The Theory of Plate Tectonics and Continental Drift

The Theory of Plate Tectonics and Continental Drift • Write down the underlined items for your notes. Feel free to put the material in your own words.

Tectonic Plates and Continental Drift Tectonic plates are pieces of the lithosphere that move around on the asthenosphere. Continental drift is the hypothesis which states the continents once formed a single landmass called Pangaea and have drifted into their current positions when Pangaea broke apart.

Tectonic plates: Pieces of lithosphere that move around on top of the asthenosphere. Take a look. Are the plates just continental or oceanic or are they a combination of both?

A Giant Jigsaw Puzzle • All of the tectonic plates fit together, like a puzzle, but are not the same size. • Along the plates’ edges (boundaries) movement occurs. • Plates move away from each other, toward each other, and slide past each other. • Earthquakes, volcanoes, & mountain ranges occur along these boundaries.

Continental Drift • Alfred Wegener; 19121915 • 1 land mass in world – called Pangaea – Drifted apart over 250 million years

Wegener’s Theory • Wegener’s theory explained and was supported by several things: – Puzzle Theory: the continents look like they fit together like a puzzle & they do. – Fossil Pattern: Fossils of the same plant & animal species are found on continents oceans apart. – Rocks: Similar types of rocks are found on several different continents.

Oceanic Crust and Sea-Floor Spreading • Problem for Wegener: – How could items as big as continents move? He couldn’t prove how the continents had separated. – Solution: sea-floor spreading • The adding of ocean crust as magma bubbles up through a midocean ridge and hardens on the ocean floor. • New crust is closer to the ridge. • Old crust farther away, indicating plates are moving apart.

Further evidence for sea-floor spreading: magnetic reversal (polarity changes). Rocks record the magnetism of the earth. This changes periodically, so the changes are seen in the rock record. The pattern of "stripes" or anomalies is symmetrical around the oceanic ridge. The youngest oceanic rocks are near the ridges with the oceanic rocks becoming older as they move away from the ridge. The black stripes represent rocks that cooled under "normal" conditions and are normally polarized, while the white stripes represent rocks that cooled under "reversed" conditions and are reversely polarized.

Mid-Ocean Ridges & Sea Floor Spreading • A chain of submerged mountain ranges that runs through the world’s oceans = mid-ocean ridge. . • Most common one = Mid-Atlantic Ridge.

Sea- Floor Spreading Sea-floor spreading is the process by which new oceanic lithosphere (ocean crust) forms as magma rises to the surface & hardens. As tectonic plates move away from one another, the sea floor spreads apart and magma fills in the gaps, creating new ocean floor— creating new crust, new Earth.

Sea-Floor Spreading • Sea-floor spreading takes place along the mid-ocean ridges all around the world.

• Mid-ocean ridges are also the site of other geological events: • Mountain building: • Where plates collide, great mountain ranges may be pushed up, such as the Himalayas or Andes. • If one plate sinks below another (subduction zone), deep oceanic trenches and chains of volcanoes are formed. • Earthquakes are by far most common along plate boundaries and rift zones, and they can trigger tsunamis.