Plate Tectonics (Ch. 7) Objectives Flashcards
What is meant by “continental drift”? Name and describe early evidence that indicated it had occurred.
Continental drift is the idea that the continents were once part of a single supercontinent (Pangaea) that broke apart about 200 MYA and drifted to their present positions.
Early evidence:
Fit of the continents, esp. along continental shelf.
Fossil match across oceans, indicating an original continuous range
-Mesosaurus, Glossopteris
Matching Geologic Features
-mountain ranges (mts. similar to Applachians are found in British Isles and Scandinavia)
-rock type (rock formations in Africa have matches in corresponding placed in S. America)
Ancient Climates
-fossils of tropical origin found in cooler climates
-evidence of glaciation near the equator
Who first proposed “continental drift”? What did he call his proposed supercontinent?
Alfred Wegener; Pangaea
What is meant by plate tectonics?
The theory that the lithosphere (the outer rigid part of the Earth) is made up of about 20 rigid sections called plates that move relative to each other. (These plates overlie the hot, plastic asthenosphere which allows the plates to slowly move.)
What are the three main types of plate boundaries?
- Name and/or identiy some features of each.
- Describe the relative motion at each boundary.
- Cite examples of each type of boundary.
Divergent
-pull directly away from each other
-most are located along the crests of oceanic ridges on ocean floor
-first upwarming, then rift/rift valley, then linear sea, then ocean
-Icealand, Great Rift Valley of East Africa, Red Sea, Gulf of California, Atlantic Ocean
Convergent
-push directly against each other
-subduction zone is a region where an oceanic plate descends beneath another plate
-Central American Trench, Andes Mts., Cascades, Aleutian Trench and Islands, Mariana Trench, Himalayas, Appalachians
Transformation Faults
-plates slide sideways past one another
-most are on the ocean floor as the fault between offset ridge segments
-San Andreas Fault in California
Sketch and label a cross-sectional diagram of a subduction zone. With what type of boundary is it associated? Where are some subduction zones located on the Earth?
Convergent
Cascade Range; Andes; Aleutian, Mariana, and Tonga islands
List, describe, and give examples of the three types of convergence.
oceanic-continental (deep ocean trench, volcanic arc)
oceanic-oceanic (deep ocean trench, volcanic island arc, Mariana Trench)
continental-continental (San Andreas Fault in California)
List and describe some more recent evidences for plate tectonics.
Paleomagnetism (“fossil magnetism”)
-polar wandering: the position of the Earth’s North Magnetic Pole appears to have “wandered” over the past several hundred million years. Previous positions of the North Pole as measured from different continents appear to be the same as they should be if the continents were originally much closer together.
-magnetic reversals
Hot Spots
-a rising mantle plume produces a chain of volcanic islands and seamounts from the Pacific plate moving over a “hot spot”; ex. Hawaiian Islands & Emperor Seamounts
Deep Sea drilling
-age of ocean sediments
-ocean basins are geologically “young” compared to the continents
-age of sea-floor increases giong away from the ridges; it is youngest at the ridge crest, oldest at the continental margin
-depth of ocean sediments
Earthquake epicenters and foci
-sediments get thicker and older going away from the ridges
-explains location and sequence of shallow focus, intermediate focus, and deep focus earthquake epicenters
-only shallow focus earthquakes are associated with divergent and transform boundaries
-deep focus earthquakes are only associated with oceanic trenches (convergent boundaries) where a plate is subducted deeply beneath another plate
What is the approximate rate of divergence across the Atlantic?
2 cm per year
