Chapter 2

Alfred Wegener

German meteorologist and polar explorer

Wrote The Origins of Oceans and Continents

He suggested land masses slowly move (continental

drift)

He hypothesized a former supercontinent, Pangaea

He produced 5 Evidences to support this

Wegener’s Evidence for Continental Drift

1. The continents seem to fit together

2. Glacial evidence of past glaciers found on four continents

3. Climate Belts Follow a specific order

4. Identical fossils found on widely separated landmasses

5. Mountain belts connect across the Atlantic

Mid-Ocean Ridges (MORs)

Elevated areas of ocean floor, usually in the middle of an
ocean

Deep Sea Trenches

Deep areas of the ocean that tend to boarder one side of a volcanic arc

Volcanic Arcs

Islands in an arc shape created by volcanism

Fracture Zones

Areas where fractured rock breaks up and moves, or offsets, the MORs

Seamount Chains

Linear islands that can erode away and be below seal level

Observation on seafloor sediment

It is thinner near MORs and thicker away from MORs

Composition of Continental crust

Mostly made of granites (felsic)

Composition of Oceanic crust

Mostly made of Basalt (mafic)

Observation on heat

Heat flow is greater near MORs

There must be a sorce of heat near MORs

Observation on Earthquakes

Earthquakes in the

Oceans occur in patterns

Seismic Belts, follow bathymetric features like MORs, Trenches and fracture zones

Observations on ridge axis

MORs have a ridge axis that is lower elevation

Seafloor Spreading

Magma rises under MORs pushing each side of the ridge axis away from each other, magma then cools to form new crust

Marie Tharp

Bruce Heezen

Harry Hess

Paleomagnetism

The study of the Earths ancient magnetic field

Magnetic Declination

Angle of the needle and a given line of longitude (reading a

compass)

Magnetic Inclination

The angle of a needle and the surface of the earth

Apparent Polar Wander

Every time a rock forms, it will “Capture” the Earth’s magnetic field in that moment

When we plot the pole based on these layers, and assume the continent is fixed, then we get an ___________ path

Two interpretations of Apparent Polar Wander

1) The pole moves relative to the continent

2) The continent moves relative to the pole

A Proof of Seafloor Spreading

The magnetic field sometimes “flips”

Magnetism in sea-floor rocks varies farther from MOR

Divergent Boundary

Forms when two plates move apart through the process of seafloor spreading

Spreading crust is filled in with magma from the asthenosphere, which creates MORs

Ocean crust grows wider

Exclusively happens in oceans

Convergent Boundary

Two plates move towards each other. At least one plate MUST be oceanic

The older, colder, more dense oceanic plate will bend down into the asthenosphere in a process called subduction

Earthquakes and the Fate of Subducted Plates

Very large earthquakes (EQs) occur where the two plates meet, due to friction

EQs also occur very deep in the subducting plate, known as the Wadati-Benioff Zone

Accretionary Prism

Scraped up sediment off the subducting plate onto
the overriding plate

Found in Convergent Boundaries

Deep Ocean Trench

Area where two plates first meet and one ocean crust is
thrust downward into the mantle

Found in Convergent Boundaries

Continental Volcanic Arc

Present on continental crust when there is O-C convergence

Island Volcanic Arc

Present on the overriding oceanic plate in O-O convergence

Transform Boundary

When two plates slide past one another. No creation and no destruction

MOST occur in Oceanic crust

Triple Junction

The meeting of any 3 plates and plate boundaries

Hot Spots

Anomalously hot mantle material buoyantly rises and melt through the crust to produce volcanoes

The crust moves over the spot to create a chain of volcanoes in time

Continental Rifting

Continental crust moves apart creating a rift

If enough rifting occurs, ocean crust will be

created and the rift will evolve into a divergent boundary

Collision

Convergence will eventually consume all the ocean crust, bringing two continental masses towards one another

Continental crust cannot sink into the mantle, so it builds upwards, creating mountains

Ridge Push

Gravity pulls down the elevated MOR, causing it to push on

the lower elevated abyssal plains

Slab Pull

The subducting slab is cold and dense and sinks into the asthenosphere, pulling all of the attached ocean crust behind it

Mantle Convection