1. Chapter 1
Earth’s Layers

2. Earth’s Materials
4.6 billion years ago, materials collided and stuck together to form Earth.
Over time, dense materials such as nickel (Ni) and iron (Fe) sank to the center of the Earth.
The lighter materials rose to the top, created the Earth’s layers.

3. Earth’s Layers
Scientists know about the layers of the Earth by studying the energy released by Earthquakes and other underground explosions.
The energy waves go faster through dense materials and solids, and slower through less dense materials.

4. Inner core ball of hot, solid metals.
Due to high pressure, the materials in the core remain solid despite the heat.
2400 km across

5. Outer core liquid metals. Lower pressure allows it to stay liquid.
2300 km thick

6. Mantle made of hot rocks that can move like a paste.
Thickest layer (2900 km).
The top is cool and rigid.

7. Crust Thin layer (6-70 km) of cool rock.
Continental crust: all continents and some major islands.
Oceanic crust: all ocean floors.
Earth’s crust is thinnest under the oceans and thicker under continental mountain ranges.

8. Lithosphere
Earth’s crust and the top of the mantle make up the lithosphere.
The most rigid layer.

9. Asthenosphere
The asthenosphere is directly below the lithosphere, and is made up of the hotter, softer part of the mantle.
This layer moves around like hot tar.

10. Chapter 1
The Continental Drift

11. Tectonic Plates
The lithosphere is broken into pieces of rock called tectonic plates.
They fit together like a puzzle.
Most large plates contain continental and oceanic crust.

12. Continental Drift
A theory stated by Alfred Wegener that Earth’s continents were once all joined together.
Several years later, evidence began to prove that this was correct.
The supercontinent is known as Pangaea and it began to split about 200 million years ago.

13. Evidence for Continental Drift
1: Fossils
Fossils of the ancient reptile Mesosaurus had been discovered in South America and western Africa.
This led people to believe these landmasses were once connected.

14. Evidence for Continental Drift
2: Climate
Today, Greenland is completely covered with ice. However, there are fossils of tropical plants along its shore.
South Africa has a warm climate today, but the rocks there show deep scratches from ice moving on the surface.

15. Evidence for Continental Drift
3: Geology
The rocks found in Brazil match up with the rocks found in western Africa.
Limestone layers in the Appalachian Mountains are exactly like limestone layers in Scotland’s Highlands.

16. The Theory of Plate Tectonics
Chapter 1
The Theory of Plate Tectonics

17. Theory of Plate Tectonics
Scientists were beginning to understand how the continents moved.
They found evidence from the sea floor.

18. Evidence from the Sea Floor
There are huge underwater mountains in the oceans, called mid-ocean ridges.
They form along cracks in the crust. Melted rock rises through the cracks, cools, and forms new oceanic crust.
The old crust is pushed away.

19. Sea Floor Spreading
In this picture, you can see the youngest oceanic crust is located near the ridges. Older crust is pushed farther out.

20. Evidence from the Sea Floor
When scientists compare the age of the rocks in the crust, they oceanic crust is much younger than the continental crust.
This implies that the oceanic crust is constantly reforming.

21. Ocean Trenches
If the sea floor is spreading, then why is Earth not getting bigger?
Even though the oceans are spreading apart, the Earth is not getting any bigger.
Old crust is being destroyed at the same rate as the new crust is being formed.

22. Causes of Plate Movement
Convection: heat transfer by the movement of a material.
The hot rock in the asthenosphere rises to the surface. At the surface it begins to cool, and then sinks down.
When this motion repeats itself, it is called a convection current.

23. Causes of Plate Movement
Slab Pull: gravity pulls the cooling plates into the asthenosphere.
Ridge Push: material from a mid-ocean ridge slides downhill and pushes on the rest of the plate.

24. The theory of plate tectonics
This theory states that Earth’s lithosphere is made up of huge plates that move over the surface of the Earth.
One plate cannot shift position without affecting the position of the other plates.

25. Chapter 1
Plates Move Apart

26. Causes of Plate Movement
Slab Pull: gravity pulls the cooling plates into the asthenosphere.
Ridge Push: material from a mid-ocean ridge slides downhill and pushes on the rest of the plate.