Plate Tectonics Unifying Theory
Explains: - Mountain belts - Ocean basins - Energy and Mineral distribution - Earthquakes and volcano distribution - Ocean circulation and climatic belts - Etc.
Continental Drift • Hypothesis by Alfred Wegener (German meteorologist, 1915) • Idea that continents are mobile; once a supercontinent Pangaea (Pangea) • Evidences found on multiple continents: 1. “Fit” of continents 2. Location of glaciers 3. Location/distribution of fossils 4. Rock types and structural similarities 5. Paleoclimates preserved in rocks
Glaciation - Reconstruction of continents into a southern Gondwanaland continental land mass explains glacial evidence - Striations on rock cut by glacial movement indicate direction of movement - Glacial tills present
Fossil Distribution (difficulty to impossibility to cross ocean)
Glossopteris
Matching of Rock Types on now separated continents across the Atlantic Geologic structures. Mountain belts. Rock types. The Appalachians. Rock ages The Caledonides
Minerals and rocks form in certain climates. Reconstruction puts mineral in the right place for formation
By the reconstruction of the continents the presence of inferred ancient deserts, evaporite deposits, reefs, and coal deposits were easily explained.
Ex. Why would Russia have coal deposits in boreal to arctic conditions?
Criticisms of Drift •
Why wasn’t the continental drift hypothesis accepted? – There were no mechanisms for moving continents. – When Wegener died, the debate did too.
• The drift hypothesis needed new and different evidence. • This was provided by: - Paleomagnetism - Age of the ocean floor - Volcano distribution - Earthquake distribution - Hot spots
Why is the red shading wider at the Nazca Plate than at the Mid-Atlantic Ridge?
• Drilling in the late 1960s recovered crust samples. – Ages increase away from the mid-ocean ridge. – Ages are “mirror images” across the mid-ocean ridge. • Strong supporting evidence for sea-floor spreading.
Composition and Layers of Earth Heat of the Earth 1. Particle collision 2. Gravity settling, 3. Radioactive decay* What are the dominant elements? Crust: Oxygen – 46%, Silicon – 28% Whole Earth: Iron – 35%, Oxygen – 30% Compositional Differences: Gravity segregation/Density Differentiation Subdivision
Composition
T° (C)
Density (g/cc)
Continental Crust
Felsic (Granite) (Al, K, Na)
0 - 1200
~2.7
Oceanic Crust
Mafic (Basalt, Gabbro) (Fe, Mg)
0 - 1200
~3.0
Mantle
Magnesium silicates (mafic)
2500 - 5000
3.3 – 5.7
Outer Core
Fe and Ni (Liquid)
5500 - 6500
9.9 – 12.2
Inner Core
Fe and Ni (Solid)
6500
12.6 – 13.0
Crust: Continental and Oceanic Lithosphere: Includes crust and upper mantle Mantle – solid to plastic Outer Core - liquid Inner Core - solid
P-Wave Travel
S-Wave Travel
“Rock” properties and wave types Dictate travel times and propagation P waves travel through all materials But S waves can’t pass through liquids
Magnetic Striping Comparable magnetic field reversals
Magnetic Field
Magnetic Striping and Polar Wandering are evidences of Plate Tectonics, but…. 1. What causes the striping? 2. What are the effects? (Auroras) 3. What about other planets?
http://odin.gi.alaska.edu/FAQ/
Solar winds combine with magnetic field and atom/molecule collisions lead to auroras
Earth: First view of both poles showing “mirror” auroras - 2001
Saturn (800 mil. mi. away from earth)
Aurora around Jupiter
Images from Hubble
Magnetic Fields?
Earth and Mars are thought to have condensed about 4.6 billion years ago from the solar nebula, a giant cloud of hot gas swirling around the young Sun. The evolution of the two planetary neighbors has been driven ever since by the loss of heat produced by radioactive decay within their interiors. Similar crusts, cores (except no longer molten on Mars), and early processes. But…. Earth is dynamic, Mars is static. What other differences?
Migrating whales and other cetaceans appear able to follow magnetic lineations in the seafloor, which are aligned predominantly north-south
Gray Whale
Using a combination of magnetic “stages” and radiometric dating hominid fossils have been age dated in Africa (Ethiopia). Dates indicate the hominid genus Homo evolved over 2 m.y. ago from an older genus Australopithecus
Earthquakes - Found to be concentrated at the areas of spreading (shallow), convergent (deeper), and transform “plate” boundaries; some occur within plates (intraplate) - Depth of quakes important too.
Hot spots are not related to plate margins. Source of magma is believed to be deep within the mantle at fixed locations. Fig. 2-14, p. 38
Plate Tectonics and the Very Unstable Earth – It’s Constantly Changing - 7 major plates - Numerous minor plates What are the boundary types and properties?
Rifting
Continental lithosphere can break apart. Continuation of this process leads to full sea-floor spreading.
Divergent Boundary: -
Shallow low mag earthquakes Mostly non-explosive volcanoes Tensional stress
Convergent – Tectonic plates move together.
The process of plate consumption is called subduction.
Mod – High mag quakes No volcanoes Shear stresses
Convergent
Present
185 m.y.
135 m.y.
Divergent Transform
Three Plate Boundary Types – Importance - Types of earthquakes and volcanoes and hazard potential - Types of rocks and minerals present - Location of resources - Control on soils and human activity/development
65 m.y.
Change over time
Type of Boundary
Example
Types of Rock/Volcanoes
Physiographic Features
Mid-Atlantic Ridge
Basalt/Shield
Mid-Ocean Ridge with Central Rift Valley
East African Rift
Basalt and Rhyolite/Shield to Intermediate
Rift Valley
Andes
Andesite and more felsic/Composite
Offshore trench, mountain belt
Aleutians, Japan
Andesite/Composite
Trench, Island Arc
Himalayas
Deformation/Minor
Mountain Belt
San Andreas Fault
Deformation/Minor or None
Fault Valley and Lateral Offset
Divergent - Oceanic - Continental
Convergent - Continent/Ocean - Ocean/Ocean - Continent/Continent Transform
Plate Boundaries and Features. Know and be able to Draw/Sketch these.
A
Continents: Felsic to Intermediate Al, K, Na Light Color
Oceans: Mafic Fe, Mg Darker Color
Mineral Resource occurrence is often predictable. Plate boundaries (convergent) are local areas for the formation of Copper, Gold, and more minerals.
