1. Table of Contents Earth’s Interior Convection and the Mantle
Drifting Continents
Sea-Floor Spreading
The Theory of Plate Tectonics

2. Exploring Inside the Earth
- Earth’s Interior
Exploring Inside the Earth
Earth’s surface is constantly changing. It looks very different today than it did millions of years ago.
Geologists (scientists who study rocks) would have to dig down 6,000 kilometers to get to the Earth’s core.
Because this would be impossible, geologists use two main types of evidence to learn about Earth’s interior

3. Exploring Inside the Earth
- Earth’s Interior
Exploring Inside the Earth
Direct evidence – from rock samples
Indirect evidence – from seismic waves.

4. Exploring Inside the Earth
- Earth’s Interior
Exploring Inside the Earth
Seismic waves – vibrations that travel through Earth carrying the energy released during an earthquake.
Can detect the structure of the planet by studying the speed and path of the waves
Determined that the Earth is made up of many layers

5. A Journey to the Center of Earth
- Earth’s Interior
A Journey to the Center of Earth
The three main layers of Earth:
crust, the mantle, and the core.
Layers vary greatly in size, composition, temperature, and pressure.
Temperature – the heat is a result of the formation of the planet and radio active materials.
Increases by about 1°C every 40 meters down

6. A Journey to the Center of Earth
- Earth’s Interior
A Journey to the Center of Earth
Pressure – force exerted on a surface divided by the area over which the force is exerted.
Increases as you go deeper into the Earth
Pressure increases the deeper you go, like that of a swimming pool

7. The Crust Crust – layer of rock that forms Earth’s outer skin
- Earth’s Interior
The Crust
Crust – layer of rock that forms Earth’s outer skin
Includes both dry land and the ocean floor.
Thinnest at ocean floors and thickest at mountains

8. The Crust Two types of crust:
- Earth’s Interior
The Crust
Two types of crust:
Oceanic crust – the crust that makes up the ocean floor
Made mostly of basalts – dark rock with a fine grained texture
Continental crust – the crust that makes up the continents
Made mostly of granites – light rock with a coarse grained texture

9. The Mantle Mantle – layer of hot semi-solid rock under the crust
- Earth’s Interior
The Mantle
Mantle – layer of hot semi-solid rock under the crust
Divided into layers based on the physical characteristics
Total about 3,000 km thick

10. - Earth’s Interior
The Mantle
Two parts to the upper mantle:
Lithosphere –where the upper most part of the mantel and the crust merge
“lithos” Greek for “stone”
About 100 km thick
Asthenosphere – under the lithosphere where rock is heated causing it to be flexible (like plastic)
“asthenes” Greek for “weak”
Still considered a solid

11. - Earth’s Interior
The Core
Lower mantle – beneath the asthenosphere, very hot solid material
Core – in the center of Earth and made mostly of iron and nickel
Consists of two parts: outer and inner
Total 3,486 km thick
Outer core – molten liquid metal
Scientists think that movements in the liquid outer core create Earth’s magnetic field.
Inner core – dense ball of iron and nickel atoms
So much pressure that it remains a solid

12. - Earth’s Interior
The Core
The core is made mostly of the metals iron and nickel. It consists of two parts–a liquid outer core and a solid inner core.

13. Temperature Inside the Earth
- Earth’s Interior
Temperature Inside the Earth
The graph shows how temperatures change between Earth’s surface and the bottom of the mantle. On this graph the temperature at the Earth’s surface is 0oC. Study the graph carefully and then answer the questions.

14. Temperature Inside the Earth
- Earth’s Interior
Temperature Inside the Earth
Reading Graphs:
As you move from left to right on the x-axis, how does depth inside the Earth change?
The depth increases.

15. Temperature Inside the Earth
- Earth’s Interior
Temperature Inside the Earth
Estimating:
What is the temperature at the boundary between the lithosphere and the asthenosphere?
About 1,600oC

16. Temperature Inside the Earth
- Earth’s Interior
Temperature Inside the Earth
Estimating:
What is the temperature at the boundary between the lower mantle and the core?
About 3,200oC

17. Temperature Inside the Earth
- Earth’s Interior
Temperature Inside the Earth
Interpreting Data:
How does temperature change with depth in Earth’s interior?
It generally increases with depth.

18. Links on the Structure of Earth
- Earth’s Interior
Links on the Structure of Earth
Click the SciLinks button for links on the structure of Earth.

19. - Convection and the Mantle
Types of Heat Transfer
Heat always moves from a warmer substance to a cooler substance.
Ex: holding an ice cube in your hand
There are three types of heat transfer: radiation, conduction, and convection.

