Think of something that humans used to believe but that they no longer hold to be true. (an opinion or “fact”) e.g.-flat earth -animal machines -alchemy (that you could turn purified metals into gold) What changed? -new evidence -new ideas about existing evidence

“…we have to be prepared always for the possibility that each new discovery, no matter what [scientific discipline] furnishes it, may modify the conclusions we draw.” -Alfred Wegener

CONTINENTAL DRIFT AND PLATE TECTONICS

Continental Drift In 1912, Alfred Wegener put forward a paper expressing his belief that the earth’s continents were moving (however, very slowly). Wegener suggested that a single “supercontinent” (a continent made up of all, or most, of the land on earth) called Pangaea had existed at some point in the past. *Turns out, Pangaea (meaning “entire earth”) did exist, between 200 and 300 million years ago.

“WHY DID HE BELIEVE THIS?” Evidence for Continental Drift 1) The fit of the continents 2) Fossil Evidence 3) Rock-type similarities 4) Evidence of Glaciers in Warm Climates

1.Fit of the Continents Wegener noticed that the continents are shaped such that they could fit together like jig-saw puzzle pieces. e.g. East Coast of South America and West Coast of Africa

2.Fossil Evidence Fossils of similar plants and animals were found on different continents, suggesting that at one point, they shared the same landmass. e.g. Glossopteris, a prehistoric (now extinct) plant in Antarctica, South America and Africa, could not have spread seeds across the ocean.

3.Rock Type Similarities Similar rock types (age and structure) were found along mountain ranges on different continents. e.g. Caledonian Mountains in Scandinavia and Ireland, the Atlas Mountains in Northwestern Africa, and the Appalachian mountains in Eastern North America.

4.Glacial Evidence There is evidence of prehistoric ice sheets having been present in Africa, South America, India, and Australia. Wegener believed that the only way this could have happened was if these continents had been located at higher (colder) latitudes historically. e.g. glacial striations (scrapings), in hot desert landscapes.

The Problem was… Wegener’s contemporaries did not share his enthusiasm for the theory of Continental Drift. They mocked and ridiculed him, because he could not explain “HOW” the continents moved. In other words, they would not believe him until he presented a cause (a force) that could reasonably and logically explain the movement of the continents.

Plate Tectonics In 1968, John Tuzo Wilson, a Canadian geophysicist who studied continental drift discovered the “HOW” that had evaded Wegener. Wilson proposed that “convection currents” in the earth’s “mantle” provided adequate energy to displace landmasses.

The Mantle The Earth’s mantle is a layer below the crust that descends to 2900 km below the surface. The upper part of the mantle is less dense than the crust above it, and the crust therefore “floats” on top of the mantle. The movement of the continents (tectonic plates) can be compared to broken slabs of ice floating on a lake in winter time.

Convection Convection is the transfer of heat though a material (i.e. water, magma, air). It creates cycles (or currents), with warmer areas rising and cooler areas falling. In the mantle, lower-lying magma is hotter and less dense, therefore it tends to rise. Magma located at higher points in the mantle is cooler and more dense, and therefore descends relative to its surrounding material.

Convection Currents Think of convection currents as giant conveyor belts. Their movement pushes and pulls the earth’s crust. (continental movement: 2-10 cm/year)

Plate Tectonics The edges of tectonic plates, where one plate meets another, are referred to as Plate Boundaries. There are three types. Convergent boundaries involve tectonic plates moving towards each other. It is typical for one plate to descend under the other in a process known as subduction. Divergent boundaries involve tectonic plates moving away from each other. Transform boundaries involve two plates scraping eachother while moving in opposite directions (or sometimes moving in the same direction at different speeds)

Plate Boundaries

India The Indian subcontinent is subducting under the Eurasian Plate, resulting in the building of the Himalaya Mountain Range.

Question from Yesterday Fastest-moving Plate: NAZCA: 160mm/year or 16 cm/year. Why might it be moving faster than other continents?

Subduction Drag Increasing weight as plate subducts.