Harry Williams, Earth Science1 LECTURE 11. PLATE TECTONICS; DIASTROPHISM Continental Drift The theory of continental drift formulated by Alfred Wegener in the early 20th century is the forerunner to the modern theory of plate tectonics. It was based on the observation that many continents "fit together like a jigsaw puzzle" (e.g. S. America and Africa) and, if joined back together, many similarities exist in rock types, geologic structures and plant and animal fossils.

Harry Williams, Earth Science2 North and South America, Europe and Africa all fit together like pieces of a jigsaw puzzle. Mountain ranges, rock types and fossils also match up.

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4 PANGAEA Wegener proposed that 200 million years ago all continents were joined together in a supercontinent he termed PANGAEA (“All land”, consisting of N. America and Eurasia to the north - LAURASIA; and the southern continents to the south - GONDWANALAND). Pangaea broke apart and the continents drifted, like icebergs in the sea, "bulldozing" their way through the denser oceanic crust below.

Harry Williams, Earth Science5 Pangaea The continents today

Harry Williams, Earth Science6 There were two major criticisms of Wegener's theory - firstly, no explanation was given for the driving force behind continental drift, and secondly, the oceanic crust is far too dense and rigid to allow the continents to "push" their way through it. The theory was largely dismissed. The explanation for the drifting of the continents had to wait until the 1940's and 1950's when exploration of the ocean floors (offshoot of submarine detection) resulted in the new theory of plate tectonics.

Harry Williams, Earth Science7 PLATE TECTONICS Recap: the upper 70 km of MANTLE and CRUST = the rigid LITHOSPHERE; beneath this is the partially molten ASTHENOSPHERE.

Harry Williams, Earth Science8 The lithosphere is not continuous, but is fractured into a number of LITHOSPHERIC PLATES (about 7 major plates). The plates are moving, as shown by the arrows, at typical velocities of cm/year (fingernail growth). It is the movement of these plates and the interaction between them that is known as PLATE TECTONICS.

Harry Williams, Earth Science9 SEA FLOOR SPREADING BY CONVECTION The exact driving force of plate motion still isn't certain, but the most likely explanation is that the plates are driven by convection currents in the asthenosphere. Plates spread outwards from mid-oceanic ridges where new ocean floor is created, and return into the earth at subduction zones were plates sink and melt. Mid-oceanic ridgeSubduction zone Convection Sea-floor spreading

Harry Williams, Earth Science10 Much important geologic activity, including earthquakes and volcanoes, is found at PLATE BOUNDARIES 3 types: Divergent – plates move away from each other. Convergent – plates move towards each other. Transform fault – plates slide past each other.

Harry Williams, Earth Science11 Divergent. Mid-oceanic ridges; shallow earth quakes, volcanic eruptions -> new sea floor consisting of BASALT (magma from asthenosphere forms basalt). Plate movement is initiated at mid- oceanic ridges; continents are carried along on top of plates; supporting evidence from "hot spots".

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Harry Williams, Earth Science13 A hot spot is a stationary plume of rising magma from the Mantle, as this periodically erupts through the overlying plate, a chain of volcanoes is formed in the direction of plate motion. E.g. Hawaiian islands

Harry Williams, Earth Science14 Convergent. Subduction zones; earthquakes occur down the subduction zone; volcanism from melting of subducted plate e.g. Cascades, Mt. St. Helens. Possible collisions include: ocean plate - ocean plate > island arcs e.g. Japan, Aleutian Islands. Ocean plate - continental plate collision > volcanic arc e.g. Cascade Range; Andes. Continent-continent collision > mountains e.g. Himalayas.

Harry Williams, Earth Science15 Island Arc. E.g. Japan. Earthquakes take place down the Wadati- Benioff zone (part of subduction zone where earthquakes occur).

Harry Williams, Earth Science16 Volcanic Arc: Cascade Range. Juan de Fuca plate

Harry Williams, Earth Science17 Continent-continent collision; suture zone.

Harry Williams, Earth Science18 Transform fault - plates slide past each other; large earthquakes are common e.g. San Andreas fault.

Harry Williams, Earth Science Loma Prieta earthquake.

Harry Williams, Earth Science20 Distribution of magnitude 5+ earthquakes, The pattern closely matches plate boundaries, particularly subduction zones and transform faults.

Harry Williams, Earth Science21 Distribution of major volcanoes. The pattern closely matches plate boundaries, particularly subduction zones. Note the Pacific “Ring of fire”.

Harry Williams, Earth Science22 Diastrophism Central to the theory of plate tectonics is the idea that the crust is subject to both vertical and horizontal movements. These movements can deform the crust by tilting it, breaking it and bending it. Tilted Beds: angle of tilt is the dip - ranges from 0 (horizontal) to 90 o (vertical). Orientation of the outcrop is the strike e.g. north- south, east-west etc.

Harry Williams, Earth Science23 Steeply dipping rock strata.

Harry Williams, Earth Science24 Folded Beds: Rocks at depth under great pressure tend to bend rather than break when subjected to stress, producing folds. Main types = syncline, anticline, monocline, overturned fold. Folds are often exposed at surface by erosion of overlying rocks, typically ridges and valleys are formed e.g. Appalachians.

Harry Williams, Earth Science25 The ridges and valleys of the Appalachians are exposed folds.

Harry Williams, Earth Science26 Faults: Brittle rock (e.g. near surface) tends to break rather than bend when subjected to stress -> fault.

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Harry Williams, Earth Science29 (Same as a transform fault)

Harry Williams, Earth Science30 Sometimes, the vertical movement of rock in a fault can result in a cliff-like feature on the surface of the Earth - a FAULT SCARP.

Harry Williams, Earth Science31 Blocks of crust can be uplifted and depressed along faults forming GRABENS and HORSTS. e.g. Death Valley is a graben surrounded by horsts on either side.

Harry Williams, Earth Science32 Death Valley, California. The flat valley floor occupies a graben and is filled with sediment. The horst rises abruptly on the far side of the valley.