20. Radiation Radiation – the transfer of energy through space
- Convection and the Mantle
Radiation
Radiation – the transfer of energy through space
takes place with no direct contact between a heat source and an object
Ex: Sunlight

21. - Convection and the Mantle
Conduction
Conduction – the transfer of heat within a material or between materials that are touching
Ex: spoon in a pot of hot soup
In conduction, the heated particles of a substance transfer heat through contact with other particles in the substance.

22. - Convection and the Mantle
Convection is caused by differences of temperature and density within a fluid.
Density – measure of how much mass there is in a volume of a substance.
Ex: A rock is more dense than the same volume of water.
As a substance is heated, molecules spread apart, causing it to be less dense, and allowing it to rise
As it cools the molecules get closer together, increasing density, and causing gravity to pull it back down.

23. - Convection and the Mantle
Convection Currents
Heating and cooling of the fluid, changes in the fluid’s density, and the force of gravity combine to set convection currents in motion.

24. - Convection and the Mantle
Convection Currents
Convection current – movement of a fluid, caused by differences in temperature, that transfers heat from one part of the fluid to another.
Continue as long as heat is added
Without heat, convection currents eventually stop.

25. Convection Currents in Earth
- Convection and the Mantle
Convection Currents in Earth
Heat from the core and the mantle itself causes convection currents in the mantle.

26. Click the Video button to watch a movie about mantle convections.
- Convection and the Mantle
Mantle Convection
Click the Video button to watch a movie about mantle convections.

27. More on Convection Currents in the Mantle
- Convection and the Mantle
More on Convection Currents in the Mantle
Click the PHSchool.com button for an activity about convection currents in the mantle.

28. - Drifting Continents
Continental Drift
Throughout centuries people have been studying maps. During this time many people have noticed the similarities between the coastlines of the continents.
The continents on each side of the Atlantic Ocean looked as though they could fit together like a puzzle piece.

29. - Drifting Continents
Continental Drift
Alfred Wegener – (1910) German scientist who hypothesized that all of the continents were once joined together in a giant continent that he named Pangaea.
Continental drift theory - idea that the continents slowly move across Earth’s surface
Pangaea was believed to exist about 300 million years ago when reptiles and winged insects first appeared

30. - Drifting Continents
Continental Drift
Wegener’s hypothesis was that all the continents were once joined together in a single landmass.

31. - Drifting Continents
Continental Drift
Wegener gathered three types of evidence to support his ideas about continental drift.
They included: land features, fossils, and evidence of climate change
Evidence from land features
Wegener found that the mountains of eastern South America matched that of western Africa
Coal in England matched that of eastern North America

32. Continental Drift Evidence From Fossils
- Drifting Continents
Continental Drift
Evidence From Fossils
Wegener also used fossils to support his idea.
Fossil – any trace of an ancient organism that has been preserved in rock.
Ex: Glossopteris (250 mill yr old fern) – found in rocks of Africa and South America

33. Continental Drift Evidence From Climate
- Drifting Continents
Continental Drift
Ex: Mesosaurus and Lystrosaurus (ancient freshwater reptiles) now separated by an ocean
Evidence From Climate
Wegener found fossil evidence of ancient tropical plants and animals in polar regions
He also found evidence of ancient arctic species in areas that are now close to the equator.

34. Evidence for Continental Drift

35. Evidence for Continental Drift
- Drifting Continents
Evidence for Continental Drift

36. Wegener’s Hypothesis Rejected
- Drifting Continents
Wegener’s Hypothesis Rejected
Many scientists opposed Wagener’s hypothesis, despite the evidence, because he was unable to explain what the force was that caused this movement.

37. Links on Continental Drift
- Drifting Continents
Links on Continental Drift
Click the SciLinks button for links on continental drift.

38. - Sea-Floor Spreading
Mid-Ocean Ridges
The East Pacific Rise is just one of the many mid-ocean ridges that wind beneath Earth’s oceans.

39. - Sea-Floor Spreading
Mid-Ocean Ridges
Mid-ocean ridge – undersea mountain chain where new ocean floor is produced
Discovered by using sonar (a device that bounces sound waves off underwater objects and then records the echoes)
Harry Hess – (American) first geologists to study mid-ocean ridges
Found evidence to back up Wegener’s Continental Drift Theory
Called this seafloor spreading

40. What Is Sea-Floor Spreading?
Sea-floor spreading – sea floor spreads apart along both sides of a mid-ocean ridge as new crust is added
Ocean floors move like conveyor belts, carrying the continents along with them.

41. Evidence for Sea-Floor Spreading
Hess found evidence to support his theory of sea-floor spreading, which included:
eruptions of molten material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves.

