Sea-Floor Spreading and Theory of Plate Tectonics
ObjectivesApproximately 50 years after Wegener’s work, more discoveries were made that proved that
Wegener was indeed correct.
As the ocean floor was being mapped, a series of underwater mountain chains or ridge were
discovered. Along the center of the ridge was a step-sided valley. It was discovered that molten
material rises from the mantle and
erupts onto the surface from the
valleys. This new material spreads
out, pushing older rocks outward.
This movement is called sea-floor
spreading. The age of rocks
supports this idea. Rocks along the
coast of continents are much older
than the rocks along the ridge. As
go outwards in both directions, the
rocks increase in age the same amount. Additionally, the rocks that from at the ridge only form when
melted material cools very quickly. This shows that melted material is rising to the surface here.
Alright, so we know the lithosphere (crust and upper mantle) is broken into sections called
plates and the plates are moving. When the plates move, three things can happen: they can pull apart
from each other, bump into each other or slide along each other. Depending on type of collision and
what type of crust it is (continental- solid land, or oceanic- land is under the ocean), different landforms
result.
If two plates pull away from each other, a divergent plate boundary forms. When the plates pull
apart, a crack appears and molten material rises up to the surface of the earth. When ocean plates pull
apart, ocean ridges pull form. When both plates are made of continental crust, rift valleys form. The
Mid-Atlantic Ridge between North/South America and Europe/Africa is an example. The Mid Atlantic
Ridge runs through Iceland which is one of the most volcanic locations on the earth. The Great Rift
Valley in Africa (where Mt. Kilimanjaro is located) is another. A small rift valley is forming along the Rio
Grande River which is located along the western boundary of Texas through New Mexico.
A transform boundary forms when two plates
slide along each other. Earthquakes commonly
happen at transform boundaries. The San Andreas
Fault found along the west coast of California is an
example of a transform fault boundary.
Convergent plate boundaries form when two plates collide. When the plates are both
continental plates, mountains form. Examples of this type of plate boundary include how the
Appalachian and Rocky Mountains formed. The Himalayan Mts. located between Nepal and India is an
active plate boundary and those mountains are still being shoved higher. As evidence that the crust was
shoved this high, fossilized fish skeletons have been found high in the Himalayans.
When two ocean plates converge, one
plate is subducted (pushed under) the other.
The one that is subducted eventually melts and
the now less dense molten rock rises to the
surface and forms island arc volcanoes. Japan,
the Aleutian Islands between Alaska and Russia,
and the US Virgin Islands all form this way.
When an ocean plate and continental plate
converge, a similar event happens. The ocean plate
is subducted under the continental plate and creates
a chain of volcanic mountains along the coast. The
Andes Mountains along the west coast of South
America and the Cascade Mountains along the
northwest coast of the United States (includes Mt.
St. Helens) form this way.
Look at the plate boundary map on the next page and try to locate each of the places mentioned
in the text and then use the key to see what type of plate boundary is located there.
Questions
Look at the map on the previous page. For each location given, tell what type of boundary is present
and the landform. If possible give the specific name of the landform (ex. don’t say volcanic mountain,
but Cascade Mountains).
1. Brazil (northern South America)
2. East of Saudi Arabia (Persian Gulf)
3. Hawaii (middle of Pacific Plate)
4. Philippian Islands (north of Australia)
Research question: Where is the Mariana Trench located? Is it along a plate boundary? If so, what kind.
How deep is the trench?
5. The process that continually adds new material to the ocean floor is called
___________________________.
6. Deep underwater canyons are called _________________________.
7. What is subduction?
8. The lithosphere is broken into separate sections called _____________________.
9. State the theory of plate tectonics.
10. Is the following sentence true or false? The theory of plate tectonics explains the formation,
movement, and subduction of Earth’s plates.
Matching
11. _____ plate tectonics
a. deep valley that forms where two plates pull apart
12. _____ fault
b. line where different pieces of the lithosphere meet
13. _____ rift valley
c. break in Earth’s crust where rocks have slipped past each other
14. Compete the table.
Type of Plate Boundary
How Plates Move
Transform boundary
Divergent boundary
Convergent boundary
15. Complete the table on convergent boundaries.
Types of Plates Converging
Ocean/ocean
Ocean/continent
Continent/continent
Landform Feature