Ch. 23 Continued

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Ch. 23 Continued
23.4 - Theory of Plate Tectonics
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Review Earths Drifting Continents
A. The Earth once had a single land mass that
broke up into large pieces, that have been
drifting apart ever since.
1. Fossils found on distant continents
suggest they were once connected.
2. Mountains, valleys and mineral evidence
suggest that the continents once fit together
like a jigsaw puzzle.
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B. Pangaea - The Earths original land mass.
C. Continental Drift - The process of the
Earths continents drifting over time to form
the modern continents.
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Earths Spreading Ocean Floor
Midocean Ridges - Underwater mountains that
have a deep crack running through their center.
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1. Rift Valley - The crack in midocean ridges.
2. Midocean ridges form the largest mountain
chain in the world (80,000km long), 20 times
the distance from L.A. to New York.
Ocean-floor Spreading - As the ocean floor moves
away on either side of the ridge, lava wells up
and hardens to form new ocean floor.
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Earths Moving Plates
A. Plates - Moving, irregularly shaped slabs
that fit together like paving stones, to form
the surface layer of the Earth.
B. Tectonics - The study of movements that
shape the Earths crust.
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Faults and Folds (Bend or Break)
The crust is under a lot of pressure and strain.
Rocks subjected to stress begin to deform into
folds, and if there is enough stress, they will
break.
Two Types of Folds Anticline - An upward fold in
a rock. (ant hill) Syncline - A downward fold
in a rock (sinning goes down)
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C. Theory of Plate Tectonics (Linking the
ideas of ocean-floor spreading and continental
drift), the theory says that Earth has evolved
over time and that this can be explained by
understanding the formation, movements,
collisions, and destruction of the Earths
crust. - Alfred Wegener
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There are seven major plates The Pacific,
North American, South American, Eurasian,
African, Indo- Australian, and Antarctic.
Plate Boundaries (3 types)
1. Divergent - Where two plates are moving
apart.
2. Convergent - Where two plates are colliding.
a. The denser plate is subducted.
b. Oceanic plates tend to be denser than
continental plates.
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c. When two continental plates (of equal
density) collide, mountain ranges are formed.
Strike-slip - Boundaries formed by lateral
faults, two plates grind together and slip past
each other horizontally.
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Transform Faults - Straight ridge sections are
offset by thin cracks.
Trench - Deep V-shaped valleys at the bottom
of the ocean, the deepest parts of the Earth.
Subduction - The process of the crust plunging
back into the Earth.
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G. Convection Currents - The movement of
materials caused by differences in temperature.
1. Convection currents are believed to be the
cause of plate motion.
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2. Magma close to the core is very hot, magma
farther from the core is cooler and more dense.
3. Gravity pulls harder on the cooler magma,
pulling toward the core. The hotter magma gets
pushed up toward the surface.
4. The hot and cool materials trade places
again and again, this cycle never stops