Intro to Oceanography - Plate Tectonics I Theory and early evidence

In what ways do the lithospheric plates interact with the neighboring plates?

(1) Divergent (2) Convergent (3) Transform

Paleomagnetism

- study of Earth's ancient magnetic field - Interprets where rocks first formed

Evidences of Continental drift

- All continents fit around the Atlantic - best fit @ 2000m -Edward Buillard - Matching mountain sequences w/ similar proportions and scale across continents - Glaciers leave glacial deposits and scarring by scraping - Edward Suess discovered ferns across continents - Fossils distributed across continents

Divergent Plate Boundaries

- Boundaries between plates moving apart - Divergent oceanic crust - for example, the Mid-Atlantic Ridge - Divergent continental crust - for example, the Rift Valley of East Africa. - Oceanic crust is formed!

Moho

- Boundary between crust and mantle - Upper mantle = denser rock - Moho's closest to the oceanic crust 4-10 km thick

Density Stratification:

- Earth has distinct layers - Each deeper layer is heavier than the next. - Density: expression of heaviness amount of mass contained in a volume - (Density = mass/volume; D=g/mL; D=g/cm3)

Earth's magnetic field and paleomagnetism

- Earth has magnetic polarity - North and South polarities - Magnetic polarity recorded in igneous rocks - Magnetite in basalt

The Tectonic System Is Powered by Heat

- Mid-ocean ridge = spreading center, very high heat flow - Subduction zones = oceanic trench site of crust destruction, very low heat flow - Subduction can generate deep ocean trenches.

Earthquakes as Evidence

- Most large earthquakes occur at subduction zones. - Earthquake activity mirrors tectonic plate boundaries.

Slap Pull/Suction

- Old, dense heavy plate sinks pulling plates behind - Conveyor belt movement - Stronger than ridge push

Convergent Plate Boundaries

- Regions where plates are pushing together. - Compression produces buckling and shortening. - Oceanic crust destroyed - Ocean trench - Volcanic arc - Great forces involved - Mineral structure changes associated

Isostatic Adjustment

- The vertical movement of the crust - The result of the buoyancy of the earth's Lithosphere, as it floats on the denser, plastic-like asthenosphere below. - Oceanic Crust is denser, floats lower in the mantle. - Continental Crust is thicker, less dense; floats higher. - Where the Continental Crust is thickest -it floats even higher... bc there is a thick mass of crustal material Beneath them, buoying them up...

Land - Continental Crust

- Thick , light (2.7 g/cm) - Granite -Course - Light

Ocean - Oceanic Crust

- Thin, dense (3.0 g/cm) - Basalt - Fine/Course - Dark

Magnetometer

- instrument towed behind ocean vessel - measures Earth's magnetic field and how it was affected by sea floor rocks

Magnetic dip

- magnetite particles in sedimentary rocks or igneous rocks such as basalt align with Earth's magnetic field - Magnetite: formed from cooling magma that contains iron oxide

Applications of Plate Tectonics

-Mantle Plumes and Hotspots +Intraplate features = Volcanic islands within a plate, Island chains - Record ancient plate motions

Geopoetry/ Sea Floor Spreading 1

1. New ocean crust formed at ridges 2. Volcanoes that form along the ridge are originally close to the surface of the ocean, where they can be eroded flat on top 3. Deep ocean trenches are where the seafloor finally descends back into the mantle 4. Mud and sediments deposited on the ocean floor thin toward the mid-ocean ridges are because the age of the seafloor gets younger, thus less and less time to accumulate sediments

What are the 3 types of Convergent Plate Boundaries?

1. Ocean-Continent - Tectonic process - Subduction - Ocean-Continent crust interaction - Old Sea floor is destroyed 2. Ocean-Ocean - Denser plate is subducted - Old Sea floor is destroyed - Deep trenches generated - Volcanic island arcs generated 3. Continent-Continent - Tectonic process - Collision - no Subduction - Continent-Continent crust Interaction - Sea Floor is not created or destroyed...

What did Hess notice?

1. mid-ocean ridges 2. deep trenches at continental margins 3. undersea volcanoes had flattened tops 4. Less sediment at mid-ocean ridges

Paleogeographic Reconstructions

180 million years ago - Pangaea separated - N. and S. America rifted from Europe and Africa - Atlantic Ocean forms - 120 million years ago - S. America and Africa clearly separated - 45 million years ago - India starts Asia collision - Australia moving north from Antarctica

Ridge Push

Buoyant magma pushes up driving plates apart

Mechanisms and Doubt

Continents "plow" through the ocean basins - Calculations are incorrect Continents moved because of gravitational attraction of equator and sun/moon. - Gravitational forces are too small.

Alfred Wegener (1912)

Could these land masses have been joined together at some distant time and subsequently drifted apart?" - Further supported continental drift

Geopoetry/ Sea Floor Spreading 2

Harry Hess (w/ the help of Tharp's maps!) - mapped the seafloor 1. Mid ocean ridges 2. Deep trenches at continental margins 3. Seafloor volcanoes w/ flat tops - Hess proposed that convection cells, in the mantle drove seafloor spreading - As rocks age and cool, they become more dense, and due to isostasy, the ocean crust sinks deeper into upper mantle. Seafloor gets deeper with distance from mid-ocean ridge (MOR)....

Heat Flow

Heat from Earth's interior released to surface

Buoyancy & Isostatic Pressure

If weight added or removed from object floating, equilibrium disturbed!

Pressure affects Melting Points

Increase in pressure = higher melting point!

Abraham Ortelius - 1596

Suggested Continental Fit - Continents 'drift'

Types of Spreading Centers

Oceanic rise - Fast spreading -Gentle slopes -East Pacific Oceanic ridge - Slow spreading - Steep slopes -Mid-Atlantic Ultra-slow -Deep rift valley -Widely scattered volcanoes Arctic and southwest India

How Fast Do Plates Move?

On average - 2-12 cm (1 - 5 inches)

Compositional Layers

Order (Top to Bottom Layers) - Continental crust (granite) - Oceanic crust (basalt) - Mantle (silicate minerals) (84% Earth's volume) - Core (iron with nickel and sulfur)

Mechanical Layers (movement)

Order (Top to Bottom Layers) - Hydrosphere (liquid) - Lithosphere (rigid) - Aestheosphere (plastic) - Mesosphere (plastic) - Outer core (liquid) - Inner core (rigid)

Marie Tharp

Pioneering Mapmaker of the Ocean Floor

Plate Tectonics and Intraplate Features

Seamounts - Rounded tops Tablemounts or guyots - Flattened tops - Subsidence of flanks of mid-ocean ridge - Wave erosion may flatten seamount.

Magnetic Anomalies

Stripes of magnetism were a historical record of new sea floor

Isostatic Rebound

Vertical movement upward - canada rose 1640- 2300 ft

Wilson Cycle

life cycle of geological movement

Plate Boundaries

records motion and rate of how fast the boundaries move