Plate Tectonics and Continental Drift

What evidence did Wegener use to support his continental drift hypothesis?

"Jigsaw puzzle" fit of Africa/South America, glaciers in unexpected locations, distribution of climate zones, fossil distribution, and matching rock types

Why was Wegener's continental drift hypothesis not initially accepted?

Wegener didn’t have any good ideas about what caused the continents to move

Where would you go to find a glacier?

Glaciers only form near the poles, not near the equator

Glaciers from ~250 million years ago had a wide distribution, were in areas that are now near the equator

Explanations?

If overall climate were colder, then other areas

would have been covered by glaciers too

If continents were in different locations 250 million years ago, glacial locations would make sense

What is "polar wander?"

Rocks of different ages from the same site can show different inclination and declination

What is the significance of "polar wander" paths?

Each continent suggests a different polar wander path, indicating that the continents have moved over time

Inclination

up and down

Declination

side to side

Paleomagnetism

Magnetic particles in rocks have ‘3-D’ orientation affected by both the declination and inclination at the time of formation

List major features of the ocean floor.

Abyssal plains, mid-ocean ridges, and deep-ocean trenches

Abyssal plains

Flat regions that are about 4-5 km below sea level (make up most of ocean floor)

Mid-ocean ridges

Long submarine mountain ranges with peaks that are 4~2 km below sea level

Deep-ocean trenches

Areas where the ocean floor is very deep (8 to 12 km below sea level) All deep-ocean trenches border volcanic arcs (chains of active volcanoes)

Where is sediment thin and where is it thick on the ocean floor?

Sediment is thin near mid-ocean ridges and thicker away from ridges

Where is heat flow high and where is it low on the ocean floor?

Heat flow is high at mid-ocean ridges and lower further away from mid-ocean ridges

What causes magnetic anomalies on the ocean floor?

Changes to the polarity of Earth’s magnetic field that have occurred in the past

Zones of strong and weak magnetism

These zones are symmetric (centered on mid-ocean

ridges)

Polarity reversals

Polarity reversals cause volcanic rocks to have reversed magnetism (declination differs by 180°)

What occurs at mid-ocean ridges according to the concept of seafloor spreading?

Rising magma erupts, cools to form new rock, and new crust moves away from ridges

How are magnetic anomalies distributed around mid-ocean ridges?

Symmetrically

According to the concept of seafloor spreading, where does the seafloor "dive" into the mantle?

At trenches

Changes in seafloor rock polarity either

add to or subtract from Earth’s magnetic field strength

What is the lithosphere?

A rigid ‘shell’ around the Earth

What are tectonic plates?

Pieces of lithosphere

plate boundaries

Breaks separating plates

What is an active continental margin?

A continental margin that is a plate boundary

What is a passive continental margin?

A continental margin that is not a plate boundary

How many major plates

12

several smaller

microplates

best way to identify a plate boundary

Earthquakes

Lithosphere is made of

both crust and the upper mantle

When ‘loaded”

asthenosphere flows

Lithosphere “floats” on

asthenosphere

Displacement =

moving

Floating solids displace a volume of fluid with mass

equal to their own mass

Why do we say that the lithosphere floats on the asthenosphere?

The asthenosphere is solid, but it flows, and thus acts somewhat like a liquid

What are the three types of plate boundaries?

Divergent, Convergent, and Transform

Describe a divergent plate boundary

Tectonic plates move apart from each other

What happens with divergent plate boundaries

Plates move apart, asthenosphere rises and melts Magma erupts, cools, forms new lithosphere

Plates move apart, more asthenosphere moves up