University of Colorado GEOLOGY 1010

Class Note 20

Plate Tectonics

Continental Drift and Sea-Floor Spreading

• The geometric fit of the continents.
• The similarity in rock age groups between adjoining regions.
• The similarity in Paleozoic fossils between adjoining areas.
• The distribution of Paleozoic glaciation in S.America, S.Africa, Australia, and India.

Although Wegener compilied solid observational evidence for the concept, he failed to articulate a mechanism by which the continents could move over a mantle that was solid enough to transmit S-waves.

The notion of continental drift gained support in the late 40s and 50s with paleomagnetic evidence for polar wandering along different paths for different continenets.

Paleomagnetics is the study of rock magnetism. Igneous rocks contain a small percentage of ferromagnetic minerals such as magnetite (Fe₃O₄), which become magnetized by the Earth's megnetic field when they cool through a critical temperature. This means that these rocks will record the direction of the field at the time they cool. If the rocks are not tilted or deformed, they can be used to record the latitude at which they were formed. The magnetized minerals also perturb the local magnetic field giving rise to magnetic anomalies.

The concept of continental drift only gained general acceptance after symmetric magnetic anomalies about the Mid Atlantic Ridge were reported in the late 1960's. These anomalies were not only symmetric about the ridge crest, but the pattern matched that of magnetic reversals known from continental basalt lava flows.

After these data were reported, different research groups rushed to test the hypothesis by as many methods as possible.

• Rocks in adjoining parts of Africa and South America were age-dated using new radio-isotopic methods: they matched perfectly.
• Rocks were dredged from the sea floor: they showed ages symmetrically increasing away from the ridge. There were no old (>250MY) rocks.
• The fit of continents was revised using the edge of the continental slope rather than the coastline: the match was nearly perfect.
• Seismic evidence suggested that crust was being subducted (returned to the mantle) at convergent boundaries to balance crust production at the ridges.
• Hess proposed a plausible mechanism for the driving force that moved continents: thermal convection in the solid mantle.

Plate Tectonics

• There appear to be 13 major plates that cover the globe.
• The plates can contain oceanic, or continental crust or both.
• New ocenic crust is created at the mid-ocean ridge (divergent boundary).
• Old ocenic crust is consumed ( subducted) at convergent plate boundaries.
• Continental crust resists subduction.
• Continent-continent convergent boundaries form major mountains.
• Ocean-ocean and ocean-continent convergent boundaries form subduction zones marked by deep ocean trenches and Benioff Zones (deep earthquake zones extending to 670km).

Class Note 21

Class Note 19

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