Plate Tectonics and Continental Drift
There are numerous seams on the surface of the Earth
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?
Answers 1. Large crustal plates at the Earth s surface move about, colliding with one another. 2. There is geographic, geomagnetic, paleontologic and other evidence that this occurs 3. Convection in the mantle is the main driver of plate movement 4. Neighboring plates move relative to one another, causing earthquakes and volcanic eruptions 5. Active plate boundaries produce mountains and trenches 6. Continents have changed position
Plate Tectonics Tectonics Movement of Earth s crust Plate tectonics Movement of discrete segments of Earth s crust in relation to one another 5
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)
Wegener s Pangea Modern reconstruction of Pangea Continental drift maps by Wegner (1915)
Continental Drift Wegner mechanism for drift was not credible Less dense silicic rocks (the continents) plowed through more dense ocean floor Earth s rotation was driving force Other scientists didn t buy it
What is the evidence for Continental Drift? 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 and similar rock types 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!!!!!!!!!
Rock type & structure evidence
Evidence for Continental Drift Glaciation Late Paleozoic glaciation Covered large portions of the southern continents Distinct glacial deposit No evidence for glaciation on northern continents at this time
Next homework is to recreate this figure P r e c a m b r i a n Eon P h a n e r o z o i c Proterozoic Archean Hadean Era Period Age (Myrs) Epoch C e n o z o i c M e s o z o i c P a l e o z o i c Geologic Time Scale Tertiary Quaternary Cretaceous Jurassic Triassic Permian Pennsylvanian Mississippian Devonian Silurian Ordivician Cambrian Neogene Paleocene Age of the Earth 4600 Myrs (4.6 Byrs) Source: Geological Society of America (1999) 0.01 1.8 5.3 23.8 33.6 54.8 65 144 206 248 290 323 354 417 443 490 543 2500 3800 Holocene Pleistocene Pliocene Miocene Oligocene Eocene Paleocene
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, which 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
Fig. 17.21. Models of plate tectonic motion
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
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
The Mid Atlantic Ridge
Line of fissures and volcanoes in Iceland
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.
East Pacific Rise Fast spreading
Ridges subside and are covered with sediment
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
JuandeFucaSmoker.avi
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 Example: East African Rift Valley
East Africa East African system extends 3000 km from Ethiopia to Mozambique Red Sea rift extends from Ethiopia to the Sinai and Dead Sea Complex volcanism throughout Oceanic crust present in SE Red Sea Lakes form in isolated down-dropped blocks Several areas below sea level
Fig. 19.33. The Red Sea
A transform fault in the Gulf of Aqaba
The parting of the Red Sea The Red Sea
Fig. 17.15. Divergent plate margins
Convergent Plate Margins Oceanic-Oceanic Seafloor Trench Shallow and deep earthquakes, 0-700 km deep Andesitic volcanoes in an island arc Example: Japan
Ocean-Ocean convergence
The Aleutian Island Chain
Seismic activity in the Aleutian Islands
The MarianaTrench http://www.ngdc.noaa.gov/mgg/image/2minrelief.html
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 Example: Cascade range
Ocean-Continent convergence
Major tectonic boundaries
Mt. Vesuvius
The people of Pompeii; mummified in 5-8 meters of hot ash in A.D. 79
The smoldering city of Pierre, Martinique
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 Example: The Himalayas
Continent-Continent Collision
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 Example: San Andreas fault and transform faults in the ocean
Fig. 19.29. Basin & Range Province
Juan de Fuca plate
Fig. 20.12b. Landforms along the San Andreas
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