2 Plate tectonics part one Flashcards
Earth’s Plates
Rigid segments of the Earth’s lithosphere
Always in motion relative to each other
Made of both oceanic and continental crust
Alfred Wegener : continental drift
Meteorologist/Climatologist/ Geologist
Studies the evidences for continental movement
The Origins of the Continents and Oceans, 1915
Proposed that all continents were once part of the supercontinent of Pangaea
Evidence for continental drift
- Continental puzzle pieces
Using the continental shelf* improves the fit
*Continental shelf: the part of the continent covered by fluctuations in sea level - Distribution of climatic belts
Glaciers
Glacial evidence in areas that do not have glaciers today
Unlikely to have glaciers, even with dramatic climate change
-Distribution of climatic belts
Coal swamps
Deserts
Reefs
Salt flats
-Distribution of fossils
Non-ocean swimming creatures found on both sides of the Atlantic and Indian Oceans
-Matching geologic units and mountain belts
Example: Appalachian Mtns (US), Atlas Mountains (Morocco), Scottish Highlands (Great Britain), and Caledonian Mountains (Norway)
Evidence against continental drift
No mechanism
HOW do the continents move?
Plow through the ocean rocks like an icebreaker ship?
Float over the oceans?
Slide over the oceans?
Expanding earth?
Mechanism for continental drift
~30 years after Wegener’s death
Seafloor spreading
The History of the Ocean Basins
1962
Harry H. Hess
Ocean floor is constantly recycled (trenches)
Ocean floor is constantly regenerated (ridges)
This “pushes” the continents around
Evidences?
Oceanic topography
Paleomagnetism: stripes on the floor because of changing magnetic poles
Age of the ocean floor
Sediment thickness on the ocean floor
Discovery of seafloor spreading
Rear Admiral Harry H Hess
Keeps the sonar on all the time
Even when there are no enemy subs around
Creates thousands of miles of surveys of ocean floor
Gathering evidence of seafloor spreading
sonar= sound navigation and ranging
Mapping the Sea Floor: Earthquakes
Odd, shallow earthquakes in the ocean
Along definite lines: plates
Ocean floor topography/mapping
Marie Tharp
Maps the ocean floors for Bell Laboratories
Publishes the first detailed topographic map of the Atlantic Ocean with Bruce Heezen (1959)
Ocean floor is not featureless
Flat plains interrupted by vast, linear mountain ranges
These mountain ranges have central rift valleys
mid ocean ridge draw
see screenshot
sea floor sediments
As you move away from the mid-ocean ridge, the sediment becomes both older and thicker
sea floor spreading
Ocean floors are spreading apart symmetrically along the ocean ridge.
New ocean floor is created by volcanoes on the mid-ocean ridge
Old ocean floor is destroyed by being shoved into the mantle at the ocean trenches
How to test sea floor spreading hypthesis
How to test if this is true?
Could check for ages of the seafloor
Expensive, time-consuming
Could check for magnetic polarity of seafloor
Easier, can do with a ship on the surface
Mapping the ocean floor: paleomagnetism
Vine and Matthews
Testing the seafloor spreading theory
Discovered:
Alternating strong and weak magnetic fields (poles switching)
Positive (strong)
Negative (weak)
Creates a pattern of stripes on the ocean floor
What powers the earth’s magnetic field?
Movement of iron molecules in liquid outer core
Earth’s magnetic field
Magnetic field is NOT parallel to the rotational axis
Our magnetic poles WANDER
Over the last 1800 years
North is always near the North Pole, but not always in the same location
