Ch.2 - Plate Tectonics & the Ocean Floor

Panthalassa

one large ocean

Sir Edward Bullard

used computer models to fit continents

Two possibilities of glaciation in now tropical regions

• glacial ice age

• some continents now in tropical regions were closer to the poles

Wegener’s proposed mechanism for continental drift

continents plow through ocean basins due to gravitational attraction of equatorial bulge and tidal forces from the sun & moon

Where did wegener think mountains formed?

at the leading edges of continents

What generates Earth’s magnetic field & polarity?

convective movement in outer core

Do magnetic objects align parallel or perpendicular to magnetic field? What type of rocks was magnetic polarity recorded in?

parallel; igneous rocks

Magnetic dip

magnetite particles in sedimentary or igneous rocks align with Earth’s magnetic field

Magnetometer

instrument towed behind ocean vessel to measure earth’s magnetic field

Magnetic anomalies

pattern of north-south magnetism strips

Harry Hess

did depth recordings to show sea floor features

2 Key ideas of History of Ocean Basins

• sea floor spreading

• convection cells in asthenosphere as driving mechanism

Mid-ocean ridge

• spreading center

• convection cells pull sea floor apart

• new oceanic crust forms at ridges

  • split apart at ridge and move toward subductions zones

Subduction zones

• oceanic trench site of crust destruction

• generates deep ocean trenches

Deep ocean trenches

• deepest parts of ocean

• cause largest earthquakes

Frederick Vine & Drummond Matthews

• analyzed igneous rock strips around mid-ocean ridge

• strong evidence for continental drift

• ocean floor act as conveyer belt carrying continents

Which ocean has most symmetric pattern of age distribution? Least symmetric?

most: Atlantic

least: Pacific (numerous subduction zones)

Which ridge rifted pangea apart?

mid-atlantic ridge

Heat flow

heat from Earth’s interior released to surface

Heat flow high or low at mid-ocean ridges? Subduction zones?

mid-ocean ridge: high

low: subduction zones

Life cycle of Ocean Basin formation

1. Embryonic - heat source/uplift

2. Juvenile - spreading & downdropping

3. Mature - mature basin with mid-ocean ridge

4. Declining - subduction zones; basin shrinks

5. Terminal - narrowing of ocean

6. Suturing - ocean disappears; continents collide

Divergent boundary

• 2 plates moving apart

• new ocean floor (lithosphere) created

• shallow earthquakes

Does faster spreading or slower spreading create more energy for earthquakes?

slower

Divergent boundary creation of ocean basin

• heat source leads to upwarping

• movement and splitting of land creates rift valley

• inc mvmt and downdropping eventually flooded by seawater & forms sea

• eventually sea becomes ocean

3 types of divergent boundary spreading centers

• oceanic ridge

• oceanic rise

• ultra slow

Oceanic ridge

• slow spreading

• steep, rugged slopes

• stronger seismic activity

• prominent rift valley

Oceanic rise

• fast spreading

• gentle slopes

• weaker seismic activity

• small rift valley

Which produces more crust: ridges or rises?

oceanic rises

Ultra slow

• deep rift valley

• widely scattered volcanoes

• arctic and southwest india

Convergent boundary

• plates move toward each other & collide

• oceanic crust destroyed

• generates ocean trench & volcanic arc

• deep focus earthquakes

Deep ocean trench marks what?

beginning of subduction zone

Volcanic arc

highly active row of volcanoes above subduction zone

3 types of convergent boundaries

• oceanic-continental:

  • subduction causes trench

  • continental volcanic arc

• oceanic-oceanic:

  • subduction causes trench

  • creates volcanic island arc

• continental-continental

  • collision creates tall mountain range

oceanic-continental

• denser oceanic plate subducted under continent

• mantle wedge: superheated by subducting plate and partially melted; rises to surface and feeds volcanoes

Does a trench develop well with more or less distance?

More; gives more time for crust to cool and contract

Transform boundary

• 2 plates move past one another

• offset segments of ocean ridges (2 divergent boundaries)

• shallow but strong earthquakes

Are transform boundary offsets parallel or perpendicular to mid-ocean ridge?

perpendicular

2 types of transform faults

• Oceanic transform fault: ocean floor only

• Continental transform fault: cuts across continent

  • ex) san andreas fault

Where do transform faults occur?

between mid ocean ridge segments

Are mantle plumes & hotspots intraplate or interplate features?

intraplate

Hotspots

intraplate regions of intense volcanic activity that remain in one location over geologic time

Nematath

chain of extinct volcanoes, records plate movement

Are seamounts and tablemounts inter or intra plate features?

intra

Seamount

tall underwater volcanoes with rounded/conical tops

Tablemount (guyots)

tall underwater volcanoes with flattened tops

Continental accretion

continental material added to edges of larger continents through plate motion

Terrane

fragments of crustal material broken off one plate & accreted/sutured onto another

All continents fit together with the least number of overlaps and gaps when the continents are matched along ____

edges at around 2,000 meters depth

The east african rift valleys are an example of a _______ plate boundary

continental-continental divergent