Chapter 18 Plate tectonics

1. Chapter 18 Plate tectonics

2. History of plate tectonics • The earth’s surface is divided into several major and minor plates and the interaction between these plates is known as plate tectonics.

3. There are 8 major and about 30 minor plates. • N. American • S. American • African • Eurasian • Indian • Pacific • Australian • Antarctic

5. Intense geologic activity occurs at the plate boundaries where plates; • Collide with one another • Move away from one another • Slide past one another

6. The concept of plate tectonics was developed in 1960s by combining two theories; • Continental Drift • Sea floor spreading

7. 1. Continental Drift: proposed by Alfred Wagener in 1912. • He found similarities in S. America, Africa, India, Antarctica and Australia.

8. On this basis he proposed; • All the continental landmasses were once joined together in one supercontinent—Pangea. All the oceans formed one super ocean—Tetheys. • The Pangea then split into a northern Laurasia and a southern Gondwanaland.

11. Revival of continental drift: Wegener’s mechanism of continental drift was not very convincing and his theory remained discarded till 1960s. • It was revived with the advent of paleomagnetism, which confirmed that the continents have moved relative to one another.

15. Additional evidence for cont. drift: • The continents fit like a jigsaw puzzle • Rock similarity • Fossils and age similarity

17. 2. Sea floor spreading: proposed by Harry Hess in 1962. • He proposed that the sea floor moves away from the crest of a mid ocean ridge and finally disappears beneath a continent or an island arc (subduction).

18. Mantle convection is responsible for ocean spreading.

20. Plates and plate boundaries • Rigid lithospheric plates move over plastic asthenosphere.

21. Plate boundaries: 3 types; • Divergent plate boundary: plates move away from one another. Also known as Constructive P.B. or spreading center.

22. 2. Convergent P.B: plates move towards one another. Also known as Destructive P.B. 3. Transform P.B: plates move horizontally past one another. Also known as Conservative P.B.

24. Do plates really move? • Plate motion is very slow but predictable. 1-10 cm/year. • Movement is measured by satellites, lasers and GPS.

25. Do plates really move? • The magnetic anomalies at the sea floor and movement along a transform fault indicate plate motion.

31. Divergent Plate Boundaries • Two plates move away from one another. • Can occur within a continent or an ocean. If it occurs in a continent—rifting.

32. Divergent Plate Boundaries • Generally basaltic magma erupts and spreads on either side of the fractures causing the plates to push away from one another.

36. Examples: • Red Sea. • East African Rift

39. A passive continental margin forms when one of the moving portions of the plate are covered by sediments.

41. A mid ocean ridge is formed when divergent P.B. occurs in the middle of an ocean—mid Atlantic ridge.

42. Convergent P.B. • Two plates move towards each other and collide forming mountains.

43. Convergent P.B. • Three types of convergent p.b; • Ocean-ocean convergence • Ocean-continent convergence • Continent-continent convergence

44. Ocean-ocean convergence: one plate subducts under the other, a trench and a volcanic island arc are formed.

48. Ocean-continent convergence: oceanic plate subducts beneath the continent and an active continental margin is formed.

50. Continent-continent convergence: neither plate subducts and a collisional mountain chain is formed.