Plate Tectonics

Rigid, rocky outer surface of the Earth Definition Tectonic: Plate: Rigid, rocky outer surface of the Earth To build or destroy The processes by which the rocky plates of the planet move and interact with each other

Internal Structure of the Earth lithosphere plates mantle inner core hot dense rock solid iron outer core liquid iron

Lithospheric Plates Continental: Oceanic: thick, light, mostly granite thin, heavy, mostly lava flows Asthenosphere Weak, plastic upper mantle

Lithospheric Plates

Diverging Plate Boundaries Where plates pull apart from each other Stretching & thinning High heat flow (New Crust Created)

Diverging Plate Boundaries Key landform: mid-oceanic ridges volcanic mt. ranges axial valley 1000’s miles long 1-2 miles tall

Converging Plate Boundaries Where plates collide Subduction zone: collision between oceanic and continental lithosphere (Crust Destroyed)

trenches & volcanic arcs Subduction zones Key landforms: trenches & volcanic arcs

Transform Plate Boundaries Where plates slide side by side Key landform: faults (Crust neither destroyed or created)

thermal convection / convection currents plate sinks and subducts Driving Force thermal convection / convection currents pushes up at ridges plates spread away hot rock cold rock plate sinks and subducts

Geologic activity at plate margins Volcanism: C, D Earthquakes: C, D, T Rock Deformation: C, D, T

Evidence: shape of continents The margins of some continents look like they could fit together First proposed by Alfred Wegner

Fossil Evidence

Nonrandom occurrence of volcanic activity Evidence: volcanoes Nonrandom occurrence of volcanic activity

Evidence: earthquakes Nonrandom occurrence of earthquakes Shallow at diverging margins deep at converging margins

Stationary upwellings of heat away from plate margins Evidence: hot spots Stationary upwellings of heat away from plate margins

Evidence: hot spots As plates move, the volcanoes are carried away from the hot spot Only volcanoes near the hot spot are are active Can determine direction and speed of the plate motion

Evidence: age of seafloor rocks Youngest rocks are found at the mid oceanic ridges Oldest rock are found farthest from the ridge

Evidence: paleomagnetism Earth has a strong magnetic field Iron rich volcanic rocks record Earth’s magnetic field at the time they form

seafloor paleomagnetic record Bands of alternating polarity Symmetrical about the mid oceanic ridges

seafloor paleomagnetic record lava forming at the ridges picks up the current magnetism, then gets pushed out of the way as newer lava forms Records of the growth and spreading of the seafloor

seafloor paleomagnetic record The alternating polarity represents complete reversals of Earth’s magnetic field

continental paleomagnetic record The location of magnetic poles on separate continents appear to wander with time If the poles haven’t moved significantly, then the continents did Use overlapping poles to reconstruct the past positions of the continents

Plate Tectonics in the PNW Cascadia subduction zone

Plate Tectonics in the PNW Cascades volcanic arc

Plate Tectonics in the PNW Juan de Fuca & Gorda mid ocean ridges

Plate Tectonics in the PNW Blanco & Medocino fracture zones

Plate Tectonics in the PNW “exotic terranes islands carried by the plates Attached to the North America continent

seafloor paleomagnetic record Bands of alternating polarity Symmetrical about the mid oceanic ridges

Evidence: paleo-environments

Evidence: distribution of mountains

Importance of Plate Tectonics