Forces Of Change
• In 1906 the booming city of San Francisco was
destroyed by an earthquake that measured 8.6 on
the Richter scale. Over 400 people were killed, and
28,000 buildings were reduced to rubble. Another
slightly less forceful earthquake struck the city in
1989, doing far less damage and claiming 67 lives.
Most people remember it because it interrupted the
World Series for 12 days while damages were
repaired in Oakland and San Francisco.
• A Layered Planet The earth is composed of three
layers: the core, the mantle, and the crust.
- The inner core is about 4,000 miles (about 6,430
km) below the surface.
- The outer core is about 1,400 miles (about 2,250
- Both are made of iron and nickel.
- The mantle is made of hot, dense rock.
- It releases 80 percent of the heat generated from
the earth’s interior.
•Many scientists believe that all the
continents once were joined and have broken
apart and drifted. This theory is called
• Plates move gradually. As they move, they
come together and pull apart.
• This movement pushes up mountains,
creates volcanoes, and produces
• These activities are called plate tectonics.
2.1 explain internal and external physical forces that impact Earth
Colliding and Spreading Plates
• One way that mountain ranges form is in
a process called subduction, when a sea
plate collides with and dives beneath a
• Continents grow in a process called
accretion, when pieces of the earth’s
crust come together slowly as a sea plate
slides under a continental plate.
• When two sea plates collide, they may
create a chain of islands.
Folds and Faults
• When plates squeeze the earth’s surface, they
create folds or bends in layers of rock.
When the layers grind or slide past each other, they
create cracks known as faults.
•Sudden, violent movements along the fault lines are
• The Ring of Fire is a zone of earthquake and
volcanic activity surrounding the Pacific Ocean.
• Cities in this area, like San Francisco and Los
Angeles, are prone to severe earthquakes.
• Volcanoes are mountains formed by
magma that has broken through the
• Volcanoes often rise along plate
• They also occur when especially hot
places deep inside the earth blast
their magma to the surface.
External Forces of Change
• Physical weathering occurs
when large masses of rock
are broken down into smaller
pieces. Chemical weathering
changes the chemical
composition of rocks.
• The movement of dust, sand,
and soil from one place
to another is called wind erosion.
• It can devastate some areas and
bring mineral riches to the soil in
• Glaciers are large bodies of ice that
move slowly across the earth’s surface.
• As they move, they carry along rocks
• Sheet glaciers are flat, broad sheets
• Mountain glaciers are located in high
• Rain, rivers, streams, and
oceans wear away soil and rock
in a phenomenon called water
• Water erosion often forms
valleys and canyons.