Basics of Plate Tectonics

Review of Density Density= mass/volume Substances of a low density will float on substances of a higher density

Internal Structure of Earth

Internal Structure Core- innermost layer; composed mostly of mixtures, or alloys, of iron Solid inner core and liquid outer core Mantle- flows like a liquid; very hot near the melting point of rocks Lithosphere- consists of the rust and the uppermost part of the mantle

Internal Structure (2) Crust- outermost layer; extremely thin and rigid Rest on solid upper mantle that floats on denser plastic middle mantle (like molten tar) Constantly changing by erosion and glacial melting reduces continental mass (floats higher on mantle) Oceanic crust- made of basalt and is denser Continental crust- made of granite, high in aluminum silicates; is less dense; average thickness is 35km

Continental Drift At the beginning of the 19th century scientists thought that the positions of the continent were fixed Scientists today realize that the plates are moving 1. Tectonics is the process of plates moving 2. As plates move they create mountains on earth and various sea floor features

Alfred Wegener 1. Proposed the idea of Pangea that all continents were once a single land mass 2. Evidence a. Fit coastlines together b. Similar fossils on the matching coastlines c. Magnetic reversals (he did not know this)

How Plates Move Plates move by the mechanism of sea floor spreading 1. Sea-floor spreading is driven by convection currents Plate Tectonics occurs at plate boundaries

Convection Currents in Mantle

Divergent Boundaries Plates move in opposite directions away from each other and rift (crack in earth’s crust); called sea-floor spreading New oceanic crust is formed here Plates move apart at the rate of 0.8”-7”/year Example: Mid-ocean ridge

Convergent Boundaries Plates Collide

Convergent (Oceanic-Continental) Ocean plate sinks and is subducted creating a trench Creates volcanic arcs/mountains on land Earthquakes can also occur Examples: Mt. St. Helens

Convergent (Oceanic-Oceanic) One of the plates will be subducted forming an island chain or arc Example: Aleutian Islands off the coast of Alaska

Convergent (Continental-Continental) As plates collide they are welded together forming mountain ranges Example: Himalaya Mountains