PLATE TECTONICS

This information should be a review of ideas and concepts taught in GE 141 or GE 131. As such, this is only a review of the concepts that will not be discussed in GE 142. A detailed discussion of Plate Tectonics that was found at the Universität Erlangen's Department of Geology Website or, if you'd prefer American-made, at the USGS (U.S. Geological Survey) in Reston.

1620 Sir Francis Bacon recognized that the outline of the southern hemisphere continents of South America & Africa were like pieces of a jig-saw puzzle.

1858 Pelligeini hypothesized N. Am./S. Am./Africa union.

1872 Elise Reclus believed that continents were in motion and this was related to mountain building, volcanism, & earthquakes.

Late 19^th C Eduard Suess reconstructed Gondwanaland based on distribution of tillites (glacial deposits).

1910 Taylor & 1912 Alfred Wegener postulated that present continents were once joined together. They were moved to their present positions and were not originally located at the same latitude & longitude.

Wegener used several independent data sets:

1. Rock sequences
2. Structures
3. Paleontological evidence:
1. Glossopteris, Lystosaurus (dicynodont); Mesosaurus (aquatic freshwater reptile); paleobiogeography through the Mesozoic
4. Paleoclimatic evidence
5. Geometrical fit
1. You can test the hypothesis of continental margin fit at an interactive site at Project Java, State University of New York, Binghampton.

Mechanism proposed to control continental movement:

• Taylor believed that forces associated with the rotation of Earth (rotational gravity & tides)
• Wegener believed that the continents could "sail" through the crust

Skepticism developed due to the problem of an appropriate mechanism.

1930 F.A. Vening Meinesz proposed the presence of convection currents in the crust after examining gravity surveys over trenches.

1950-1960 FAMOUS (French-American Midocean Undersea Survey) discovered paleomagnetism (magnetism imparted to ancient rocks at the time of crystallization). This is remnant magnetism.

• Examination of the paleomagnetic patterns across the ocean floor indicated that there were periods of varied intensity caused by reversals in the polarity of Earth's magnetic field (normal & reversed [where the sum of the remnant magnetism measured is less than the present strength and therefore weaker] & vice versa)

Magnetic remnants are oriented as stereopairs on either side of mid-oceanic ridges. Driving force related to the development of rising mantle plumes at one margin of a convection cell developed within the mantle (discussion concerning focus in asthenosphere or within the entire mantle or stacked convection cells).

Plates move along the top of the convection cell parallel to the direction of radial flow. The movement of heat in the Earth is governed by Geothermal Theory.

• Competing hypotheses include the "slab-pull" (sinking of the plate at a subduction zone) and
• "ridge-push" (spreading centers are high on the ocean floor, have low-density roots causing them to spread out transmitting push).

Independent Tests of Tectonic Theory

1. Paleontological: fossil planktonic organisms found in drill core were no older than 200 MY BP. Sea floor sediments were relatively thin and thinned towards the mid-ocean ridge. This trend was followed by an increasingly younger-age to the plankton.
2. Physical Measurement: during the Apollo Mission 3 reflectors were placed on the moon; the time it takes for laser beams "fired" from earth to the moon and return can be used to calculate the distance between the laser device & lunar reflector. Accuracy 3 cm.
3. Seismological Evidence: assessing the distribution of earthquake foci around the globe. Benioff seismic zones (seismic shear zone traced to depths of 700 km) mark the positions of subducted plates beneath overriding plate.
4. Gravity: measurements over deep sea trenches are low (negative gravity anomaly) because of low-density rock beneath the surface. Their presence reflects the fact that belts are underlain by rocks much lighter than those at depth on either side.
5. Hot Spots: chains of volcanic islands and seamounts (submerged volcanoes) aligned on the sea floor. Hawaiian islands formed over such a hot spot with the oldest radiometrically dated islands farthest from the source (40 MY BP).

Plate Boundaries

Divergent Boundaries

• Divergent boundaries display tensional geological structures (e.g., normal faulting & graben formation).
• New crust is added to the trailing edge of each separating plate.
• Crust is basalt and, hence, responsible for the formation of new ocean basins.

A spherical earth requires adjustment of a rigid crust. Therefore, offsets in the crust exist termed transform faults.

Convergent Boundaries

• Convergent boundaries are where leading edges of two plates come in contact with each other.
• High frequency of earthquakes and are associated with deep sea trenches (subducted ocean crust) or folded mountain ranges (compressional forces).
• Structural features of these zones are varied.

Tectonic behavior at convergent boundaries include:

1. Continent:continent resulting in folded mountains (igneous granite cores). Rocks of each are sutured together.
1. Example is the Himalayas
2. Ocean:ocean resulting in a deep ocean trench (rate of plate movements affects the features)
1. Example is found in southwestern Pacific
3. Continent:ocean resulting in development of a deep sea trench associated with volcanic terrains. Melange structures, ophiolite suites, & blue schist (blue amphiboles [high p, low t]) facies.
1. Example is the Andes.

Shear Plate Boundaries

• Where one plate moves laterally along another.
• High frequency of earthquakes due to stresses developed between two moving rock bodies.
• These are the active segments of transform faults where there is no new crust formed or old crust consumed.
• Example: the San Andreas fault.

Over the past 750 Million Years, the relative plate positions have changed. View an animation of Phanerozoic Plate Tectonics.

© Copyright 1998-2004 by Robert A. Gastaldo. All rights reserved. No part of these lecture notes may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the author.