Plate tectonics

Part One - Major features of the earth

Elevations and the crust

(a) Name three ways that the crust in the ocean basins differ from the crust of the continents.

(b) Why does oceanic crust produce low elevations and continental crust produce high elevations?

Active vs. passive margins

Not all of the continent-ocean margins are plate tectonic boundaries. The character of margins depends on whether there is an active plate boundary (active margin) or no present-day plate boundary (passive margin). For example, the east coast of U.S., a passive margin, has a broad flat shallow continental shelf that grades into the continental slope and rise. In contrast, active margins at convergent boundaries may have a narrow shelf (or none at all) that drops abruptly into a narrow very deep trench (for example the west coast of South America). The trench is where the subducting plate is bent beneath the overriding plate as it is forced beneath. Trenches may also occur where two oceanic plates converge at the plate boundary.occur at

(a) Based on the character of the shelf (figure 18.3) name the type of plate boundary - active subduction zone or passive

South coast of Australia

West coast of Africa

Southwest coast of India

Aleutian islands

Southeast coast of South America

(b) Some active margins also have a transform fault at the continent-ocean boundary.

Give one example:

Mountain chains

Some of the major actively forming mountain chains of the world are the Himalayas, Andes, and the European Alps.

(a) On the basis of these examples, at what type of plate boundary do mountain chains form?

(b) Except for a small segment (where the small Juan de Fuca plate is subducting beneath the pacific northwest), the west coast of North America is a transform boundary. Given your answer to question (a), then how can you explain the presence of a very large mountain chain along the west coast of the U.S. and Canada.

Part two - Calculating rates of plate motions

Problem 1 - refer to the diagram and text on the following page assume that the hot spot is fixed

(a) What is the direction of movement of the Pacific plate over the last 43 million years?

(b) What is the average rate of motion of the Pacific plate over the last 43 million years?

(c) What is the direction of movement of the Pacific plate between 43 and 73 million years?

(d) What is the average rate of motion of the Pacific plate between 43 and 73 million years?

(e) What does the bend in the volcanic chain tell you about plate motions?

Problem 2

Using the magnetic seafloor stripes shown in the map combined with the time scale showing magnetic reversals, calculate the following: HINT:

(a) What is the average rate at which the 2.50 million year old oceanic crust has moved away from the ridge axis since it formed? Answer in both km/my and cm/year

(b) What is the rate at which points A and B have been moving apart over the last 2.50 million years?

(c) Was the spreading rate faster before or after 1.75 Ma? Explain

(d) Below the diagram draw the way the magnetic stripes would look if the spreading rate decreased to the south-south east.

North