Evidence for Plate Tectonics

• Shape of Continents• Rock Types• Fossils• Glaciers• Mountain Chains• Location of Earthquakes• Mid-ocean ridges and ocean floor rock ages• Magnetic Polarity Reversals

Shape of the Continents

• The edges of the continents appear to fit together like the pieces of a puzzle

Alfred Wegener - Continental Drift

• Noticed that continents appear to fit together

• Hypothesized that continents were once together and had drifted apart

• Idea was called “Continental Drift”

Continental Drift

Permian225 m.y.a.

Jurassic150 m.y.a.

Triassic200 m.y.a.

Cretaceous65 m.y.a.

Present Day

Rock Type Evidence

• Same sequence of rocks of same age can be found all over the world– Indicates that when these rocks

formed all these places were connected

Fossil Evidence

• Same fossils found on

many different continents– Fossils of organisms that

could not fly or swim

between continents– Continents were together when these animals

lived, so they could walk from one continent to another

Glacier Evidence

• Glaciers leave marks on rocks called striations that show which direction they move

Glacier Evidence

• Striations have been found in places too warm to have glaciers now

• Striations point in different directions

• If all the continents were together at the south pole, striations point in the same direction

Mountain Chains

• Mountain ranges around the world appear similar to each other in composition and age.

• If we reconstruct the past arrangement of the continents, these ranges formed as one mountain range and were later split

Mountain Chains

• This is true of mountain ranges all over the world

Location of Earthquakes

• Do earthquakes appear randomly on this map?

• NO!

Location of Earthquakes

• Earthquakes form a definite pattern- we know now that these are the edges of “plates” of rock that fit together to form the earth’s crust

Mid-Ocean Ridges

• First discovered when scientists started to map the bathymetry of the oceanfloor

• Long chain of underwater mountains that stretches around the entire earth

Ocean Floor Rock Ages

• Rocks all over the ocean floor were dated using absolute dating

Ocean Floor Rock Ages

• Rocks closest to the mid-ocean ridges were youngest

• Rocks farthest from the mid-ocean ridges were oldest

• Pattern of ages is the same on both sides of the ridges

• Indicates that new rocks are forming at the mid-ocean ridges!

The Theory of Plate Tectonics

• Explains all evidence• Similar to continental drift, but more complex• Earth’s crust is composed of “plates” that make up

the crust under the ocean and on the continents• Continents DO NOT float on the oceans• Plate boundaries do not always occur at the edges of

continents• Earthquakes and volcanoes occur where two plates

meet

Plate Tectonics Today