Constructive Forces

Building the Crust

I. Plate Tectonics

A. Moving Plates- current theory is that Earth’s surface is

composed of sections of the lithosphere called plates

- Lithosphere is composed of the crust and the rigid mantle

- The study of the movement of these plates is called plate tectonics

- original concept was proposed by Alfred Wegener as part of his theory of continental drift

Plate Tectonics (contd.)

- Pangaea - original supercontinent

- Concept that all of the continents were combined at one time to make one, giant landmass

- Geologic forces caused Pangaea to split up

Plate Tectonics (contd.)

B. Plate Thickness- lithospheric plates

are approximately 100 km thick

- ocean basin is mostly basalt

- continental crust is mostly granite

Plate Tectonics (contd.)

C. Plate Movement- asthenosphere -

partially melted layer of the mantle that the lithosphere floats on

- convection currents occur inside the asthenosphere

Plate Tectonics (contd.)- hot material moves

toward the crust because it is less dense

- As material rises, ir cools and becomes more dense

- More dense material sinks back towards the bottom of the asthenosphere

- Movement of mantle material pushes the lithospheric plates that float on top of it

Plate Tectonics (cpnyd.)

D. Evidence of Plate Tectonics

1. Africa and S. America fit together like the pieces of a jigsaw puzzle

2. Earthquakes and volcanoes occur along plate boundaries

Plate Tectonics (contd.)3. Paleomagnetism -

oceanic bedrock shows patterns of magnetic reversal that could only be caused by seafloor spreading

4. Seafloor spreading - oceanic crust is growing at the mid-ocean ridges - crust increases in age as you move away from the ridges

Plate Tectonics (contd.)II. Types of Plate

BoundariesA. Divergent - where two

or more plates are moving away from one another

- occurs primarily at the mid-ocean ridges

- ex.: Mid-Atlantic Ridge- Great Rift Valley

(Africa)

Plate Tectonics (contd.)B. Convergent - where two or

more plates are colliding1. Subduction Boundary -

where continental and oceanic plates collide

- oceanic crust is more dense than continental crust, so it sinks underneath the continental crust and is pushed into the mantle

Plate Tectonics (contd.)- as the oceanic crust melts,

pockets of magma are created

- Magma rises through the crust at the boundaries and forms a volcanic mountain on the overriding continental plate

- Also forms deep sea trenches where the oceanic crust plunges into the mantle

- Ex.: Pacific NW - Cascade Mtns.

Plate Tectonics (contd.)C. Transform Boundary- where plates slide past

one another- Can be right-moving or

left-moving- frequently create faults -

cracks in the crust where movement occurs

- May see faults further inland from the plate boundary

- Ex.: San Andreas fault, CA

Plate Tectonics (contd.)

III. Moving Plates- plate movement is

evidenced from the occurence of earthquakes and volcanoes

- different hypotheses about the causes of plate movement

Plate Tectonics (contd.)A. Mantle Convection- asthenosphere is semi-

solid, like slush or putty- as heat radiates from

the mantle, it causes movement in the asthenosphere

- Creates convection currents

- Movement is caused by changes in density

Plate Tectonics (contd.)

- Hotter material is less dense - floats upward

- rises, cools, and sinks back down towards the bottom of the asthenosphere

Plate Tectonics (contd.)B. Ridge Push- occurs at mid-ocean ridges- As new rock is formed, it is

less dense and hotter than older rock

- New rock slides down the side of the ridge

- Cools and hardens - becomes more dense

- Creates force pushing on lithosphere from the force of gravity pulling on the denser rock

Plate Tectonics (contd.)C. Slab Pull- at subduction boundaries, one

plate is colder and more dense than the other

- One plate sinks into the mantle

- The edge of the sinking plate is still colder and more dense than the mantle

- As the plate sinks into the mantle, it pulls the rest of the plate behind it

- Considered to be stronger than ridge push

Plate Tectonics (contd.)IV. Continental GrowthA. Craton - ancient

continental cores- Continents were

originally much smaller

- large amounts of crustal material has been added to the continental plates over time

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Plate Tectonics (contd.)

B. Sources of Crustal Material

1. Deep sea sediments

2. River sediments (both make sedimentary rock)

3. Volcanic eruptions

Plate Tectonics (contd.)

C. Thin-skinned thrusting

- pushing of thin, horizontal sheets of rock from continental margins over great distances along level fault surfaces

Plate Tectonics (contd.)

D. Terranes - large block of a lithospheric plate that has been moved and attached to the edge of a continent