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Chapter 13 – Plate Tectonics – Key Ideas
What is Plate Tectonics?
The Earth’s surface is made up of moving, solid pieces called plates (Reference
Tables). Some of these plates are moving together and some are moving apart.
Plate tectonics is the study of the formation and movement of these plates.
 The Earth’s plates are part of the lithosphere. The lithosphere considered to
be the crust and the upper part of the mantle.
 The oceanic crust is mainly made of the igneous rock basalt, while the
continental crust is made of the igneous rock granite.
 Oceanic crust is denser than continental crust.
How Plates Move
The lower part of the mantle is called the athenosphere. This layer has similar
composition to the upper mantle, but it is partially melted. This makes the
athenosphere like a very thick “liquid” (you can compare solid rock to very thick
mud to give yourself a picture in your mind).
 The “liquid” property of the athenosphere allows it to flow very slowly.
 You can think of the lithospheric plates as “floating” on the athenosphere.
 The heat from inside the Earth causes convection currents to form in the
athenosphere.
 Where the convection currents rise, new material (molten rock) forces the
plates apart.
 Where the convection currents are sinking, the plates are moving together.
Diagram:
Evidence of Plate Movement
1. The shape of the west coast of Africa and the east coast of South America
seem to fit together like a puzzle. It is believed that they were once
together, but moved apart over time (Continental Drift).
2. Similar fossil remains in Africa and South America of a reptile that is
found nowhere else in the world.
3. Distinctive rocks were also found in Africa and South America in regions
where the two continents were most likely joined together.
4. The majority of earthquakes and volcanoes are found in belts that occur
along plate boundaries (where the plates are moving). Ex. the Pacific Ring
of Fire.
5. Magnetic polarity reversals are recorded in rocks on the ocean floor as the
sea floor spreads apart. During certain periods in Earth’s history, the
magnetic fields have reversed (a compass would point south during a
reversal). Minerals containing iron found in the igneous rocks on the
ocean floor would shift before the magma cooled and hardened. If the
polarity changed, the new magma would cool with the minerals shifted in
the opposite direction. This change is recorded in the rocks. (LAB)
6. Heat flowing out of the rocks is greatest where the rocks are spreading
apart (rising convection current), and decrease as you move away from the
center (sinking convection currents).
Plate Boundaries (Reference Tables)
1. Diverging Boundaries- this is where the plates are moving apart at
spreading centers (on the ocean floor). These boundaries form mid-ocean
ridges or rises (like underwater mountain ranges). Between the peaks of
the ridge are valleys called rift valleys. Ex. mid-Atlantic Ridge, East
Pacific Rise.
Diagram:
2. Sliding Boundaries- as the name implies, this is where two plates are
sliding past each other. The sliding movement often causes earthquakes to
occur. This happens along faults. A fault is nothing more than a crack in
the Earth’s crust where movement has occurred. Ex. North American Plate
and the Pacific Plate are sliding past each other along the San Andreas
Fault in California.
3. Converging Boundaries- this is where two plates are coming together.
 Collision boundary- this is when the two plates that are moving
together are both continental plates. The collision causes the plates
to form a single, larger continent and the crust is pushed upward
into a mountain range. Ex. Himalayan Mountains (including Mt.
Everest). The Indian Plate and the Eurasian Plate are still moving
together, so Mt. Everest is still slowly getting taller!!
Diagram:

Subduction boundary- this is when one of the plates plunges under
(subducts) the other plate. This occurs between two oceanic plates,
or an oceanic and continental plate. The most common
characteristic of a subduction boundary is the formation of a deepsea trench. These trenches are the deepest locations in the ocean
(Ex. the Mariana Trench is approx. 35,000 feet deep! Mt. Everest
could be put into the trench and the peak would still be about 1
mile below the ocean’s surface!!)
*Because oceanic crust is denser than continental crust, the oceanic
crust will always subduct underneath the continent.
**Volcanoes always seem to form at subduction boundaries. If two
oceanic plates converge, volcanic islands will form. If one of the
plates is continental and the other oceanic, the volcanoes will form
along the edge of the continent.
Diagram: