The earth’s crust is made of twelve plates. These plates float around on the mantle’s flowing rock.

The earth’s crust is made of twelve plates. These plates float around on the mantle’s flowing rock.

A theory of Earth Science that explains both the “shape” of the sea floor and of land forms.

This theory was developed largely from observations of the sea floor.

So how did the theory of Plate tectonics develop?

Observation: Continents seem to be able to fit together!! Coincidence??? Or were they once connected

1858 Geographer Antonio Snider-Pellegrini made these maps

What is a theory?

•In science, it is an explanation for observations of nature.

•A theory in science must be supported by a great deal of evidence. •In everyday language the word “theory” may be used to mean a guess, a hypothesis, a prediction or something that we’re very unsure of. People often say, “Oh it’s just a theory!”. But that is not its meaning in science. A scientific theory is one that most scientists accept.

Theories in science•A scientific theory may be a “currently accepted theory”, a “widely supported theory”, a “discarded theory”, or a “new theory”.

The Development of a theory

Alfred Wegener 1912

•German meteorologist.

•He was the first to suggest the hypothesis of continental drift to explain Pellegrini’s observations that continents fit together like puzzle pieces.

The Hypothesis of Continental Drift

•Continents were once together in a super continent he called Pangea

•Then broke up into two super continents call Laurasia and Gondwanaland

•Continents can move by plowing through the crust like ships plowing through water in the sea

•Continents are still moving today

Continental “drift” as Wegener imagined it.

Evidence to support the hypothesis of continental drift

• People don’t just propose a hypothesis without evidence to support their ideas!

What evidence besides the puzzle pieces observation could have led Wegener to propose this hypothesis???

1. Evidence from Fossils

• Animals living today on distant continents (ex: Africa & South America) are all different from one another.

• Fossils show that animals and plants long ago were similar. This implies the continents were connected.

• In the millions of years since separation, species evolved differently on different continents.

• Tropical fossils are found in many polar areas suggesting that those places were once closer to the equator.

Wegener’s evidence for Continental Drift

2. Evidence from Glaciation

• “Scratches” made on bedrock by glaciers on different continents “line” up when the continents are placed together.

• On some continents, glacial “striations” visible today make “no sense”. The run east-west rather than north-south.

• Evidence of glaciers is found in tropical areas suggesting that those areas were once further north or south.

• This suggests that the continents were connected when the striations were made

Glacial striations

3. Evidence from rock and mineral formations• Coal deposits had been found in Antarctica. • Coal deposits only form in the tropics suggesting

that Antarctica had once been in the tropics.• Rock composition in certain areas does not match

surrounding rock but does match rock type of a distant continent.• Ex: Rocks of Northern Newfoundland do not match NA

but are similar in structure and composition to those of Scotland. This suggests that a small bit of what is now Scotland became attached to NA when the two split.

Unfortunately for Wegener, the scientific community did not accept his ideas.

His proposed theory was rejected because:

•Evidence was considered too “circumstantial”

•Not enough evidence

•Wegener’s theory could not explain what forces could cause continents to “travel” large distance over the surface of the earth.

•Without harder evidence and without a believable reason why continents might be moving, people were not convinced

1930

Wegener left for Greenland to find more evidence to support his hypothesis.

He was never seen again.

This is the last photo of Wegener taken with a native guide.

His continental drift hypothesis was hotly debated for the next 30 years.

New Evidence to Support Wegener’s hypothesis

•After World War I the newly invented “sonar” showed the ocean floor to be much more rugged than was previously thought.

•In 1947 the survey ship Atlantis found that the sediment layer on the floor of an ocean was very thin near mid-ocean ridges but that it became thicker as you got farther from the ridge.

•Deep sea sediments are produced by the accumulation of dead plants and animals over long periods of time. So very little sediment implies very new sea bottom!

• In the 1950’s a great undersea mountain range (named the mid-ocean ridge system) that extends through all oceans was discovered. It zigzags between the continents with an average height of 4,500 m. The mountain range was found to be very “seismically active”.

• Also in the 1950’s many very deep ocean trenches were discovered, mapped and found to also be very seismically active.

• Radioactive dating showed that no ocean rocks are older than about 200 million year whereas continental rocks close to 4 billion years old are common.

Mid Ocean Ridges:

Radioactive dating showed that rocks are youngest near mid ocean ridges and that they get older the farther you get from the ridge

Pangaea means supercontinent, one huge landmass.

200 million years ago all of the present-day continents combined to form a single supercontinent called Pangaea.

http://www.ucmp.berkeley.edu/geology/anim1.html

Divergent Boundary:

the boundary between 2 plates that are spreading apart, known as sea-floor spreading.

An example is the Mid-Ocean Ridge.

Diagram of a Mid-Ocean Ridge.

Convergent Boundary: the boundary between 2 plates

that are moving towards each other.

Also called a Subduction Zone.

Cross-section of a Subduction Zone.

Diagram of what happens at a Subduction Zone.

Transform Boundary: a boundary between 2 plates that

are sliding past one another.

An example is the San Andreas Fault.

Diagram of a Transform Plate Boundary.

13 Major Tectonic Plates of the World.

Earthquakes

Earthquakes- are caused when a plate (part of the crust) lurches downward or undergoes structural transformation

http://www.abag.ca.gov/bayarea/eqmaps/animation.html

http://www.pbs.org/wnet/savageearth/animations/earthquakes/

Volcanoes

Volcanoes- some of the crust will melt when it subducts (plunges downward) forming magma. In some places this magma rises through to the surface and causes a volcanic eruption.

http://www.pbs.org/wnet/savageearth/animations/volcanoes/index.html

Ring of Fire- A circle of violent geological activity surrounding much of the pacific ocean