Questions and Topics

Plate Tectonics and Continental Drift

Questions and Topics 1. What are the theories of Plate Tectonics and Continental Drift? 2. What is the evidence that Continents move? 3. What are the forces that drive plate tectonics? 4. What happens at the boundaries between plates? 5. How do the different types of plate boundaries impact the regional geology and geomorphology? 6. How has continental drift affected the positions of the continents over time?

Plate Tectonics Tectonics Movement of Earth s crust Plate tectonics Movement of discrete segments of Earth s crust in relation to one another 4

Continental Drift Alfred Wegener (1880-1930) Proposed that all of the continents were once part of a large supercontinent - Pangaea Based on: Similarities in shorelines Distinctive rock and fossil groups found in Africa & South America

Continental drift maps by Wegner (1915)

Continental drift maps by Wegner (1915)

Continental Drift Wegner proposed a mechanism for drift Less dense silicic rocks plowed through more dense ocean floor Earth s rotation was driving force Although supporting evidence existed, the theory was not widely accepted

Evidence for Continental Drift Paleontological Similarity of fossils on opposite sides of the Atlantic Ocean Plants and land dwelling animals No mechanism to transport across ocean Glossopteris on all southern continents Divergence of species following break-up

Paleontological evidence

Evidence for Continental Drift Rock type & structures Distinct rock type and geologic structures on both sides of the Atlantic Ocean Cape fold belt and equivalent S.Africa & Argentina Appalachian Mtns and equivalent U.S., Canada, Scotland & Norway Only occur in rocks > 145 mya

Continental Drift Geologic Similarities Brazil and South Africa have nearly identical geologic sequences Similar in Antarctica and India Glacial sediments Coal 12

Rock type & structure evidence

Evidence for Continental Drift Glaciation Late Paleozoic glaciation Covered large portions of the southern continents Distinct glacial deposit Glacial striations indicate direction of movement No evidence for glaciation on northern continents at this time

Reconstruction from glacial deposits

Evidence for Continental Drift Paleoclimate Evidence of extreme changes in climate as compared to the present Coal deposits in Antarctica Evidence from evaporite deposits, eolian deposits & coral reefs Paleoclimate reconstruction shows strange patterns unless continents are moved

Fig. 17.6. Paleoclimate evidence

Paleomagnetism Magnetization of ancient rocks at the time of their formation Declination Angle that a compass needle makes with the line running to the geographic north pole Rocks lock in this orientation at formation 20

Reconstruction from paleomagnetic data

Geology of the Ocean Floor Paleomagnetism Fe rich rocks are weakly magnetized by the Earth s magnetic field as minerals form Orientation of magnetic field is preserved Magnetic field orientation varies with position on Earth s surface

Modern Plate Tectonic Theory Original evidence for continental drift was from continental rocks Technological advances in the 1950 s and 1960 s allowed investigation of the sea floor Geophysics & paleomagnetism provided new data

Geology of the Ocean Floor Topography of the ocean basins Basins are divided by a large ridge system Ridge system is continuous around the entire globe Central rift valley within the ridge

Geology of the Ocean Floor Physical properties Composed of basalt Younger in age than most continental rocks Oceanic crust is thinner than continental No evidence of crustal deformation folded mountains

Crustal Properties Crust Density Composition Thickness Age continental ~2.8 g/cm 3 Felsic Thick: 20-70 km Old: up to 4 Byrs oceanic ~3.2 g/cm 3 Mafic Thin: 2-10 km Young: <200 Mys

Geology of the Ocean Floor Seafloor spreading proposed by Hess (1960) Considered new data on ocean floor Proposed mechanisms of: Mantle convection Rifting and volcanism along ridge system Continents pushed along w/ spreading seafloor Recycling of oceanic crust by subduction

Geology of the Ocean Floor Vine & Matthews (1963) tested Hess s hypothesis using magnetism Magnetic polarity reversals recorded in ocean floor basalt Magma cools forming new crust Polarity at time of cooling preserved Old crust pushed aside

Geology of the Ocean Floor Magnetic polarity stripes in ocean crust parallel ridges Symmetrical on either side of the ridge Polarity chrons give age of seafloor Increases away from ridge Rates of plate motion may be calculated

Fig. 17.10. Patterns of magnetic reversals

The Mid Atlantic Ridge

Age of the sea floor

Geology of the Ocean Floor Seafloor sediments support plate tectonic theory Youngest sediments resting directly on basalt near the ridge Sediment just above the basalt gets older moving away from the ridge Accumulation rates of ~3 mm/1000 yr

