1. Geography: Chapter 2, Lesson 2
Forces of Change
Geography: Chapter 2, Lesson 2

2. Lesson Introduction Who in here has ever felt an earthquake?
Has anyone seen the effects of a flood, a tornado, or a hurricane?
Were the changes that you saw from the effects of these destructive natural occurrences permanent or temporary?
Such occurrences have been going on for millions of years and many have had lasting effects on Earth’s structure and surface

3. Earth’s Structure
For hundreds of millions of years, the surface of the Earth has been in motion
Pressures build up slowly inside the Earth and are then released in sudden events, such as volcanic eruptions and earthquakes
Wind and water also leave a very strong mark on the surface of the Earth, though erosion

4. The Earth is composed of three main layers: the core, the mantle, and the crust

5. Earth’s Structure: The Core
At the very center of the Earth is a super hot, but solid, mass of iron and nickel called the inner core
Outside of the solid inner core is a liquid outer core
The outer core is also made of the same materials as the inner core, but is not under the same intense pressure; that is why it is liquid rather than solid

6. The Earth’s Core is Hot Hot Hot! 7000 degrees!

7. Earth’s Structure: The Mantle
Next to the outer core is a thick layer of hot, dense rock called the mantle
The mantle is made of silicon, iron, magnesium, oxygen, and other elements
The dense mixture is soft enough to rise, cool, sink, warm up, and rise again
The mantle releases most of the heat that is generated by the Earth’s interior

8. The Earth’s Mantle is constantly circulating, leading to all sorts of things happening on the surface

9. Earth’s Structure: The Crust
The outer layer is the crust, which is the hard rocky shell on the Earth’s surface
The crust may be as thin as 2 miles under oceans and as thick as 75 miles under mountains
The crust is broken into more than a dozen great slabs of rock called plates that float on the mantle
Because the plates are really floating, they do move around and bump into each other
Many scientists believe that at one point many years ago, all the continents were once connected as one HUGE landmass, called Pangaea
The theory that all these continents are now drifting apart is called continental drift

10. Pangaea was once a super-continent, and has broken into our current seven continents

11. Earth’s Structure: Plate Tectonics
Plate tectonics refers to all the activities of continental drift and magma flow
Plates move by about an inch a year
With the plates bumping into each other, that causes mountains, trenches, and earthquakes
When plates split apart from each other, molten magma comes up from the mantle and volcanoes may be formed
Scientists theorize that plates move primarily due to the heat given off by the interior of the earth, sort of like how boiling water is quite active on the surface

12. Tectonic Plates are bumping into or ripping apart from other plates

13. Colliding and Spreading Plates: Subduction
Mountains are formed in areas where the giant plates collide
The Himalayan mountains contain some of the tallest mountains in the world, including Mount Everest, and these peaks are getting higher, as tectonic plates continue to push against each other
If a heavy oceanic plate collides with a lighter continental plate, the heavier plate dives beneath the lighter plate in a process called subduction
In subduction, the oceanic plate plunges deep into the Earth’s interior, becoming molten material
Then as magma, the molten material rises and bursts through the crust to form volcanic mountains

14. Subduction can lead to a lot of earthquakes

15. Colliding and Spreading Plates: Accretion
Another process that happens from plates colliding is called accretion (pronounced uh-KREE-shun)
Accretion is a slow process in which an oceanic plate slides under a continental plate, levels off underwater mountains, causing debris to spread and can cause continents to grow outward
Many scientists believe the western coast of North America spread outward into the Pacific Ocean due to accretion

16. In an Accretion Wedge, the ocean crust can be sent to the surface to create land forms, mountain ranges, and even islands

17. Colliding and Spreading Plates: Spreading
Oceanic plates may also spread apart in a process called spreading
In spreading, the magma comes up through the tear between the plates and rip the plates farther apart
The magma then cools when it rises and builds undersea mountains and even islands if they are tall enough mountains
This is happening in the middle of the Atlantic Ocean, forming the Mid-Atlantic Ridge, pushing Europe and North America away from each other