42. Evidence for Sea-Floor Spreading
Evidence From Molten Material
1960’s – submarine Alvin found rocks along the ocean floor running parallel with the ridge
These type of rocks only form when molten material hardens quickly after erupting under water
Evidence From Magnetic Stripes
rock that makes up the ocean floor lies in a pattern of magnetized “stripes.”
record of reversals in Earth’s magnetic field.

43. Evidence for Sea-Floor Spreading
Evidence From Drilling Samples
Glomar Challenger – (1968) drilling ship that obtained rock samples from the ocean crust
Found that the rocks closest to the mid-ocean ridge were “younger.”
The farther away from a ridge the samples were taken, the older the rocks were

44. Subduction at Trenches
Deep-ocean trench – deep valley along the ocean floor beneath which oceanic crust slowly sinks toward the mantle.
The oceanic crust bends downward into the mantle
Subduction – process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle.
Occurs at a convergent plate boundary
Trench
Subduction

45. Subduction at Trenches
- Sea-Floor Spreading
Subduction at Trenches
In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle through deep-ocean trenches.

46. - Sea-Floor Spreading
Growing an Ocean
Because of sea-floor spreading, the distance between Europe and North America is increasing by a few centimeters per year.

47. More on Sea-Floor Spreading
Click the PHSchool.com button for an activity about sea-floor spreading.

48. Click the Video button to watch a movie about sea-floor spreading.

49. How Plates Move
Plate tectonics – theory that pieces of Earth’s lithosphere are in constant motion, driven by convection currents in the mantle.
As the plates move, they collide, pull apart, or grind past each other, producing spectacular changes in Earth’s surface.
Changes include volcanoes, mountain ranges, and deep-ocean trenches.

50. - The Theory of Plate Tectonics
How Plates Move
The theory of plate tectonics explains the formation, movement, and subduction of Earth’s plates.

51. Plate Boundaries
The edges of Earth’s plates meet at plate boundaries that extend deep into the lithosphere.
Fault – break in Earth’s crust where masses of rock slip past each other.

52. Plate Boundaries There are three kinds of plate boundaries:
- The Theory of Plate Tectonics
Plate Boundaries
There are three kinds of plate boundaries:
divergent boundaries, convergent boundaries, and transform boundaries.
A different type of plate movement occurs along each type of boundary.

53. - The Theory of Plate Tectonics
Calculating a Rate
To calculate the rate of plate motion, divide the distance the plate moves by the time it takes to move that distance.
Rate = distance/time
For example, a plate takes two million years to move 156 km. Calculate its rate of motion.
156 km/2,000,000 years = 7.8 cm per year

54. Calculating a Rate Practice Problem
- The Theory of Plate Tectonics
Calculating a Rate
Practice Problem
The Pacific plate is sliding past the North American plate. It has take ten million years for the plate to move 600 km. What is the Pacific plate’s rate of motion?
60,000,000cm ÷ 10,000,000years = 6 cm/yr

55. - The Theory of Plate Tectonics
Plate Boundaries
Divergent boundary – plate boundary where two plates move away from each other.
Most divergent boundaries occur along the mid-ocean ridges where sea-floor spreading occurs.
Rift valley – deep valley that forms where two plates move apart.
Ex: Great Rift Valley in Africa

56. - The Theory of Plate Tectonics
Plate Boundaries
Convergent boundary – plate boundary where two plates move toward each other.
The density of the plates determines which one comes out on top.
Ex: Oceanic – Continental collision:
Oceanic crust is more dense, therefore subduction occurs
Ex: Continental – Continental collision:
Neither is more dense, therefore mountains form
Ex: Appalachia Mountains

57. - The Theory of Plate Tectonics
Plate Boundaries
Transform boundary – plate boundary where two plates move past each other in opposite directions.
Earthquakes often occur along transform boundaries, but crust is neither created nor destroyed.
Ex: San Andreas Fault, California

58. - The Theory of Plate Tectonics
Continental Drift
It has taken the continents about 225 million years since the breakup of Pangaea to move to their present locations.

59. Continental Drift Activity
- The Theory of Plate Tectonics
Continental Drift Activity
Click the Active Art button to open a browser window and access Active Art about continental drift.

60. Graphic Organizer Type of Boundary Type of Motion Effect on Crust
Feature(s) Formed
Plates slide past each other.
Crust is sheared.
Strike-slip fault
Transform boundary
Plates move together.
Subduction or mountain
Convergent boundary
Mountains, volcanoes
Plates move apart.
Mid-ocean ridge, ocean floor
Divergent boundary
Crust pulled apart