Plate Geography Lithosphere is divided into individual plates Boundaries based on structural features, not land and ocean Plates are outlined by ridges, trenches and young mountain belts Plates are not permanent features

Major tectonic boundaries

Divergent Plate Margins Oceanic-Oceanic Crust Mid-oceanic ridge with central rift valley Shallow earthquakes, less than 100km Basaltic lavas

Fig. 17.15. Divergent plate margins

Passive Continental Margin

Size comparison of various volcanic features

Divergent Plate Margins Continental-Continental Crust Rift Valley Shallow earthquakes, less than 100km Basaltic and Rhyolitic volcanism New material rising from the mantle produces basaltic lavas Thinning continental crust melts to produce rhyolitic lavas & instrusions East African Rift Valley

Convergent Plate Margins Oceanic-Oceanic Seafloor Trench Shallow and deep earthquakes, 0-700 km deep Andesitic volcanoes in an island arc Japan

The Aleutian Island Chain

Seismic activity in the Aleutian Islands

Oceanic-Oceanic and Oceanic-Continental Subduction

Convergent Plate Margins Oceanic-Continental Subduction Zone Shallow and deep earthquakes, 0-700 km deep Andesitic volcanoes in a continental arc Cascade range

Convergent Plate Margins Continental-Continental Intensely folded and thrust faulted mountain belts Metamorphic rocks dominate Sediments accumulated along continental margin are squeezed Igneous rocks commonly included Granitic magmas

Convergent plate boundaries

Transform Fault Margins Transform faults are large vertical fractures or faults in the crust Movement along faults is side to side May extend for long distances In oceanic crust, deep valleys are formed Transform faults may extend onto continents San Andreas fault

Juan de Fuca plate

Rates of Seafloor Spreading FAST SLOW (East Pacific Rise) (Mid Atlantic Ridge) ~10-20 cm/year ~1-2 cm/year Life of a person 100 years 10 meters 1-2 meters Civilization 10,000 years 1 km 100-200 m Modern Humans 100,000 years 10 km 1-2 km Stone tools 1,000,000 years 100 km 10-20 km Width of the Pacific Ocean ~ on the order of 10,000 km (16,000 miles) wide. How long would it take to create this much ocean crust.

Rates of Plate Motion Two ways to look at plate motion Relative velocity the movement of one plate relative to another Age of seafloor / distance from ridge Absolute velocity compares plate movement to a fixed position Use hotspots as fixed points of reference Rates vary from 1 to 20 cm/yr

Fig. 17.20. Rates of plate motion around the world

Where do we see deep earthquakes? What is happening there?

Tectonic Mechanisms Convection of heat from the core and mantle drives tectonics Convection cells bring new material to the surface Old crust is pushed away from ridges Subduction carries cool crust back into the mantle

Fig. 17.21. Models of plate tectonic motion

Tectonic Mechanisms Plates are active participants in the convection process Slab pull dense ocean crust descends under its own weight Ridge push gravity pulls lithosphere down & away from ridge Friction resistance to movement from various sources

More evidence. Mantle Plumes and Hot Spots Mantle plumes may form hot spots of active volcanism at Earth s surface Approximately 45 known hotspots Hot spots in the interior of a plate produce volcanic chains Orientation of the volcanic chain shows direction of plate motion over time Age of volcanic rocks can be used to determine rate of plate movement Hawaiian islands are a good example

Fig. 22.1. Hawaiian Island chain

The World s Hot Spots

Fig. 22.21. Cenozoic features of NW U.S.

Plate Motion GPS Global Positioning System Earth-orbiting satellites identify motion Transmitter on satellite Ground-based receiver Average rate 5 cm/year 80

Tectonic setting Rock/sediment type Lavas and pyroclastics Basalts (Ophiolites) Marine sediments (cherts, limestones, red clays) Turbidites, clays, silts, sands Granites, Rhyolite and pyroclastics Mafic Felsic

Composition of the Ocean Crust Seismic surveys suggest oceanic crust is ~7 km thick and comprised of three layers First layer is marine sediment of various composition and thickness (extensively sampled) Second layer is pillow basalt overlying basaltic dikes (extensively sampled) Third layer is thought to be composed of sill-like gabbro intrusions (not directly sampled) Ophiolites are rock sequences in mountain chains on land that are thought to represent slivers of ocean crust and uppermost mantle Composed of layers 1-3 overlying ultramafic rock

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