Plate TectonicsChapter 3

Map is from United States Geological Survey

By 1700, suspicions that continents moved began to grow. Even Benjamin Franklin believed that continents drift. In 1855, Antonio Snider published a sketch showing how the Americas and Africa could fit together, jigsaw-puzzle fashion. In 1915, Alfred Wegener published the formal theory of continental drift.

Plate Tectonics

Continental Drift

• By 1700, map make had improved to the point that coastlines were mapped with high precision. Improved quality of maps contributed to the growing idea that continents were once connected, then drifted apart

Plate Tectonics

Tectonics is the study of large-scale movement and deformation of the earth’s outer layers

Plate tectonics relates such deformation to the existence and movement of rigid “plates” over a weak or partly molten layer in the earth’s upper mantle

Plate Tectonics

Continental drift, although controversial at first, was proven in 1960 with the discovery of paleomagnetic reversals in seafloor crust. This important discovery proved that sea floor crust was growing from spreading centers. Sea floor crustal growth causes continents to move. In 1965, the Canadian geologist, John Tuzo Wilson, coined the phrase, plate (piece) tectonics (motion; Greek).

Figure 3.6

Rock Response to Plate Tectonics

• Stress –force applied on a rock– Compressive stress – squeeze or compress

an object– Tensile stress – pull or stretch an object– Shearing stress – different parts of an object

move in different directions or at different rates

• Strain – results from stress; is the change in shape or size of an object because of the stress it experienced

Figures 3.2 a and b

Strain• Temporary or permanent• Elastic deformation – temporary strain,

object recovers original size and shape once the stress is removed– Elastic limit – strain that becomes permanent

in an object once limit of recoverable strain has been exceeded

– Plastic deformation occurs in materials once elastic limit has been exceeded

– Brittle deformation occurs at the limit of strength of the material, a rupture or a break occurs

Figure 3.3

Lithosphere and Asthenosphere

• Earth’s crust and upper most mantle are solid and compose the lithosphere– Stresses cause brittle and elastic deformation

• Beneath the lithosphere is a plastic layer called the asthenosphere

• Lithospheric plates can move over this plastic layer; plate tectonics plausible

• Boundaries of the plates are active with earthquake and some with volcanic activity

Figure 3.4

Evidence for Plate Tectonics• Earthquakes and volcanoes• Sea Floor topography

– Trenches– Ridges

• Paleomagnetism• Magnetic patterns imprinted on oceanic crust• Curie temperature• Magnetic reversals• Magnetic polar wandering curves

• Sea Floor Spreading• Age of the sea floor• Other evidence

– Fit of continents, GPS data, and more …

Figure 3.5

Figure 3.7

Figures 3.8 a and b

Figure 3.9

Figure 3.10

Other Evidence for Plate Tectonics• Distribution of rocks representing ancient

deserts, sea shores, tropical areas, glaciated areas, swamps, and equatorial regions

• Location of fossils that were originally restricted in their distribution but now separated by oceans and on separate continents

• Fit of continents reveal super continent of Pangaea

• Recognition of plate boundaries

Figure 3.13

Figure 3.14

Mesosaur

• Mesosaur from a quarry near Sao Paulo, Brazil. This fossil is that of an adult. The size of the specimen is approximately 35 cm across. Photo by J. Carr.

Glossopteris

• Glossopteris. This fossil of a now-extinct plant is from Australia and is about 15 cm across. Photo by J. Carr.

Figure 3.6

Figures 3.15 a, b, and c

Plate Boundaries• Divergent Plate Boundary

– Lithospheric plates move apart; form oceanic ridges

– Upwelling of asthenosphere injects magma forming oceanic ridges and new oceanic crust

– Forces plates apart– Sea floor spreading occurs

• Transform Boundaries – short segments of a ridge– Transform faults offset ridge– San Andreas Fault – transform fault under

continental crust

Figures 3.16 a and b

Plate Boundaries• Convergent Plate Boundaries

– Lithospheric plates move toward each other– Higher density oceanic crust overridden by

low density continental crust– Subduction zone forms and produces a trench– Subduction of older oceanic crust balances

the spreading seafloor equation– Subduction zones are active geologic places

• Volcanism• Earthquakes• Island arc formation

Figures 3.18 a, b, and c

Figure 3.19

Tectonics• Convection cells operate in mantle• Upwelling of heat and magma occurs at

divergent plate boundaries– New oceanic crust formed– Oceanic crust pushed away from spreading centers

• Hot spots located independent of plate boundaries– High heat flow radiate from them– Volcanic activity associated with them

• Hawaiian Islands• Yellowstone

Figure 3.20

Figure 3.21

Figure 3.22

Figure 3.23