Continental Drift and Seafloor Spreading

The Moving Earth:
Continental Drift and
Seafloor Spreading

The Puzzle of the Continents

●Have you ever noticed how some
continents look like they could fit together
like puzzle pieces?

●Scientists noticed this too, especially
between South America and Africa

●In 1912, Alfred Wegener suggested all
continents were once connected

●This connected supercontinent was called "Pangea"

●Think: What other continent edges look like
they might fit together?

Multiple Choice

Who got credited with the idea that all continents were once connected in a supercontinent?

• Alfred Wegener

• Alfred the Butler.. ya' know.. the one from Batman

• Alfred Hitchcock

• Arnold Werthers

Evidence from Land
Features

●Mountain ranges on different continents
line up when put together

●Similar rocks and minerals found on
different continents

●Coal deposits found in places that shouldn't
have them

●Question: Why would finding the same type
of coal in Africa and South America be
important evidence?

Multiple Select

Which were land features that were used as evidence for Pangea?

• Rocks & Minerals

• Coal

• Mountains

• Hills

Clues from Ancient Life

●Fossils of the same plants and animals found on different
continents

●Glossopteris: A fern-like plant found on 5 different continents

●Mesosaurus: A freshwater reptile found in both South America
and Africa

●How could these animals and plants be in such different
places?

Multiple Select

What are 2 reasons that South America and Africa might have been a lot closer?

• Mesosaurus fossils on both continents

• They look like they fit together like a puzzle

• Mountain ranges on both continent coastlines

• There is no proof that these specific continents were right next to each other

Climate Tells a Story

●Some places have fossils that don't match
their current climate

●Example: Tropical plant fossils found in the
Arctic

●This means these places must have moved
over time

●Think: Why can't the Arctic have had a
tropical climate in the past?

Multiple Select

What were the 3 evidences that supported Wegener's hypothesis?

• Fossils

• Climate

• Land Features

• Earth's Rotation

Fill in the Blank

Although Wegener could not come up with a mechanism to explain how the supercontinent became smaller continent pieces, his hypothesis until this day is referred to as:

(two words)

Introducing Mid-Ocean
Ridges

●Scientists discovered huge mountain
chains under the oceans

●The Mid-Atlantic Ridge runs down the
middle of the Atlantic Ocean

●These ridges stretch for 70,000 kilometers
around Earth

●If Earth were a baseball, the ridges would
look like the seams!

Seafloor Spreading in Action

●New rock forms at mid-ocean ridges

●Molten rock rises up through cracks

●Rock spreads out on both sides like a conveyor belt

●Question: If new rock keeps forming, why don't the oceans keep
getting bigger?

Fill in the Blank

What is the mechanism that scientists believe caused not only mid-ocean ridges to form, but also the continents to "drift" apart?

Ocean Trenches:
Earth's Deepest Places

●Ocean trenches are deep valleys in the
ocean floor

●They're where old ocean floor sinks back
into Earth

●This process is called subduction

●The ocean floor gets recycled here

How Subduction Works

●Ocean floor gets cooler and denser as it
moves away from ridges

●Dense ocean floor sinks under lighter rock

●This can create volcanoes on land or in the
ocean

Multiple Select

Subduction has to do with the ocean floor's

• temperature

• density

• texture

• color

Multiple Choice

Why does older ocean floor sink better than newer ocean floor?

• Newer ocean floor is more buoyant because it is colder.

• Older ocean floor is denser and has had more time to cool.

• Newer ocean floor is made of lighter materials than older ocean floor.
• Older ocean floor is warmer, making it less dense.

The Atlantic vs. Pacific Oceans

●The Atlantic Ocean is growing wider (2-5 cm per year)

●The Pacific Ocean is getting smaller

●The Pacific has more subduction zones

Multiple Choice

Which ocean do you think is older

• Pacific Ocean

• Atlantic Ocean

Earth's Future Changes

●Continents are still moving today

●The Pacific Ocean may close up in the
future

●A new supercontinent might form

●How do you think Earth's continents will
look 100 million years from now?

Poll

How do you think the Earth will look in a million years because tectonic plates are still moving?

• Form another supercontinent

• Around the same

• Aliensss

• No idea