Earth overall average density = 5.5 g/cm 3 Temp increases with depth, the thermal gradient 30 0 C/km Pressure and the density also increase with

Transcription

1 Plate Tectonics

2 Earth Earth overall average density = 5.5 g/cm 3 Temp increases with depth, the thermal gradient 30 0 C/km Pressure and the density also increase with depth Spheroid: with a longer major axis along the equator, a shorter minor axis in polar direction The surface relief is around 12 miles

3 Major Landform Features Continent Shields: the extensive flat region of complexly deformed ancient crystalline rocks (igneous & metamorphic basement complex) formed b/w 1-4 BYO Stable Platforms: The areas have been stable during the last 500 MYO Mountain belts: the rugged terrain, the most active geologic regions, typically along margins of continents Ocean Floor The oceanic ridge: The most striking feature with peaks rise about 2 miles and extends approximately 70,000km (more than 40,000miles), and a central cleft runs through the axis of the ridge Trenches: the lowest areas on the earth, adjacent to chains of volcanoes Continental t Margins: transition zone b/w continent t and ocean basin, including continental shelf and continental slope Abyssal floor: the broad deep ocean basins (abyssal hills & abyssal plains)

4 Earth Interior Structure (1) Through h geophysical studies of gravity, density Particularly through seismic waves, Abrupt changes in seismic-wave velocities that occur at particular depths suggests that Earth must be composed of distinct layers Through geochemical studies of rocks Through the studies of meteorites Two basic models of earth s interior by chemical & physical properties

5 Earth Interior (2) Earth interior: Three principal layers by chemical properties Crust Mantle Core Earth interior: i five major divisions i i by physical properties Lithosphere Continental crust, oceanic crust Moho discontinuity Upper rigid mantle Asthenosphere Mesosphere Outer core Inner core

6 Discovering Earth s Major Boundaries The Moho (Mohorovicic discontinuity) Discovered in 1909 by Andrija Mohoroviĉiĉ Separates crustal materials from underlying mantle Identified by a change in the velocity of P waves

7 Earth Interior (3) Crust: the outermost rigid layer, approx miles thick Continental crust: lighter and thicker than oceanic crust,,granitic composition Oceanic Crust: Heavier and thinner, basaltic composition Mantle: the most massive layer, 80% of the earth s volume, ultramafic rocks composition Upper rigid mantle: part of the lithosphere with the crust Asthenosphere: plastic (semi-liquid) layer from km, also known as LVZ (lower velocity zone for seismic waves) Mesosphere: bulk of the lower mantle Core Outer core: liquid, S-waves does not transmit through it Inner core: highly compacted Ni & Fe alloy, solid & dense layer

8 Continental Drift: An Idea Before Its Time Alfred Wegener First proposed continental drift hypothesis in 1915 Published The Origin of Continents and Oceans Continental drift hypothesis The supercontinent called Pangaea began breaking apart about 200 million years ago Pearson Education, Inc.

9 Tectonic Processes (1) Continental Drift Proposed by Alfred Wegener (1915). He proposed a supercontinent concept (Pangaea) existed back MYA The Pangaea started to break up and drifted in relation to each other, forming today's continental llandmasses Fit of continents like a jigsaw puzzle along the continental shelf Similar rocks and fossils on both sides of Atlantic Ocean Ancient glacial erosion in South Africa, Australia, India Apparent polar wondering

10 A Scientific Revolution Begins During the 1950s and 1960s, technological strides permitted extensive mapping of the ocean floor. The seafloor spreading hypothesis was proposed p by Harry Hess in the early 1960s. In 1963, Vine and Matthews tied the discovery of magnetic stripes in the oceanic crust near ridges to Hess s concept of seafloor spreading.

11 Tectonic Processes (2) Seafloor Spreading s Harry Hess proposed that sea floor does not stand still as a whole piece, and it consists of blocks that move relative to each other New oceanic crust forms at mid-ocean ridges and spreads outward Paleomagnetic data The age of the rocks on the sea floor The age and thickness of deep-sea sediments Oceanic ridge and fracture zones

12 Plate Tectonics (3) Tectonics lithosphere broke into a number of rigid plates bounded by ocean spreading ridges, trenches, faults/rifts, and mountain ridges. These plates floats and move on the asthenosphere No major deformation processes within each plate, and major tectonic deformation concentrates along the plate boundaries Plates move relative to each other at approxi. 2-17cm/year, The driving mechanism for the movement is earth interior convection, ridge push & slab pull Distribution of global earthquakes, volcanoes, mountain belts

13 Plate Tectonics: The Earth s major plates New Paradigm Seven major lithospheric plates Plates are in motion and are continually changing in shape and size. The largest plate is the Pacific plate. Several plates include an entire continent plus a large area of seafloor Pearson Education, Inc.

14 Plate Tectonics: The New Paradigm Plate boundaries Interactions among individual plates occur along their boundaries. Major types of plate boundaries: Divergent plate boundaries (constructive margins) Convergent plate boundaries (destructive margins) Transform fault boundaries (conservative margins)

15 Plate Boundaries Divergent (constructive): plates move away from each other, and new crust is created Convergent (destructive): plates move toward each other and parts of the plates are destroyed d by subduction (one plate dives under another) Transform (conservative): Plates slide horizontally past one another, no major crust destruction or creation

16 Divergent Boundaries Continental Rifting: Formation of continental rift valley or new ocean basin Mid-oceanic i ridge: formation of new ocean crust (seafloor spreading Basaltic Volcanism Shallow earthquake

17 Convergent Boundaries Three subtypes C - C: A continental plate collides with a continental plate Crust thickening & mountain building process Suture Zones & remnants of oceanic crust (ophiolite) Mountain building & shallow earthquakes C-O: continental to oceanic plate Subduction trenches and volcanic mountain arc system Benioff seismic zone, from shallow to deep earthquakes Andesitic volcanism Mountain building O-O: oceanic to oceanic plate Creation of volcanic Island Arcs Backarc shallow sea, forearc trench & accretionary wedge Benioff seismic zone

18 Transform Boundary Plates slide horizontally past one another, no major crust destruction or creation Shear zone--conservation of Lithosphere Transform faults Most join two segments of a mid-ocean ridge along breaks in the oceanic crust known as fracture zones. Af few (h (the San Andreas Fault and the Alpine Fault of fnew Zealand) cut through continental crust. Horizontal Displacement of Land Masses Shallow and Intermediate Depth Earthquakes, San Francisco (1906), Loma Prieta (1989), Northridge (1994)

19 Red: segment ruptured 1906 S.F. earthquake What Is the Earthquake History of the San Andreas Fault and Related Faults? Black and colored lines: recently active faults Blue: creeping segment Moderate EQs along eastern zone Parkfield segment (north end of orange) ruptures every ~20 years Orange: last fully ruptured in 1857 ( Big One for L.A.)

20 What Drives Plate Motions? Mantle convection: convective flow in the mantle is the basic driving force of plate tectonics. t Forces that drive plate motion: Slab-pull Ridge push 2011 Pearson Education, Inc.

21 Hot Spots (Mantle Plumes) Relatively stationary with respect to the mantle Occur in the interior of plates or plate boundaries Mantle plumes Long-lived structures Some originate at greater depth Explain the distribution patterns of intra-plate volcanoes & the creation of sea mount chains Examples: Hawaii Island chain, Iceland, Yellowstone

22 , Directiion Of plate movement Ar,ea of: actiive vollcanism,q I km 50 I