3-1. Continental Drift

A. Based upon the fit of continental outlines and fossil and geologic evidence, Alfred Wegner proposed his hypothesis of continental drift. According to Wegner, the continents are sections of a past super continent called Pangea, which broke apart and the fragments plowed through the oceanic crust ,.to their present locations

3-2. Sea-Floor Spreading

B. Sea floor spreading demonstrates that the sea floor moves apart at the oceanic ridges and new oceanic crust is added to the edges.

1. Rift valleys along oceanic ridge crests indicate tension, are bounded by normal faults, and are floored by recently-erupted basaltic lava flows.

2. Axis of the oceanic ridge is offset by transform (strike-slip) faults which produce lateral displacement.

3. Whereas oceanic ridges indicate tension, continental mountains indicate compressional forces are squeezing the land together.

C. The geomagnetic field is the magnetic field of the Earth.

1. Magnetometers detect and measure Earth’s magnetic field.

2. Moving across the ocean floor perpendicularly to the oceanic ridges, magnetometers alternately record stronger (positive) and weaker (negative) magnetic fields (called magnetic anomalies) in response to the influence of the sea floor rocks.

3. Magnetic anomalies and the rocks causing them form parallel bands arranged symmetrically about the axis of the oceanic ridge.

4. As basaltic rocks crystallize, some minerals align themselves with Earth’s magnetic field, as it exists at that time, imparting a permanent magnetic field, called paleomagnetism, to the rock.

5. Periodically Earth’s magnetic field polarity (direction) reverses poles.

D. Because of their paleomagnetism, rocks of the sea floor influence the magnetic field recorded by magnetometers.

1. Rocks on the sea floor with normal polarity paleomagnetism locally reinforce Earth’s magnetic field making it stronger and producing a positive anomaly.

2. Rocks on the sea floor with reverse paleomagnetism locally weaken Earth’s magnetic field, producing a negative anomaly.

3. Rocks forming at the ridge crest record the magnetism existing at the time they solidify.

4. Sea floor increases in age away from the ridge and is more deeply buried by sediment because sediments have had a longer time to collect.

5. Rates of sea-floor spreading vary from 1 to 10 cm per year for each side of the ridge and can be determined by dating the sea floor and measuring its distance from the ridge crest.

6. Continents are moved by the expanding sea floor.

3-3. Global Plate Tectonics

E. Because Earth’s size is constant, expansion of the crust in one area requires destruction of the crust elsewhere.

1. Currently, the Pacific Ocean basin is shrinking as other ocean basins expand.

2. Destruction of sea floor occurs in subduction zones.

3. Seismicity is the frequency, magnitude, and distribution of earthquakes. Earthquakes are concentrated along oceanic ridges, transform faults, trenches and island arcs.

4. Tectonism refers to the deformation of Earth’s crust.

5. Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench downward in the direction of the island arc.

d. Subduction is the process at a trench whereby one part of the sea floor plunges below another and down into the asthenosphere.

F. Earth’s surface is composed of a series of lithospheric plates. Plate edges extend through the lithosphere and are defined by seismicity.

1. Plate edges are trenches, oceanic ridges, and transform faults.

2. Seismicity and volcanism are concentrated along plate boundaries.

3. Movement of plates is caused by thermal convection of the "plastic" rocks of the asthenosphere which drag along the overlying lithospheric plates.

4. Mantle plumes originate deep within the asthenosphere as molten rock which rises and melts through the lithospheric plate forming a large volcanic mass at a "hot spot."

G. Wilson Cycle refers to the sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins.

- Stages in basin history are:

1. Embryonic—rift valley forms as continent begins to split.

2. Juvenile—sea floor basalts begin forming as continental sections diverge.

3. Mature—broad ocean basin widens, trenches develop, and subduction begins.

4. Declining—subduction eliminates much of sea floor and oceanic ridge.

5. Terminal—last of the sea floor is eliminated and continents collide forming a continental mountain chain.