Forces of ChangeGeography: Chapter 2, Lesson 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

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

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

Crust

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

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

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

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

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

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

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

Tectonic Plates are bumping into or ripping apart from other plates

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

Subduction can lead to a lot of earthquakes

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

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

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

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

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

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

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.

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

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.

Chemical weathering from the acid and carbon dioxide in rain is evident in this statue

Mechanical Weathering

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

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

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

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

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

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

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

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