18. Spreading can cause new mountains and islands to emerge through the separating plates

19. Folds and Faults
Moving plates sometimes squeeze the Earth’s surface until it buckles
This activity forms folds, or bends, in layers of rock
In other cases, plates may grind or slide past each other, creating cracks in the Earth’s crust called faults
One famous fault is the San Andreas Fault
Faulting happens when the folded land cannot be bent any more, and it breaks
The broken rocks continue to rub against each other as the plates slide by each other
As these rocks catch each other and then let go, sudden earthquakes appear

20. The San Andreas Fault created the Cajon Pass, and runs though Wrightwood and much of California

21. Earthquakes and Volcanoes
While most earthquakes are not even noticed, or cause minimal disruption, some can dramatically change the surface of the land
One earthquake in Alaska in 1964 caused the ground to lurch upward by 38 feet (roughly equivalent to a 4 story building)
Many earthquakes and volcanoes occur along the Pacific Ring of Fire, along the outer edge of the Pacific plate, in places like California, Alaska, Japan, and New Zealand
While most volcanoes occur between plates, they may also occur at hot spots where a plate may just melt. This is how Hawaii was created, which is in the middle of the Pacific Plate.

22. The Pacific Ring of Fire is so named because of the unusual number of earthquakes and volcanoes along the edge of the Pacific Plate

23. External Forces of Change
In addition to all the internal forces within the earth (plate tectonics, geothermal heat), there are external forces that can change the Earth as well, such as wind and water
Wind and water movements involve two processes: weathering breaks down rocks and erosion wears away the Earth’s surface
Mechanical weathering occurs when large rocks are physically broken by a process such as when water seeps in and freezes, with the expansion of the ice wedging the rock apart
Chemical weathering occurs when rain containing carbon dioxide or acid dissolves softer rocks. This process has created many caves.

24. Chemical weathering from the acid and carbon dioxide in rain is evident in this statue
Mechanical Weathering

25. External Forces of Change: Erosion
Wind erosion carry dust, sand and soil from one place to another
Plants help protect the land from wind erosion by tying down the soil with roots
Land can thus be vulnerable where people cut down trees and plants

26. Huge clouds of soil were blown away in Kansas, Oklahoma and Texas when overgrazing and improper farming combined with a windstorm to form The Dust Bowl, aggravating the conditions of the Great Depression

27. External Forces of Change: Glaciers
Another source of erosion is glaciers, which are large bodies of ice scraping across the Earth’s surface
When glaciers melt and recede, they may leave piles of rocks and debris called moraine
There are two types of glaciers: sheet glaciers and mountain glaciers
Sheet glaciers are flat broad sheets of ice that advance a few feet in the winter and recede in the summer, often large blocks break off and become icebergs
Sheet glaciers cover most of Greenland and all of Antarctica
Mountain glaciers are more common than sheet glaciers, and carve out huge U- shaped valleys wherever they slide

28. This is typical moraine and the U-shaped valley of a glacier

29. External Forces of Change: Water Erosion
As rainwater flows downhill, it carries away soil and even cuts into the Earth
The water first forms a gully and then eventually a V-shaped valley, possibly eventually becoming a canyon, like the Grand Canyon
Waves on the beach also cause water erosion, turning rocks into sand, and then carrying that sand somewhere else

30. The Grand Canyon was created by years of erosion by the Colorado River

31. Soil Building How do we get soil?
Soil is the product of thousands of years of weathering, erosion, and biological activity
Weathering breaks down solid rock into smaller pieces
Worms and other organisms break down organic matter– dead plant and animal material– while also adding nutrients to the soil and creating passages for air and water
Five factors influence what kind of soil will be formed
Climate (wind, temperature, and rainfall) determines the type of soil that can develop
Topography– the shape and position of Earth’s features– affects surface runoff of water, drainage, and water erosion
Geology determines the original rock that is there, which influences the depth, texture and drainage of the soil
Biology– living and dead animals– adds organic matter to the soil
Time is another factor that influences what kind of soil may be created

32. Rich, life-giving, soil is created from years of erosion, weathering, and biological activity