Origin of a theory:

Continental Drift to

to Plate Tectonics

Sea floor Spreading

Geologic Processes Earth's major geological zones: core: solid center surrounded by molten layer, mostly iron and nickel;

mantle: solid zone with hot, partly molten "plastic" upper layer known as asthenosphere; rich in __________________________ __________________________;

crust: outermost thin layer, location of mineral resources & soil. lithosphere: solid crust and uppermost part of mantle. This layers is divided into several Tectonic Plates

Plate Tectonics • A consistent global framework that accounts for most of

Earth’s features and processes • Replaced a mixture of vague, disconnected explanations • Yet, was accepted only after decades of skepticism

Plate Tectonics Explains:

1. The matching of the coastlines of Africa and Europe with those of South and North America.

2. The presence of mid-ocean ridges.

3. Volcanic mountain chains and patterns of earthquakes around the Pacific basin.

Geology as a Science James Hutton (mid 1700s):

1. “…the present is the key to the past” = ___________________. (Replaced ____________________)

2. Most observed processes are slow, with gradual effects = Earth is OLD.

3. “…no vestige of a beginning, no prospect of an end” = Earth is OLD

Geologic Cycles • Examples such as the hydrologic cycle, the

rock cycle, etc. came to dominate geology. • Recycling of “plates” would seem to be a

good fit with this line of thinking. • Problem: Lack of a good mechanism to

explain large-scale motions of plates.

Continental Drift • Alfred Wegener (1880-1930) was a

polar researcher, primarily a ______________. His expedition was the first one to ever over-winter on the Greenland ice sheet.

• Wegener and others (1915) proposed that the continents had once been together (Pangaea = “_____________”) then split and drifted apart.

• First split was north (Laurasia) and south (Gondwanaland) with the expanding Tethys Sea in between.

Wegener’s Evidence 1) _________________ --seem to fit together

(Bacon, 1620). Even better if continental shelves are used.

Wegener’s Evidence 2. _____________(including

mountain ranges): types, features, history.

Matching coastlines of South America and Africa, as well as matching geology for older rock units on the two continents.

Wegener’s Evidence 3. ______________: flow directions and positions

Wegener’s Evidence 4. Distribution of animal and plant ____________.

Wegener’s Evidence Put together, it was pretty compelling, but…

No Adequate Mechanism Proposals for Continental Drift included (and were dismissed as): 1. _______ (too weak). 2. Rotational effects (too weak). 3. ______________(Impossible! Continents are too

weak, would be shredded. Like “pushing a brick of cheese through a concrete wall”).

Wegener died in 1930 while attempting to collect more evidence.

• Mapping of the ocean basins revealed chains of ridges like the stitching on a baseball, offset by faults and filled with volcanic activity.

• In 1962, _____________ published “History of Ocean Basins,” which furthered the idea that oceans were spreading at the mid-ocean ridges. Sea floor spreading became an accepted idea.

Post-WWII Discoveries

Magnetic Reversals and Seafloor Stripes

• Iron-bearing minerals in volcanic lavas will record the direction of Earth’s magnetic field at the time they freeze into rock (upon ____________).

• Since Earth’s field reverses, the spreading sea floors act as giant tape recorders. _____________________can be determined.

Post-WWII Discoveries

Inclination of Earth’s magnetic field varies with latitude. It is parallel to the surface at the equator and vertical over the magnetic poles.

Magnetic Striping off the coast of the Pacific Northwest.

More Evidence: _______ of Sea Floor

More Evidence: Location of Known ________________

More Evidence: _____________ Locations

Major Plates on Earth

Plate Tectonics: the Prevailing Theory By the mid-1960’s, mantle convection was accepted as a viable mechanism for the movement of the crustal plates, and the theory of Plate Tectonics was established.

Plate Tectonics

Plate boundaries can be seen as undersea mountain chains (Mid Ocean Ridges—diverging plate boundaries), deep sea trenches (converging boundaries), or major faults that primarily separate sections of mid-ocean ridges (transform plate boundaries).