11/14/17. Con;nental DriB Continents have slowly moved over geologic time

Transcription

1 GEOL 100 (Planet Earth) #23 - Grand Unifica;on Theory (s) Con;nental DriB Continents have slowly moved over geologic time Sea-floor Spreading Ocean crust forms at oceanic ridges, moves away from there, destroyed as it descends into mantle at trench Theory Tectonic plates slowly move around globe; revolu;onary idea, most important concept in geology Theory - Overview theory = outer por;on of Earth consists of number of thick slabs of rock (tectonic plates) that slowly move around globe; In map view, tectonic plate can consist en;rely of oceanic crust or both oceanic and con;nental crust. Tectonic plates change size over ;me; some get bigger, others get smaller Theory - Overview In cross-sec;on, tectonic plate = lithosphere - ~cool, outer rigid layer where rocks can break, ~100 km thick, includes en;re crust, outer part of mantle; Below lithosphere is asthenosphere - layer of ~hot, weak rock that flows, from ~100 - ~300 km in depth, only mantle rock 1

2 Theory - Overview Geological ac;vity (mountains, volcanoes, earthquakes) occurs mainly at tectonic plate boundaries. Earth s ac;ve volcanoes, earthquake epicenters Theory - Overview Boundaries of tectonic plates are defined by earthquake loca;ons Earth s mountain belts, volcanoes, tectonic plate boundaries Plate boundaries don't always occur at margins of con;nents, e.g., eastern North America Boundaries Three types of plate boundaries: Divergent, Convergent, Transform Plates move apart; oceanic ridge - undersea volcanic mountains and riy valley. Oceanic crust (basalt) produced at ridge due to rising magma ( pressure causes mel;ng); cooled rock moves laterally away from ridge. Mantle (low in silica and water) melts, producing non-explosive lava flows (pillow basalt). Get minor EQs as lava reaches surface. Divergent boundary/oceanic ridge wraps around globe Examples = Mid-Atlan;c ridge Iceland Oceanic ridge shown in red 2

3 Boundaries - Divergent Black smokers (hydrothermal vents) = very hot, undersea hot springs/geysers Black smokers = seawater sinks through cracks in oceanic crust, superheated and rises producing jet-stream of very hot water; black from sulfide minerals Black smokers = abundant life deep in ocean due to chemosynthe;c bacteria (energy from changing chemical oxida;on state) Video of Black smokers: haps:// (Miracle Planet - The Heat Within, 15:43-17:25) Op;onal video of Black smokers and associated life: hap:// (Greatest Discoveries/Science Channels, 2:30) Plates come together Subduc;on zone - old ocean lithosphere is subducted (descends back into mantle, melts), destroyed giant tube worms Oceanic trench (deep linear trough) = where subduc;on begins. Subducted crust (silica- and water-rich) melts producing explosive volcanoes (and volcanic mountain range); Ocean lithosphere subducted = amount created at ridge Many earthquakes (major, minor) as rocks grind past each other 3

4 Several kinds of convergent boundaries: a) Ocean/Con;nent Convergence = Subduc;on zone; denser, oceanic plate subducts below con;nental plate; con;nental crust (granite) less dense than ocean crust (basalt), con;nental plate not subducted Example = Andes Mountains Margin of Pacific Ocean = "Ring of Fire" (lots of EQs, volcanoes), convergent boundary at most loca;ons Example = Cascade Mountains b) Con;nent-Con;nent Collision Zone - If all ocean lithosphere subducts, get con;nent/con;nent collision; con;nental plate cannot subduct (~low density), tremendous mountain range with lots of deforma;on (faults, folds), major and minor EQs, no volcanoes. Boundaries - Convergent Con;nent/Con;nent Collision Zone Example = Himalaya Mountains 4

5 Boundaries - Transform Two plates grind past each other, horizontal mo;on; Major fault (transform fault); Not crea;ng or destroying crust, no mountains or volcanoes; Grinding two blocks of crust past each other, lots of EQs (major, minor). Boundaries - Transform Example = San Andreas Fault, CA s - Hot Spots Some volcanoes (Hawaii) occur in middle of tectonic plate; hot spot - localized zone of hot, upwelling mantle rock. s - Hot Spots Hot-spot volcanoes probably due to mantle plumes. As plate moves over hotspot, chain of ex;nct volcanoes ("hot-spot track") forms. Plate motion Mantle plume Moving lithosphere Asthenosphere Seamount of volcano 1 Extinct volcano 2 Active volcano 3 Time s - Hawaii Hot Spot Pacific tectonic plate moved over sta;onary hot spot producing chain of basalt volcanic islands (Hawaii), seamounts (Emperor); progressively older to northwest. Mo;on - Speed Plates move at speeds of 2-10 cm/year (by age of ocean crust, accurate posi;ons on Earth from laser-satellite data) 5

6 Mo;on - Over Geologic Time Plates move 2-10 cm/year; Over geologic ;me, Earth's surface constantly changing s Theory - Suppor;ng Data EQs, Volcanoes and Mountain Ranges (tectonic plate margins) - occurrence and origin Evidence for Con;nental DriY - fit of con;nents, paleoclimates, fossils, dis;nc;ve rock sequences Evidence for Seafloor Spreading - ocean floor bathymetry, polar wander paths, marine magne;c anomalies Many Other Data - occurrence and origin of sedimentary and metamorphic rocks Mo;on - Future Projec;ons Con;nent posi;ons 50 million years in future Mo;on - Future Projec;ons California won t fall into Pacific Ocean Mo;on - Future Projec;ons California 6