Plate Tectonics

Midterm

What observations did early cartographers make that supported plate tectonics?

They noticed that some continents, especially Africa and South America, have matching coast lines that fit together like puzzle pieces.

Who was Alfred Wegener?

A German scientist who proposed the idea of moving continents in the early 1900s.

What is Continental Drift?

A hypothesis stating that continents were once joined together and have slowly moved apart over time.

What fossil evidence supported Continental Drift?

Identical fossils of reptiles and plants found on continents that are now separated by oceans.

How did rock and mineral formations support Continental Drift?

Matching rock types and mountain ranges were found on different continents, indicating they were once connected.

What climate evidence supported Continental Drift?

Fossils of ferns and coal deposits found in Antarctica suggest it once had a warm climate.

Why was Wegener’s hypothesis NOT accepted at first?

He could not explain how continents moved, and there was no known force to drive their motion at the time.

Why is Continental Drift accepted today?

New evidence such as seafloor spreading, magnetic patterns, and plate tectonics explains how continents move.

What is seafloor spreading?

The process by which new oceanic crust forms at mid-ocean ridges and moves outward, pushing plates apart.

Is the ocean floor flat?

No. It contains trenches, mid-ocean ridges, and other features.

What does sonar tell us about the ocean floor?

Sonar maps the shape and features of the seafloor, revealing ridges, trenches, and depth variations.

What is an isochron map?

A map that shows the ages of oceanic rocks, with younger rocks near ridges and older rocks farther away.

Why is one side of the seafloor a mirror image of the other?

New crust forms at the ridge and spreads outward evenly on both sides.

Name two ways the seafloor is symmetrical.

1) Rock ages are the same distance from the ridge on both sides
2) Magnetic stripe patterns are mirrored

How does crust thickness change with distance from a ridge?

The crust becomes thicker as it moves farther away from the ridge.

Where is Earth’s magnetic field generated?

In the outer core, from the movement of molten iron and nickel.

What is normal vs. reversed polarity?

Normal polarity matches today’s magnetic field direction; reversed polarity is when Earth’s magnetic field flips.

How does ocean crust record Earth’s magnetic changes?

As magma cools, iron minerals align with Earth’s magnetic field, preserving magnetic patterns in the crust.

Where is new crust created and destroyed?

Created at mid-ocean ridges; destroyed at subduction zones.

What are the three main types of plate boundaries?

Convergent, Divergent, and Transform.

What happens at a convergent plate boundary?

Plates move toward each other due to compression forces.

What happens at an oceanic–oceanic convergent boundary?

One plate subducts, forming deep-sea trenches and volcanic island arcs.

What happens at a continental–continental convergent boundary?

Plates collide and form mountain ranges (no subduction).

What happens at a continental–oceanic convergent boundary?

The oceanic plate subducts, forming volcanoes, trenches, and earthquakes.

What happens at a divergent plate boundary?

Plates move apart due to tension forces, creating mid-ocean ridges or rift valleys.

What happens at a transform boundary?

Plates slide past each other, causing earthquakes but no crust is created or destroyed.

What force is believed to cause plate motion?

Convection currents in the mantle, along with ridge push and slab pull.