EXERCISE 5 –
PLATE
TECTONICS
TOOLS REQUIRED
CALCULATOR
NAME ______________
DATE ______________
CLASS ______________
THE THEORY OF PLATE TECTONICS
It has long been recognized that the geographic features of the earth are not
uniformly distributed, and that certain geographic features seem to match across
land masses separated by the oceans. In 1912, a German meteorologist
suggested that the continents had moved horizontally, and that certain
geographic anomolies could be explained by continental movements. He called
the process continental drift.
After exploration of the ocean basins revealed long mountain ranges, and long
trenches, the theory of sea floor spreading was proposed in the 1960’s to
explain the geography of the ocean basins. We have previously examined the
general ocean basin geography in Exercise 4. Sea floor spreading exlplained the
evolution of the ocean basins by proposing that the ocean floor separated at the
mid ocean ridges, creating new ocean floor, and subsequent “subduction” at the
trenches destroyed ocean floor.
The two theories were merged into a unifying model to explain the evolution of
the earth’s crust called plate tectonics. In essence, the earth’s crust is divided
into a number of large “plates” that are rigid, and that move across the earth very
slowly creating plate boundaries.
These plate boundaries can be summarized as:
Mid Ocean Ridges – where plates separate, and new magma from the
mantle of the earth upwells to create new ocean floor.
Trenches – where one plate dives or “subducts” under another plate,
plunging into the mantle to be destroyed.
Transform Faults – where plates slide pst eachother laterally.
Each of these plate boundaries created geologic hazards. At the Mid Ocean
Ridges, there are volcanic activity and shallow focus earthquakes. At the
Trenches there are deep focus earthquakes and volcanic mountains and/or
islands in line with the trench. At transform faults, earthquakes are common.
Below is a map of the major plates of the lithoshere:
Critical evidence for plate tectonics comes from magnetic studies of the basalts
that make up the ocean plates. These basalts contain iron rich minerals that
retain the magnetism of the earth at the time they solidified. About every half
million years or so, the field reverses itself. Any basalts formed during a
reversed magetic field would have a “reversed” magnetic field, and this weak
difference can be detected by sensitive magnetometers. Such studies have
been made of all the oceans, and the pattern of magetic reversal has been
matched to the time scale for the reversals. By perfoming a distance / time
calculation, the spreading rates of the mid ocean ridges can be caclulated.
Here is a schematic sketch showing the major features of plate tectonics, and the
“magnetic stripes” generated at the ridges:
normal / reversed
basalts
How the reversals can be correlated to known time scale of reversals:
ACCRETION OF CONTINENTS
Continental rocks are lighter (lower density) than the plates. The
continents are said to “ride” the plates as they move. If a subduction zone
causes two continental masses to collide, they are not drawn into the mantle
because of the lower density. Instead, continental collisions cause the rocks to
weld together, and create exotic terranes at the edges of the continents. As an
example, examine the accreted terranes along the northwestern Canadian
continental margin:
QUESTIONS AND PROBLEMS
1. Name the type(s) of plate boundary (ridge, trench, transform fault) between the
Pacific and North American plates at each of the following locations.
Southern California _____________________
Northern California, Oregon, Washington ______________
Aleutian Alaska _________________
2. What type of plate boundary dominates the circum-Pacific belt?
3. In physics we talk about stress (force). What type of stress (tension,
compression, shearing) characterizes each type of plate boundary?
Ridge ___________
Trench ___________
Transform fault ____________
4. The schematic below is an “overhead” view of a mid-ocean ridge with a
transform fault. Draw in arrows showing the relative plate motions, and indicate
where earthquakes would occur due to shearing forces:
ridge
ridge
5. Carefully examine the diagram below. A time scale is provided, along with
correlated distances from the mid ocean ridge (right side of diagram) away from
the ridge for each ocean. Essentially, the field work has been dome for you; all
you have to do is choose a distance from the ridge for each of the oceans, use
the time scale to get the age of the rocks, and determine the spreading rate in
cm/year for each of the oceans. The assumption is that ocean floor at any
distance from the ridge was created at the ridge, and has moved due to sea
floor spreading. (Note – your calculations will only be for one side of each mid
ocean ridge. The actual spreading rate of each ocean will be twice your answer)
Useful stuff : there are 10,000 cm in one km
The time scale is in millions of years
South Atlantic (math setup):
Half spreading rate__________cm/yr
North Pacific (math setup)
Half spreading rate ___________ cm/yr
Pacific Antarctic (math setup)
Half spreading rate ____________cm/yr
6. The San Andreas fault exists because part of southern California is on the
North Pacific plate, and the rest of California (and the rest of North America) is
moving with the Atlantic plate. Assuming parallel motion (it’s not), what is the
relative movement along the San Andreas fault, based on your calculations?
Setup and work:
7. The Atlantic Ocean is about 2,800 km wide at the equator. Using twice your
spreading rate for the Atlantic from question 5, determine how many years it took
for the Atlantic to open to its present width:
Setup and work:
8. Examine the drawing below of the Pacific Ocean
EMPEROR
SEAMOUNTS
HAWAIIAN
ISLANDS
The Hawaiian Islands are volcanic, but do not seem to be associated with
any plate boundaries. They are caused by a “hot spot” in the mantle (refer to
class discussion). Looking carefully at the Hawaiian Islands AND the submerged
seamounts, they are in a line. The Emperor seamounts, however, are at a sharp
angle to the Hawaiian Islands. If they were created by the same hot spot, how
do you explain the sharp change in lineation of the two chains? (think about it)
9. If plate motion stopped, what would Earth eventually look like? That is, how
would it change physically, and what phenomena would increase, decrease, or
cease altogether?