Chapter 10: Plate Tectonics
Vocabulary 1. continental drift 2. mid-ocean ridge 3. sea-floor spreading 4. paleomagnetism 5. supercontinent 6. rift 7. magma 8. plate tectonics 9. lithosphere 10.asthenosphere 11.divergent boundary 12.convergent boundary 13.transform boundary 14.oceanic crust 15.continental crust 16.earthquakes 17.rift valley 18.subduction zone 19.island arc 20.fracture zones 21.convection 22.slab pull 23.rifting 24.terrane 25.supercontinent cycle 26.Pangaea 27.Panthalassa 28.Laurasia 29.Gondwanaland
I.Continental Drift A.Definition: hypothesis that a single large landmass broke up into smaller landmasses to form the continents, which drifted to their current locations 1. Proposed by Alfred Wegener, 1912 B.Evidence 1. Fossils of the same species were found on separate continents, like South America & Africa -- no evidence of land bridges & unlikely that organisms traveled across the Atlantic therefore, the continents must have been joined at some point
2. Rocks on separate continents were similar in age & type 3. Mountain ranges seemed to form continuous chains when the continents were envisioned together 4. The same types of glacial debris were found in S. America & Africa -- so these areas were once covered by the same climate
C.Mechanism 1. Wegener proposed that the continents plow through the rock of the ocean floor -- no evidence, found to be physically impossible 2. Discovery of mid-ocean ridges (underwater mountain ranges that have narrow, steep valleys in the center) provided evidence for continental drift a. Sediment on sea floor is thinner closer to a ridge -- suggests that sediment near ridges is younger/newer than sediment further away b. Oceanic rock is very young -- oldest is about 200 million years old, whereas continental rock can be up to 4 billion years old
3. 1950s: the idea of sea-floor spreading was suggested a. Valley at the center of the mid-ocean ridge is a rift (crack in Earth s crust) b. Through the rift, molten rock (magma) rose to fill the crack; then, the magma cooled & solidified to form new oceanic crust c. If the ocean floor is moving, maybe the continents are too!
D. Paleomagnetism 1. Definition: magnetism of rock due to presence of iron in solidified magma & in Earth s core 2. Earth s magnetic field has reversed several times in Earth s history depends on the flow of iron-rich minerals 3. Rocks with certain magnetic orientations are grouped together chronologically, which can help determine the age of the ocean floor 4. This supported the idea of sea-floor spreading
II. Theory of Plate Tectonics A.Background 1. Earth s interior can be divided into 3 zones based on composition & 5 zones based on structure 2. Compositional Zones: a. Crust: thin, solid, outermost layer -- oceanic (dense, rich in iron & magnesium, 5-10 km thick) & continental (low density, silica-rich, 15-80 km thick) b. Mantle: denser than crust & 2,900 km (~1,802 mi) thick c. Core: center sphere made of iron & nickel & 7,000 km diameter
3. Structural Zones a. Lithosphere: solid, outer layer of Earth that consists of the crust & the rigid upper part of the mantle; 15 to 300 km thick b. Asthenosphere: solid, plastic layer of the mantle beneath the lithosphere; made of mantle rock that flows very slowly, which allows tectonic plates to move on top of it; about 200 to 250 km thick c. Mesosphere: strong, lower part of the mantle between the asthenosphere & outer core; begin right below the asthenosphere, goes to a depth of about 2,900 km
d. Outer Core: liquid e. Inner Core: solid, begins at a depth of 5,150 km
B. Plate tectonics: theory that explains how large pieces of the lithosphere, called tectonic plates, move and change shape 1. Tectonic plates (both oceanic & continental) ride on the liquid asthenosphere 2. About 15 major tectonic plates have been identified 3. Plates are often bordered by major surface features, such as mountain ranges or oceanic trenches
C. Types of Plate Boundaries 1. Divergent boundary: two plates moving away from each other a.magma rises to the surface and cools to form warm, light rock; this rock sits higher than the surrounding sea floor because it is less dense b.this usually occurs on the sea floor at mid-ocean ridges; forms a narrow valley where the plates separate (rift valley)
2. Convergent boundary: two tectonic plates that are colliding; can happen in 3 ways a.oceanic-oceanic: one plate subducts under the other plate, and a deep-ocean trench forms; a chain of volcanic islands, called an island arc, may form (example: Japan) b.oceanic-continental: denser oceanic lithosphere subducts, or sinks, under the less dense continental lithosphere; region along this plate boundary is called a subduction zone c. Continental-continental: colliding edges crumple and thicken, which cause uplift that forms large mountain ranges (example: Himalayas)
3. Transform boundary: two tectonic plates that are sliding past each other horizontally a. Plate edges at a transform boundary scrape against each other in sudden bursts of motions that are felt as earthquakes (example: San Andreas Fault) b. Can also occur along mid-ocean ridges; short segments of transform boundaries there are called fracture zones
D. Causes of Plate Motion 1. Movement of tectonic plates is due to convection: movement of heated material due to differences in density that are caused by differences in temperatures 2. Occurs in convection cells: cooler, denser material sinks, and warmer material rises above it 3. This happens in the mantle/asthenosphere, and as the material in the mantle moves, the tectonic plates move along with it
4. Ridge Push: force exerted by one tectonic plate on another, causing the second plate to be pushed away from the mid-ocean ridge a.newly formed rock at a mid-ocean ridge is warmer and less dense than older rock nearby. b.as the newer rock cools and becomes denser, it begins to slide down the slope between the lithosphere and asthenosphere, causing ridge push
5. Slab Pull: force exerted by one part of a tectonic plate on the rest of the plate, causing the whole plate to be pulled into the asthenosphere a.when the lithosphere cools and becomes dense enough, it begins to subduct into the asthenosphere. b.as the leading edge of the plate sinks, it pulls the rest of the plate along behind it (slab pull)
III.The Changing Continents A. Reshaping Earth s Crust 1.Continents are always changing gaining material, losing material, and changing shape over millions of years 2.Rifting: process by which Earth s crust breaks apart; can occur in oceanic or continental crust a. Reason unknown b. Thick continental crust prevents the heat from the mantle from escaping; maybe as heat builds up, continental crust weakens & splits
3. Continents can also change by gaining material a. Terrane: piece of lithosphere (crust) with a unique geologic history different from the surrounding lithosphere, which become part of a continent at convergent boundaries in a process called accretion b. Characteristics: i. Different fossils ii.faults at boundaries iii.different magnetic properties
B. Effects of Continental Change 1.Climate Change: changes based on locations & geologic features mountains affect air flow, wind patterns, precipitation 2.Biological Change: populations are separated when continents drift can lead to natural selection & evolution!
C. Supercontinent Cycle 1.Definition: process by which supercontinents form, break apart, & reform over millions of years 2.Cycle: plates move toward convergent boundaries & collide, then heat from the mantle builds up & causes rifting to break them apart 3.Pangaea: supercontinent formed during the Paleozoic Era, surrounded by a large ocean called Panthalassa a.broke up during the Mesozoic into two smaller supercontinents: Laurasia (north) & Gondwanaland (south)
b. Through rifting Laurasia separated into North America & Europe, with the area between becoming the North Atlantic Ocean c. Gondwanaland separated into South America, Africa, India, Australia, and Antarctica, also forming the South Atlantic Ocean d. Fun fact: India was its own continent before it collided with Asia 4. More recent plate movement caused the Rockies, Andes, Alps, and other bodies of water to form 5. Future: at current plate movement rates, South America & Africa will collide in 150 million years, and another massive supercontinent will form in 250 million years