Earth's Structure

• Geology means study of the Earth.• Earth means the ground.• The Earth’s structure is constantly

changing due to volcanoes, glaciers, geysers, earthquakes, landslides, and other plate movements.

Pieces of a puzzle…• Have you ever noticed how much the coast line of the

continents seem to match up like puzzle pieces? • Alfred Wegener was one of the scientists who noticed

this. He saw that Paris and Washington, D.C., seemed to be moving apart by about 15 feet each year while San Diego, California and Shanghai, China got about six feet closer.

Other evidence…

• On top of that, Wegener learned that related species, too small to swim the oceans, were found on different continents, as were similar fossils.

Pangaea• In 1912 Alfred proposed that the continents we

know today were once all attached in a single landmass he called Pangaea (Greek for "all earth").

• They were surrounded by one global ocean, but then broke apart and somehow "drifted" to their separate places on the globe. Wegener could not come up with an acceptable way to explain how the continents moved, however.

• We still don’t have one theory to explain the movement (some believe it is caused by heat from the core and others believe gravity is the cause).

Today, most scientists believe that Pangaea was…

the super continent that formed about 225 million years ago. Before Pangaea, the continents were grouped much differently.

Fast forward about 40 years…• Harry Hess developed a theory

called plate tectonics in the 1960’s. He said the Earth’s plates were moving.

• He based his ideas on Alfred Wegener’s theory of Continental Drift.

• Today, we know that the plates are moving about 2 cm per year.

What moving plates do…

Harry called this process sea floor spreading, saying that theocean floor was moving kind of like an escalator. New floorforms at volcanoes or rifts while the old floor sinks into a trench.This made sense to him because the fossils on the ocean floor weremuch younger than ones found on higher land elevations. There wasalso a lot less sediment on the ocean floor because of this new land formation.

Plate TectonicsTectonic means “build.”Seven major tectonic plates make up Earth’s crust.

Types of Plate Boundaries:

• Convergent – plates come together (mountains, trenches)

• Divergent – plates pull apart (volcanoes, rifts)

• Transform Fault – plates move side by side (earthquakes)

Volcanoes• They usually occur at plate boundaries.• Convergent boundaries usually produce

them.• Hot spots also create volcanic islands (like

Hawaii).

An earthquake is:

• the amount of energy released by vibrations from movement of plates.

• This usually occurs on a fault line (crack in Earth’s crust) usually on a transform fault boundary.

Appalachian Mountains were formed as a result of shifting of continents long ago.

How old is the Earth?

• We estimate that the Earth is 4.6 billion years old. How do we know?

• Two major ways: Relative age and absolute age of rocks help us figure this out.

• Relative age is describing a rock’s age compared to one that we know is older. Most rocks are layered (older rocks are generally deeper).

Absolutely how old???

• Absolute age refers to radiometric dating. Different elements change over time if they are radioactive (into other elements).

• We can determine the age of rocks and fossils by identifying the radioactive elements that the rocks contain.

• The oldest rocks are from Greenland (3.8 byo) and the oldest minerals are from Australia (4.2 byo).

Geologists divide the Earth’s time into 6 major eras:

• Cenozoic (65 million years total)

• Mesozoic (183 million years)• Paleozoic (295 million years)• Proterozoic (1.9 billion years) • Archaean (1.3 billion years)• Hadean (700 million years)

Geologic Time

• We live in the Cenozoic Era and the Quaternary time period.

Scientists believe that different life forms were formed in different time

periods (fill in the missing parts on your notes

sheet).