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Transcript
PLATE TECTONICS BHS Earth & Space Science EVIDENCE OF CRUSTAL MOVEMENT • The rock layers at Earth’s surface are constantly in motion. • Principle of original horizontality – all rocks form in horizontal layers called strata (singular: stratum). • But we often see deformed strata. • (Oh noes!) STRATA EVIDENCE OF CRUSTAL MOVEMENT • There are three types of deformed rock. • 1) Folded rock strata • Lateral forces push together and deform folded rock either upwards or downwards. • An anticline forms an arch. A syncline forms a bowl-like depression. SYNCLINE EVIDENCE OF CRUSTAL MOVEMENT • 2) Tilted rock • Strata tilt angularly upwards or downwards. TILTED ROCK EVIDENCE OF CRUSTAL MOVEMENT • 3) Faulted rock • A fault is where a large crack occurs in the rock and the strata move up or down along the fault. • Normal fault • Hanging fault TOP: NORMAL FAULT BOTTOM: HANGING OR REVERSE FAULT EVIDENCE OF CRUSTAL MOVEMENT • Location of fossils in sedimentary rock reveals that the Earth’s crust is dynamic, e.g. marine fossils on Mt. Everest. • Benchmark elevation markers are placed all over the world. Measurements taken over time reveal that positions of the markers are changing. • Satellite data can show how fast tectonic plates are moving. EVIDENCE OF CRUSTAL MOVEMENT • A geosyncline: a large shallow ocean basin where sediments accumulate near the edges of the continents. • The weight of the sediments causes continent to rise upward. GEOSYNCLINE EVIDENCE OF CRUSTAL MOVEMENT • Isostasy: the Earth’s crust is in a state of equilibrium. The lighter continents float on the mantle, and the denser ocean basins sink into the mantle. • If a plate sinks somewhere, a plate somewhere else must rise. • It’s like squeezing a water balloon. ISOSTASY CONTINENTAL DRIFT • Alfred Wegener, a meteorologist, in 1912 formulated a theory called continental drift that explained how continents were connected and drifted apart. • He studied fossils from Africa and South America and found many of the same species (indicator species). DISTRIBUTION OF THE FERN GLOSSOPTERIS CONTINENTAL DRIFT • Geologists shrugged and hypothesized that ancient land bridges connected the continents but disappeared long ago. • Rock types were also similar and scars were left on rocks by glaciers, with the direction of the scarring matching on both continents. CONTINENTAL DRIFT • Wegener’s theory was rejected for two reasons: • He was a meteorologist, not a geologist. People can be protective of their niche. • He failed to provide the mechanism for drift – what could move a continent? • His theory was dismissed, and he died in 1930. In 1960, it turned out he was right. Oops. SEAFLOOR SPREADING • After WWII, the US military used sonar technology (developed to find enemy submarines) to take a look at the ocean floor. • Underwater mapping revealed mountain ranges called mid-ocean ridges. • New crust was being formed at the center and moving outward from the ridge. SEAFLOOR SPREADING • Evidence included alternating bands of magnetic crystals – and magnetic north and south reversed periodically. SEAFLOOR SPREADING • Scientists dated rock from mid-ocean ridges and the older rocks were farther from the ridge center and cooled as they moved outward. SEAFLOOR SPREADING • This indicated large convection cells within the Earth’s mantle, and proved Wegener’s theory right. THE THEORY OF PLATE TECTONICS • Earth’s surface is composed of large, moving plates, floating on semi-liquid asthenosphere. • The plates’ edges are called plate boundaries, and can be detected by presence of volcanoes and earthquakes. • 15 major plates and many more minor ones. They move 1 to 10cm/yr. DIVERGENT PLATE BOUNDARIES • Two plates moving away from each other that create new crust. • Divergent boundaries form mid-ocean ridges underwater, and rift valleys on land. CONVERGENT PLATE BOUNDARIES • When two boundaries collide: 3 types. • Continental-Continental: forms mountains. • Oceanic-Oceanic: Subduction forms trenches and volcanic arcs. • Oceanic-Continental: Subduction forms trenches and volcanic mountain ranges TRANSFORM PLATE BOUNDARIES • When two plates slide along one another (a lateral fault) • Great tension is built up along the fault due to friction, then released as an earthquake! – San Andreas fault, CA – Golden fault, CO MANTLE CONVECTION • Heated material rises, spreads out, cools, then sinks. • The Earth is like a giant lava lamp. With people and stuff. • Hot spots: where magma rises and breaks through crust. – Some are along plate boundaries (mid-ocean ridges), others are in the middle of plates. – Yellowstone, Iceland & Hawaii MANTLE CONVECTION HOT SPOT A SPOT WHERE MAGMA REACHES THE SURFACE IN THE CENTRAL PART OF A PLATE INSTEAD OF ALONG THE EDGE WHICH STATE IN THE U.S.A IS A RESULT OF A HOT SPOT UNDER A MOVING PLATE? WHICH NATIONAL PARK IN THE U.S.A IS A RESULT OF A HOT SPOT UNDER A MOVING PLATE? SEVERAL PLATE BOUNDARY FEATURES