S.Amer
Africa
A ridge goes down the
middle of the ocean.
not cutting across!
Modern Evidence for Continent Movement #1 detailed study of the mid-ocean ridge
- Paleomagnetism - Molten rock material will align its iron atoms with current north and south poles as it solidifies. Over time, the N and S poles have reversed positions at irregular intervals of 10 thousand to a million years (this means that a compass which points N now might point S in 10 thousand years). This record of reversals, captured in igneous rocks, is referred to as paleomagnetism.
Paleomagnetism- rocks tell you which way compass needles would
have pointed over millions of years. We can take
Samples of the rock and measure their magnetism.
These two bands, for example, were born at the same time (“twins
separated at birth”) and have spread away. They preserve the
record of which way compasses would have pointed when they crystallized
Youngest rock
Modern Evidence for Continent Movement #2
- Patterns of global volcanic activity
Modern Evidence for Continent Movement #3
Can you identify the mid-Atlantic Ridge ?
Distribution of earthquakes
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And a debunked hypothesis no, it wasn’t the moon breaking off!
Modern Evidence for Continent Movement #4
- Actual measurements of continental separation and movement together – Scientists can bounce laser beams from one station to a satellite and back to a second station. The delay in transfer indicates the change in distance between the stations. This was first measured in 1984.
Prince George’s Community College PSC 1210 Barbara A. Gage
Young igneous rock in the center. At the mid-ocean ridge
Older as you move away.
Possible conversion into metamorphic rock at the edges
Earth’s Plates
Prince George’s Community College PSC 1210 Barbara A. Gage
Prince George’s Community College PSC 1210 Barbara A. Gage
So far, we have only learned about the crust. The outer edge
The earth is really like an egg:
Yolk, egg white, egg shell (crust)
Continental drift now more than a hypothesis. It’s a fact.
But its not a theory……….. Why do continents drift????
http://scign.jpl.nasa.gov/learn/plate1.htm
Prince George’s Community College PSC 1210 Barbara A. Gage
Prince George’s Community College PSC 1210 Barbara A. Gage
Earth’s Layers (based on seismology)
CRUST
- Two forms - continental and oceanic
- Continental crust is composed of less dense rock rich in silicates; thicker than oceanic
- Oceanic crust is basaltic and denser than the continental crust; fewer silicates
Prince George’s Community College PSC 1210 Barbara A. Gage
Earth’s Layers (based on seismology)
MANTLE
- Composed of iron-rich silicates
- Has an upper layer that is “plastic” or semi-fluid
- Has a higher temperature than the crust
Prince George’s Community College PSC 1210 Barbara A. Gage
Earth’s Layers (based on seismology)
CORE
- At the center of Earth
- Has two sections, outer and inner cores composed mainly of iron and nickel
- Outer core is molten while inner core is solid
- Fluidity of outer core may account for Earth’s magnetic field
Prince George’s Community College PSC 1210 Barbara A. Gage
Another Way to View Earth Layers
- Lithosphere - composed of crust and uppermost section of the mantle; rigid layer that composes the plates
- Asthenosphere - “plastic” region in the upper part of the mantle under the lithosphere; the plates “ride” on the asthenosphere
Prince George’s Community College PSC 1210 Barbara A. Gage
Prince George’s Community College PSC 1210 Barbara A. Gage
(granite)
(basalt)
Plate Motion
- Plates can move away from each other at divergent plate boundaries or spreading centers.
- Plates can move together in convergent plate boundaries.
- Plates can slide past each other in transform fault boundaries.
Prince George’s Community College PSC 1210 Barbara A. Gage
Divergent Plate Motion
- Tensional forces stretch the lithosphere
- New Earth materials are formed between plates where hot mantle material rises into stretched area
2.5 cm/year
2.5 cm/year
Each year Atlantic Ocean gets wider by 5 cm.
Rock born in the year 2000, is now 2. 5 cm/yr x 20 years = 50 cm away from
the middle. 100 cm apart from its “twin separated at birth”
After 10000 years, its 50,000 cm = 500 meters (about 1/3 of a mile)
Your homework wants you to look at millions of centimeters to see how
long that should take to move that far and compare with current estimates.
Birth of
New rock
Simple model of diverging plate- sea floor spreading with pieces of paper
Rock is born in the middle
Divergent Plate Motion: spreading
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Go visit the mid-Atlantic
ridge (in Iceland)
Spreading about
5 cm= 2 inches a year
Convergent Plate Motion
- Causes collision of plates with compressional forces. Convergence leads to subduction of one plate under another
- Oceanic crust may go under continental crust generating a trench and causing mountain building
- Granite and continental crust is made here- underground melting and crystallization
Convergent Plate Motion
- Continental crust may go under other continental crust with mountain building. Uplift and huge mountains occur here
Prince George’s Community College PSC 1210 Barbara A. Gage
Convergent Plate Motion
- The Indian plate collided with the Eurasian Plate 40-50 million years ago to form the Himalayas. These mountains are still growing about 1 cm/year because of the “push” from the Indian Plate.
Prince George’s Community College PSC 1210 Barbara A. Gage
Convergent Plate Motion
- Ocean crust may go under ocean crust in a trench causing oceanic volcanoes and a deep ocean trench.
Prince George’s Community College PSC 1210 Barbara A. Gage
Transform (Side) Fault Boundary
- Plates may slide past each other laterally with no subduction
- Seismic activity is high along these slip boundaries. But not
volcanic activity.
Prince George’s Community College PSC 1210 Barbara A. Gage
Transform Boundary
- The San Andreas Fault in California is an example of this boundary
Prince George’s Community College PSC 1210 Barbara A. Gage
Hot Spots – not really a plate
- Sometime a plate will pass over an active spot beneath that “punches through” the lithosphere as a volcanic eruption. The Hawaiian Islands formed (and are still forming) this way.
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Prince George’s Community College PSC 1210 Barbara A. Gage
Prince George’s Community College PSC 1210 Barbara A. Gage
*
You know how plates move. But why do they move?
Just like boiling water, convection is how heat is transferred from
the Earth’s core to the crust. Its like the mantle is “boiling”!
In my little paper toy, where would slits A and C be on this diagram?
A
C
1. They are part of plates floating on the plastic upper mantle-
the asthenosphere
2. The movement of the plates results from convection currents in
the mantle as heat is transferred from the earth’s core to the
crust
3. Igneous and metamorphic rocks are formed at plate boundaries.
a. Granite at subduction zones where melting and crystallization occur
underground
b. Basalt at divergent boundaries, where lava comes to
surface
c. Metamorphic rock formed from heat and pressure at subduction zones
This is the theory of Plate Tectonics
Come back Wednesday
