Plate Tectonics
Continental Drift
Around 1912, a German scientist named Alfred Wegener theorized that all of the Earth's
continents were once joined together in a single, large landmass. He further proposed that the
continents have separated and collided as they have moved around over the last few million
years. He called this theory continental drift. He provided several pieces of evidence to support
his theory:
1) Continent Shapes- The continents appear to be shaped in such a
way that they would fit together nicely, like a jigsaw puzzle.
2) Rock Formations- There are rock formations on different
continents that match up beautifully when the continents are put back
together.
3) Fossils- There are fossils found on different continents that would also match up nicely if the
continents were all once together.
People of the time mostly thought Wegener was crazy!
New Evidence
In the 1950's, scientists discovered some surprising evidence in support of Wegener's theory.
While mapping the ocean floor, scientists discovered two important, and unexpected things:
First, the age of the rocks that make up the ocean floor gets older as you move away from the
ridges at the center. This meant that the youngest rocks were found near the ridges, and the
oldest rocks near the continents.
Below is a graph of the rock ages for the map on top.
Second, there are stripes of alternating magnetic polarity on each side of the ridge. When the
molten rock hardens, the magnetic minerals in the rock align themselves with the Earth's
magnetic field. Scientists discovered that the Earth's magnetic field has reversed itself many
times, at intervals of roughly every 100,000 years. The pattern they observed makes sense if the
ocean floor is being formed at the ridge and gradually pushed outward in both directions.
These discoveries gave rise to the now respectable science of Plate Tectonics. This is the theory
that the Earth's seemingly solid crust is actually made up of several pieces, or plates, that move
around independently.
Plate Boundaries
The places where the different plates meet, called plate boundaries, are where the tectonic
action really is. There are three basic types: convergent, divergent, and transform boundaries.
Convergent Boundaries: This a when two plates are moving toward each other, as shown
above.
If the two plates are of relatively low, and similar densities, the plates will form a Collision
Boundary.
In this scenario, the crust is forced upward by the collision, resulting in mountain building. The
diagram above shows how this type of collision between India and China forced the formation of
the Himalayan Mountains
If one of the plates is more dense than the other, as happens when oceanic and continental crust
meet, then the more dense plate will be forced under the less dense plate. This forms a trench,
or deep valley, where the plates meet. This is called subduction, and is shown in the diagram
above. This often results in a chain of volcanoes running parallel to the trench.
Divergent Boundaries: As you might expect, this is essentially the opposite of a convergent
boundary. This occurs when two plates are moving away from one another, as shown above.
This is seen at mid-ocean ridges and rifts.
Transform Boundaries: This type of boundary forms when two plates are sliding past one
another. The diagram above illustrates this motion. The most popular example of this is the San
Andreas Fault in California .
All of the different boundaries and their locations are found on page 5 of the Earth Science
Reference Tables, shown below. Notice the key that shows the different boundaries and their
symbols.
Tectonic Forces
The movement of the plates is driven by convection currents in the mantle. These currents
cause the solid plates to float along on top of the semi-molten mantle material.
Sometimes, there is an opening in the middle of a plate that allows the molten material to flow
through it. This is called a hot spot, and usually results in a chain of volcanic islands that form as
the plate moves over the hot spot. The Hawaiian Islands are a great example of this.
Plate Tectonics
As you studied volcanoes, igneous, metamorphic and sedimentary rocks, and
earthquakes, you learned how these topics are related to plate tectonics. In this
chapter we take a closer look at plates and plate motion. We will pay particular
attention to plate boundaries and the possible driving mechanisms for plate motion.
The history of the concept of plate tectonics is a good example of how scientists
think and work and how a hypothesis can be proposed, discarded, modified, and
then reborn. In the first part of this chapter we trace the evolution of an idea how the earlier hypothesis of moving continents (continental drift) and a moving
sea floor (sea-floor spreading) were combined to form a theory of plate tectonics
1. Continental drift was proposed by Alfred Wegner in the early 1900s based on
the apparent fit of continental coastlines, similar fossil plants and animals on
widely separated continents, distribution of Paleozoic glaciations and
paleoclimatology, and apparent polar wandering.
2. Wegner proposed that all continents had once been connected in a
supercontinent called Pangaea, that broke apart to form the present continents.
