earth science - plate tectonics

plate tectonics

  • is the theory that earth’s outer shell is divided into several plates that glide over the mantle

  • the plates act like a hard and rigid shell compared to mantle

lithosphere

  • a strong outer layer that includes the crust and outer part of mantle which is 100km thick

asthenosphere

  • located below the lithosphere

  • it is a portion of the earth’s mantle that flows like molten plastic despite being solid

  • a malleable allowing the lithosphere to move around

types of plate tectonics

divergent boundary

  • occurs when two tectonic plates move away from each other

convergent boundary

  • oceanic-continental boundaries | are when an oceanic and continental plate converge and denser oceanic plate is subducted below the continental plate

  • oceanic-oceanic boundaries | when two oceanic plates converge and one plate is subducted under the other and in the process a deep oceanic trench is formed and can also result in undersea volcanoes

transform boundary

  • occurs when two plates slides past each other

  • plates are split into pieces and carried in opposite directions

plate motions

  • where plates push together or converge causing mountains to rise and continents to fracture

  • causes oceans to form where plates pull apart or diverge

continent accretion theory - james dwight dana

  • the continents have always been stationary with the gradual addition of new material around a central nucleus

continental assimilation hypothesis

  • the ocean areas accumulated the denser elements then subsided to form basins

expanding earth theory

  • the present continents split apart with the expansion of earth

  • the continents combined could cover half of the current earth’s surface area

continental drift theory - alfred wegener

  • the continents once existed as a single landmass which is called pangaea

  • the pangea is covered by one big ocean called panthalassa

  • a sea called tethys divided the pangaea into two huge landmasses: laurentia/laurasia (north) & gondwanaland (south)

  • the drift started around 200 million years ago (mesozoic era, triassic period, late triassic epoch)

evidences for continental drift theory

  1. the fit of continental shorelines

  2. distribution of glacial sediments

  3. paleoclimate

  4. distribution of fossils

  5. distribution of rocks

paleoclimate

  • one piece of evidence wegener provided was the paleoclimatic indicators that linked america and africa

  • he noted that glacial, tropical rain forest, and desert deposits were aligned when the continents were places together

seafloor spreading - harry hess (1960)

  • is a geologic process in which tectonic plates split apart from each other as a result of mantle convection

  • continually adds new material to the ocean floor while pushing older rocks away from the ridge

  • occurs in divergent plate boundaries

  • formation of mid-ocean ridges (large mountain ranges) arise from the ocean floor

  • ocean floor moves like a conveyor belt carrying continents with it

  • new ocean floor forms along cracks in the ocean crust as molten material erupts from the mantle spreading out and pushing older rocks to the sides of the crack; new ocean floor is continually added by the process of seafloor spreading

sonar

  • a device that bounces sound wave off underwater objects and then records the echoes of these soundwaves

  • the time it takes for the echo to arrive indicates the distance of the object

mid-ocean ridges

  • a continuous range of underwater volcanoes that wraps around the globe like seams on a baseball stretching nearly 65,000km

subduction

  • a process where the ocean floor sinks beneath a deep ocean trench and back into the mantle

  • allows part of the ocean floor to sink back into the mantle