Study Guide
Unit V – Plate Tectonics/Earthquakes
To prepare for the Unit V test, be sure you can meet these objectives:
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Explain the process of how Earth’s interior became layered and the property that
determined the order of the layers;
Earth began as a ball of molten (melted) rock; it separated into layers as the densest
material was pulled by gravity to the core and the least dense material rose to the
surface, forming a thin crust.
Recall the different ways Earth’s interior is layered near the surface (crust and
mantle; lithosphere and asthenosphere);
Explain the type of change that marks the boundary between the crust and mantle;
The boundary between the crust and mantle is sharp and well-defined; it is marked by
a change in the composition of the rocks.
Recall the two types of crust and how their densities are different;
Oceanic crust (denser); Continental crust (less dense)
Explain the type of change that marks the boundary between the lithosphere and
asthenosphere;
The lithosphere includes all of the crust plus the upper mantle – it is solid and brittle.
The tectonic plates are made of the lithosphere layer. The asthenosphere includes
part of the mantle – it is fluid. The boundary between them is marked by this change
in behavior.
Recall which layer makes up the tectonic plates;
The tectonic plates make up the lithosphere.
Discuss the six main ideas that make up the theory of plate tectonics;
1. Earth’s surface (lithosphere and asthenosphere) is dynamic;
2. the lithosphere is broken into large plates;
3. the plates are in constant motion and move independently of each other;
4. convection currents in the asthenosphere move the plates in different directions;
5. ocean floors are spreading apart at a central ridge and sinking (subducting) at
the opposite edges;
6. the plate boundaries (edges of plates) are very active tectonically
Recall that plate boundaries are easily located by observing the patterns of
earthquakes, volcanoes and mountain ranges around the globe;
9. Locate on a labeled world map the major tectonic plates and the types of boundaries
between them;
Note: arrows indicate direction of plate movement
10. Describe Alfred Wegener’s theory of continental drift, including the significance of
Pangea;
About 250 million years ago all the continent were assembled as one supercontinent
called Pangea and have since “drifted” to their current positions
11. Discuss how each of these observations supports Wegener’s theory:
o The apparent fit of the coastlines on either side of the Atlantic Ocean
Suggests the continents were once next to each other.
o Matching plant and animal fossils on separate continents
o Rock beds of the same type and age on separate continents
These both also suggest the continents were at one time connected
o Evidence of tropical plants in Antarctica
Suggests Antarctica was at one time at a different latitude
o Glacial striations in Africa and S. America
Suggests these continents were at a latitude closer to the South Pole
12. Explain why Wegener’s theory was at first not accepted by other scientists;
Wegener could not come up with a viable mechanism for moving the continental
masses such great distances
13. Describe Harry Hess’s theory of sea floor spreading;
In the 1960’s Hess studied the sea floor and developed the idea that magma rising to
the surface was forcing the oceans to split open and move apart
14. Discuss how each of these observations supports Hess’s theory:
o The presence of a continuous system of mid-ocean ridges
Represents the boundary between plates and the location of the spreading center –
continuous volcanic activity is located here.
o The age of the sea floor
Sea floor is youngest at the mid-ocean ridge and gets progressively older moving away
from the ridge in either direction, as predicted by sea floor spreading, where the
older crust is being pushed away from the ridge by the magma rising to the surface
and forming new crust..
o The symmetrical pattern of magnetic reversals recorded in the ocean crust
The symmetrical pattern suggests the matching bands of rock formed at the same
time and moved in opposite directions away from the central ridge.
15. Describe how scientists today continue to collect data to support the theory of plate
tectonics;
GPS satellites monitor the precise latitude and longitude of stations on the plates
16. Recognize illustrations and/or descriptions of the different types of plate
boundaries, including divergent; convergent (ocean-continent, ocean-ocean, and
continent-continent); and transform;
17. Describe the direction of plate motion on either side of each type of plate boundary;
Divergent: plates moving away from each other
Convergent: plates moving toward each other
ocean-continent = ocean crust subducting beneath continental crust because the
ocean crust is denser and therefore it sinks;
ocean-ocean = ocean crust subducting beneath ocean crust
continent-continent = two continental plates colliding with intense mountain
building activity
Transform: two plates sliding past each other in opposite directions
KEY WORDS:
plate tectonics: the theory that Earth’s surface is broken into a number of pieces, called
plates, that move independently of each other and are constantly changing the features
at Earth’s surface.
Crust: the thin (5 to 50 km thick) outer layer of Earth; may be either continental
(making up the land above sea level) or oceanic (making up the sea floors)
Mantle: the layer beneath Earth’s crust – the boundary between the crust and mantle is
sharp and well-defined by the change in the composition of the rocks
Lithosphere: the solid, rigid upper mantle and crust – this layer makes up the tectonic
plates
Asthenosphere: the semi-molten layer beneath the lithosphere – this is where convection
currents that move the tectonic plates above are located
Density: Earth’s interior separated into layers based on differences in the density of the
material; also, this property controls which plate is subducted at a convergent boundary
Plates: Earth’s lithosphere is broken into large pieces, called plates
Dynamic: constantly changing – Earth’s surface is dynamic
continental drift: Wegener’s theory that the continents had at one time formed one,
giant continent that eventually “drifted” apart
sea floor spreading : Hess’s theory that the mid-ocean ridges were the sites of magma
rising to the surface, forming new crust and pushing older crust apart
earthquake: the release of energy that occurs when pieces of Earth’s crust move; plate
boundaries are marked by intense earthquake activity
tectonics: literally, mountain-building; tectonics refers to all the activity that occurs at
plate boundaries, such as earthquakes, volcanoes and mountain-building
Alfred Wegener: German scientist who developed the ideas of continental drift and
Pangea in the mid-1910’s
Harry Hess: American scientist who developed the idea of sea-floor spreading
Pangea: name given to the “supercontinent” that consisted of all the major continents
assembled as one approximately 200 million years ago.
magnetic reversals : periodic reversals in the polarity of Earth’s magnetic field –
magnetic reversals are recorded in oceanic crust (minerals align with the magnetic field
as they cool and crystallize), and a symmetrical pattern is seen on opposite sides of midocean ridges
symmetrical patterns: matching patterns in the age of rocks and in magnetic reversals on
opposite sides of the mid-ocean ridges
plate boundary: the boundary between two tectonic plates
divergent: plate boundary where the plates are moving away from each other (ex. midocean ridge)
convergent: plate boundary where the plates are moving toward each other (ex.
trenches)
transform: plate boundary where the plates are moving past each other in opposite
directions
mid-ocean ridge: a continuous system of ridges running throughout the oceans where
magma is rising to the surface and forcing the plates apart
subduction: the movement of one plate beneath another at a convergent boundary
volcanic arc: a chain of volcanoes that forms above a convergent boundary when the
subducting plate begins to melt and the magma rises to the surface (may be a continental
volcanic arc or an island arc)
rift valley: the valley that forms as two plates begin to spread apart from each other,
especially on a continent; at a divergent plate boundary
magma: molten (melted) rock beneath Earth’s surface