Wegner thought the continents moved across stationary oceanic crust. His ideas
received little support when proposed because he could provide no mechanism that
allowed continents to plow through ocean crust.
3. Paleomagnetism is the study of the ancient magnetic fields of the earth.
Magnetized minerals preserve a record of the direction of the magnetic pole and
their distance from it at the time of their formation. Paleomagnetic data revived
interest in continental drift by demonstrating polar wandering and supporting the
reconstruction of Pangaea.
4. Other recent evidence for continental drift includes better continental margin
fits, similar rock contacts and age relations between continents when fitted
together, glacial movements indicated by striations, and sources of boulders in
ancient tills, and similar geologic sequences including metamorphic rocks in Brazil
and Gabon.
5. The idea that the sea floor spread away from mid-oceanic ridges and was
subducted beneath a continent or island arc as a result of mantle convection was
proposed by Harry Hess in the early 1960s.
6. Sea-floor spreading explains processes at the mid-oceanic ridges as the result
of rising mantle: the existence of the ridge itself, high heat flow, basaltic
volcanism, a rift valley and shallow-focus earthquakes.
7. Sea-floor spreading explains processes at the oceanic trenches as the result of
descending oceanic crust: existence of the trench itself and volcanism.
8. Sea-floor spreading explains the young age of the sea floor, loss of older oceanic
crust, and increasingly older oceanic crust away from the ridge crest.
9. Plate tectonics is the theory that the earth's surface is divided into a few large,
thick plates that move and change size. It combines the older ideas of continental
drift and sea-floor spreading. Plates are formed by lithosphere (crust and
uppermost mantle) and are carried along by the asthenosphere to a depth of about
200 km. New lithosphere is added along the ridges at the trailing edge of the plate
and lost to subduction. Plate boundaries are either diverging, converging or
transform.
10. Sea floor magnetic anomalies were symmetrical with respect to the mid-oceanic
ridge crests and matched the pattern of magnetic reversals discovered previously
in stacked lava flows. Spreading rates are 1 to 6 cm/year. The hypothesis also
allows prediction of the sea floor age based on magnetic anomalies that can be
tested with samples recovered by deep-sea drilling.
11. Diverging plate boundaries experience extension that produces normal faults,
shallow-focus earthquakes, rift valleys, basaltic volcanism, crust thinning, uplift,
and creates new ocean basins. Whether rifting causes uplift, or vice versa is
unclear.
12. Transform boundaries allow plates to slide past one another. These boundaries
exhibit strike-slip motion and may connect two ridge segments, a ridge and a
trench, or two trenches. The straight course of these faults resolves mechanical
constraints caused by divergence along curved boundaries.
13. Ocean-ocean convergence is characterized by andesitic to basaltic island arcs
and trenches
14. Ocean-continent convergence exhibits an active continental margin associated
with young volcanic and some metamorphic mountain belts and trenches.
16. Continent-continent convergence passes through the stages exhibited by
ocean-continent convergence, but results in a suture zone of young mountains in
continental interiors marking the former subduction site, thickened continental
crust, and shallow focus earthquakes. Ex. the Himalayas. Collision zone not
subduction zone -- no trench.
18. Plate tectonics explains consistently: distribution of basaltic and andesitic
volcanoes, shallow-, intermediate-, and deep-focus earthquakes, young mountain
belts, mid-oceanic ridges, oceanic trenches, and fracture zones.
19. Convection currents in the asthenosphere cause mantle movement. The
overlying plates are carried along with mantle movement.
20. Mantle convection may result in mantle plumes or hot spots. They are
stationary with respect to moving plates and produce hot spots, such as
Yellowstone, Iceland and the Hawaiian Islands. Mantle plumes may also be
responsible for the initial fracturing of the lithosphere causing divergence. (e.g.
Red Sea region).
Divergent
Boundary
2 plates move away from
each other
Mid-ocean ridges, rift
valleys - basaltic magma
creating new crust
Transform Boundary
2 plates slide past one
another
San Andreas Fault, midocean ridges
3 types of convergent boundaries - destroying crust




ocean crust - ocean crust
2 plates move together






ocean crust - continental
crust
2 plates move together


older, denser crust subducts
trench
curved volcanic
island arc
andesitic to basaltic
magma
ex. Japanese island
arc, Aleutian Islands
shallow,
intermediate, deep
earthquake focus
oceanic crust subducts - denser
trench
andesitic magma
metamorphic rock
ex. Andes Mtns.,
Cascades
shallow,
intermediate, few
deep earthquake
focus



continental crust continental crust
2 plates move together


No subduction - no
trench
Double crust
thickness
Mountain range
which may have
marine fossils from
oceanic crust that
became narrow
(Think India plate
movement)
Himalayas
shallow earthquake
focus