Plate Tectonics Continental Drift
Developing the Theory • (1) demonstration of the ruggedness and youth of the ocean floor; • (2) confirmation of repeated reversals of the Earth magnetic field in the geologic past; • (3) emergence of the seafloor-spreading hypothesis and associated recycling of oceanic crust; and • (4) precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges.
Developing the Theory (continued) • (1) demonstration of the ruggedness and youth of the ocean floor. • (See next slide)
Plate Motions • There are four types of plate boundaries: • Divergent boundaries -- where new crust is generated as the plates pull away from each other. • Convergent boundaries -- where crust is destroyed as one plate dives under another. • Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other. • Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are unclear.
Mid-Atlantic Ridge Example Red triangles denote active volcanoes
Aerial view of the area around Thingvellir, Iceland, showing a fissure zone (in shadow) that is the on-land exposure of the Mid-Atlantic Ridge.
Divergent Boundary – East Africa Map of East Africa showing some of the historically active volcanoes(red triangles) and the Afar Triangle (shaded, center) -- a so-called triple junction (or triple point), where three plates are pulling away from one another
Continental-continental convergence The collision between the Indian and Eurasian plates has pushed up the Himalayas and the Tibetan Plateau
The Himalayas: Two Continents CollideThe 6,000-km-plus journey of the India landmass (Indian Plate) before its collision with Asia (Eurasian Plate) about 40 to 50 million years ago. India was once situated well south of the Equator, near the continent of Australia.
Transform Boundaries The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many fracture zones (transform faults) that scar the ocean floor and offset ridges (see text). The San Andreas is one of the few transform faults exposed on land.
San Andreas fault Aerial view of the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo.
Rates of Motion • The Arctic Ridge has the slowest rate (less than 2.5 cm/yr) • The East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).
The Long Trail of the Hawaiian Hotspot Map of part of the Pacific basin showing the volcanic trail of the Hawaiian hotspot-- 6,000-km-long Hawaiian Ridge-Emperor Seamounts chain.
Plate Tectonics & People • Natural hazards • Earthquakes • Volcanic eruptions • Tsunamis • Natural resources • Fertile soils • Ore deposits • Fossil fuels • Geothermal energy
Earthquakes Aerial view, looking north toward San Francisco, of Crystal Springs Reservoir, which follows the San Andreas fault zone.
Earthquakes (continued) Map of the San Andreas and a few of the other faults in California, segments of which display different behavior: locked or creeping
Fault Creeping Left:Creeping along the Calaveras fault has bent the retaining wall and offset the sidewalk along 5th Street in Hollister, California (about 75 km south-southeast of San Jose). Right:Close-up of the offset of the curb.
Earthquakes & Volcanoes • Christopherson TextChapter 12 pp 375-end-of-chapter
Epicenter and Focus • Focus • Location within the earth where fault rupture actually occurs • Epicenter • Location on the surface above the focus
Types of Faults Faults are classified on the basis of the kind of motion that occurs on them • Joints - no movement • Strike-slip - horizontal motion (wrench faults)
Types of Faults • Joints - No Movement • Strike-Slip - Horizontal Motion (Wrench Faults)
Right Lateral Strike Slip San Andreas 21 feet in 1906
Dip-Slip - Vertical MotionNormal Fault (Extension) Alaska, 1964 - up to 150 Ft
Seismic Risk Level Maps for the U.S.Probable ground acceleration in 50 years. Blue = small, red = large
Seismic Risk Level Maps for the U.S.Probability of Damage in 100 Years. Blue = Negligible, Green = Low, Red = High.
M 7.9 Earthquake on November 3, 2002 • The largest earthquake known to occur in the world this year struck central Alaska on Sunday, November 3. The epicenter of the Nov. 3 temblor was located approximately 75 miles south of Fairbanks and 176 miles north of Anchorage. It struck at 1:12 PM local time, causing countless landslides and road closures, but minimal structural damage and amazingly few injuries and no deaths.