Continents change position over time 1.2 outline Continents change position over time

I. Continents join together and split apart Alfred Wegener proposed the theory of continental drift Continental drift: hypothesis that Earth’s continents are moving apart from a single landmass (Pangaea)

A. Pangaea and Continental Drift Pangaea: all continents joined together to create huge supercontinent

Evidence from the Sea Floor II. The theory of plate tectonics explains how plates and their continents move Evidence from the Sea Floor Mid-ocean ridges: huge underwater mountain ranges where new oceanic crust is created Sea-Floor spreading occurs when melted rock is pushed up to create new oceanic crust; old crust is pulled away to make room for new material

A. Evidence from the Sea Floor (cont.) Age of Sea Floor shows that the youngest rock is closest to the ridge, while the oldest rock is farthest away Ocean Trenches are deep canyons in the sea floor. Dense oceanic crust is being destroyed here; old crust is being destroyed at the same rate as the new crust is forming

B. Causes of Plate Movement Convection: heat transfer by the movement of material Convection current: when the hot, soft rock rises, cools and sinks, then is heated and rises again

C. Putting the Theory Together Theory of plate tectonics: states that Earth’s lithosphere is made up of huge plates that move over the surface of the Earth Two plates moving apart are the South American and African plates Two plates pushing together are the South American and Nazca plates

C. Putting the Theory together (cont.) Two plates scraping past each other are the North American and Pacific plates Where tectonic plates meet, major earthquakes, volcanoes, and mountain ranges appear

Review Answers sea-floor spreading, different ages of rocks, mid-ocean trenches & ridges Circulation of heated and cooled material forms convection currents that can carry tectonic plates.