Earth s Layered Structure, Earth s Internal Structure, Plate Tectonics

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1 Earth s Layered Structure, Earth s Internal Structure, Plate Tectonics Chs.1&2 Earth s Layered Structure High-velocity impact of debris + radioactive decay => increase in T => Fe & Ni melt & sink => Inner core (Fe rich) Chemical differentiation => Primitive crust (enriched in Si & Al) Mantle Gas => primitive atmosphere Earth s Internal Structure & Plate Tectonics 1

2 Earth s Internal Structure Physical properties: Lithosphere (solid & rigid) Astenosphere (solid, but mobile) Lower mantle (solid) Outer core (liquid) Inner core (solid) Chemical composition: crust mantle core Earth s Crust Oceanic Av. 7 km (5 miles) basalt Continental Av. 35 km (22 miles) >70 km (40 miles) in some mountainous regions Many rock types (esp. granodiorite) 3.0 g/cm million years old 2.7 g/cm 3 4 billion years old Earth s Internal Structure & Plate Tectonics 2

3 Earth s Mantle >82% of Earth s volume Solid, rocky To a depth of 2900 km (1800 miles) Upper Mantle Crust-mantle limit to 660 km (410 miles) Lithosphere stiff Crust + Uppermost mantle Av.100 km thick >250 km below the oldest portions of continents mechanically detached from asthenosphere Asthenosphere weak Down to 350 km Uppermost part = near its melting point => may flow Lower Mantle 660 km to 2900 km (410 to 1800 miles) Gradually strengthens w. depth Very hot rocks Capable of very gradual flow Earth s Internal Structure & Plate Tectonics 3

4 Earth s Core Fe-Ni alloy Minor amts. of O 2, Si & S Extreme pressure Density: 11 to 14 g/cm 3 Outer core Liquid 2270 km (1410 miles) thick Movement of metallic Fe => Earth s magnetic field Inner core Solid 1216 km (755 miles) radius Immense pressure Plate Tectonics Chapter 2 Image source: Before 1960 s the accepted theory was that ocean basins and continents had fixed geographic positions Earth s Internal Structure & Plate Tectonics 4

5 Continental Drift Alfred Wegener First proposed hypothesis, 1915 in The Origin of Continents and Oceans Continental drift hypothesis Supercontinent Pangaea (= all lands) began breaking apart ~ 200 million years ago Continents drifted to present positions Continents broke through the ocean crust ~200 Million Years Ago Redrawn from The 2012 Origin Pearson of Education, Continents Inc. and Oceans, 1915 Earth s Internal Structure & Plate Tectonics 5

6 Wegener s continental drift hypothesis Evidence: 1. Fit of continents 2. Fossils match 3. Rock types and structures match 4. Ancient climates Main objection: why were the plates moving? 1. Fit of South America and Africa Wegener used the present-day shorelines to match the continents This image is a match using the continental slope at about 900 m depth 2012 Pearson Education, Inc. Earth s Internal Structure & Plate Tectonics 6

7 2. Fossils Match Organisms that could not cross the ocean: Mesosaurus = freshwater reptile found only in Permian nonmarine deposits of E S. America and SW Africa Lystrosaurus = land reptile that lived in S Africa, India and Antarctica Glossopteris = seed fern that lives in subpolar climates; seeds too large to be carried by the wind; found in Africa, Australia, India, S.America (& Antarctica) Other explanations for the occurrence of similar species on different continents 2.2 billion years old igneous rocks in Brazil closely resembling to rocks of the same age in Africa Mountain belts that reappear across the ocean 3. Match of Geologic Features Earth s Internal Structure & Plate Tectonics 7

8 Wegener s Matching of Mountain Ranges on Different Continents Evidence for the presence in the past of a glacier that caused 2012 Pearson striations Education, Inc. and grooves 4. Paleoclimates Evidence for a Glacial period in Late Paleozoic for South America, 2012 Pearson Africa, Education, India Inc. & Australia Earth s Internal Structure & Plate Tectonics 8

9 Late Paleozoic icesheet situated close to the South Pole Weak parts in Wegener s hypothesis No credible mechanism for the continental drift Wegener proposed the idea that gravitational forces of the Sun and Moon are responsible for the movement of the plates Continents broke through the oceanic crust Plate Tectonics The New Paradigm More encompassing than continental drift Earth s lithosphere (rigid outer shell) consists of several plates that are moving slowly are mostly beneath the ocean Largest = Pacific plate Earth s Internal Structure & Plate Tectonics 9

10 Earth s Lithospheric Plates Major Intermediate-size Small Asthenosphere Exists beneath the lithosphere Hotter and weaker than lithosphere Allows for motion of lithosphere Plate boundaries All major interactions among plates occur along their boundaries Plate boundaries: 1. Divergent 2. Convergent 3. Transform fault Earth s Internal Structure & Plate Tectonics 10

11 Types of plate boundaries A. Divergent plate boundaries (constructive margins) Two plates move apart Mantle material upwells new seafloor Form ocean ridges & continental rifts Ocean ridges = Elevated areas of the seafloor Crest 2 to 3 km higher than the adjacent ocean Widths 1000 to 4000 km Rift valley on some ridge segments High heat flow and volcanism Av. rate of spreading: 5 cm/yr (2 /yr.), Var cm/yr. (1-6 /yr.) Thickness of the oceanic crust increases with its age Continental rifts form at spreading centers within a continent (e.g.: East African Rift) Anthony Philpotts Earth s Internal Structure & Plate Tectonics 11

12 B.Convergent plate boundaries (destructive margins) Plates collide, an ocean trench forms and lithosphere is subducted into the mantle Deep-ocean trenches = large linear depressions, very long and deep Ex: Peru-Chile (4500 x 8 km), Marianna, Tonga Collisions: 1.Ocean - continent 2.Ocean - ocean 3.Continent continent 1. Oceanic-continental convergence Denser oceanic slab sinks into the asthenosphere Pockets of magma develop and rise Continental volcanic arcs form Ex: Andes, Cascades, Sierra Nevada system 2. Oceanic-oceanic convergence Two oceanic slabs converge and one descends beneath the other Volcanoes may form on the ocean floor Volcanic island arcs form 100 to 300 km from trench Ex: Aleutian, Mariana, and Tonga islands Earth s Internal Structure & Plate Tectonics 12

13 3. Continental-continental convergence When subducting plates contain continental material, two continents collide Can produce new mountain ranges such as the Himalayas The Collision of India and 2012 Asia Pearson Produced Education, Inc. the Himalayas (Before) The Collision of India and Asia Produced the Himalayas (After) Continent to continent collision: formation of the Himalayas Earth s Internal Structure & Plate Tectonics 13

14 C. Transform fault boundaries (conservative plate margins) Plates slide past one another No new crust is created or destroyed Transform faults Most join two segments of a mid-ocean ridge Aid the movement of oceanic crustal material Earth s Internal Structure & Plate Tectonics 14

15 Earth s Internal Structure & Plate Tectonics 15