Recall Continental

Drift?

Seafloor Spreading

• Nazi hunting and better maps of the ocean floor– Discovery of Mid-Atlantic Ridge

• Magnetism and more Nazi hunting– Discovery of magnetic stripes of alternate

polarities on the ocean floor

• Even more Nazi hunting– Seafloor sediment thickens with distance from the

MAR

The Crust Can’t Grow on and on…• If new oceanic crust is

constantly added at mid-ocean ridges, why doesn’t Earth grow?

• Look to the maps…– Earthquake locations

Odd Patterns…

• Scientists in Japan noticed odd patterns when they plotted earthquake foci– Appeared to lie along a down-sloping plane

• Volcanoes and ocean trenches coincided with these earthquakes

• It seemed like the ocean crust was being pushed down into the mantle, creating earthquakes as it did so

Red lines represent Ocean trenches

How Could This Happen?

• Remember the Two Types of Crust?– Oceanic crust

• Dense, thin (~4-7 km), iron-rich• Oldest oceanic crust is 180 million years old

– Continental crust• Less dense, thick (35-70 km), silicon-rich• Oldest continental crust is 4 billion years old

– Ocean plate forced to sink when encountering material of lesser density (ocean/continental)

The Grand Unifying Theory…of Geology

• Theory of Plate Tectonics

• The rigid lithosphere is broken up into ~16 tectonic plates– May include both oceanic and

continental crust– Each plate moves as one giant

mass– Collisions responsible for

earthquakes and volcanoes

How do plates interact?

Three main types of interactions

• Divergent plate margins:– Plates move apart, new lithosphere is created

• Convergent plate margins:– Plates come together and one plate always subducts

beneath the other back into the mantle

• Transform-fault margins:– Two plates slide past one another

Divergent Plate Margins• Within an ocean basin, marked by a mid-

ocean ridge that exhibits volcanism and earthquakes

• Also known as spreading centers

Ocean-Ocean Convergent Margin• When two ocean plates meet, one descends

or subducts beneath the other►Depends on which is older colder denser

Subduction Leads to Melting

• As plate enters the asthenosphere – Increase in pressure + temp forces out water into

asthenosphere above it causing it to melt– This creates a chain of volcanoes (island arc)

Ocean-Continent Convergence

• Plates with continental edges override ocean edges because they are less dense– Again, the ocean crust melts as it subducts, giving water

to the asthenosphere which also melts– Coastal trench, huge earthquakes/volcanoes on land– Melting along continent edges richer in silica (Si) which

makes them viscous and explosive

Continent-Continent Convergence

• Neither plate edge can subductNeither plate edge can subduct– Creates a sort of double crust forming world’s Creates a sort of double crust forming world’s

highest mountain rangeshighest mountain ranges

The Himalayas

Transform-Fault Margins

• When two plates slide past each other lithosphere is not created or destroyed– The rocks on either side

of the faults are often different ages due to displacement

– Large earthquakes

Hot Spots?

• Some volcanoes occur at distances from plate boundaries…– Hot columns of rock

from the lower mantle

– Act like a blowtorch on plate moving above

– Mostly volcanism– Ex: Hawaii

Hawaii 0.8-present

Maui <1.0Molokai 1.3-1.8

Oahu 2.2-3.3

Kauai 3.8-5.6

Direction of plate motion

Decreasing age

Hot Spot

Loihi