Internal Forces of
Change
Types of Boundaries

Convergent
– Come together

Divergent
– Pull apart

Transform
– Slide past each
other
http://www.cotf.edu/ete/modules/msese/earthsysflr/plates1.html
http://www.wwnorton.com/college/geo/egeo/animations/ch2.htm#9
Convergent Boundaries

Subduction
– One plate dives beneath another
– Forms volcanic mountains

Accretion
– As one plate slides beneath another, a trench
is formed
– Overriding plate scrapes off the subducted
plate, leveling off undersea mountains and
ridges

Folding
– Two continental plates collide, forming
mountains
http://www.uky.edu/AS/Geology/howell/goodies/elearning/module04swf.swf
http://www.wwnorton.com/college/geo/egeo/animations/ch2.htm#9
Divergent Boundaries
 Spreading
– Two sea plates pull apart
– Magma rises, creating ridges or
undersea mountain ranges (“new” land)
http://www.uky.edu/AS/Geology/howell/goodies/elearning/module04swf.swf
http://www.wwnorton.com/college/geo/egeo/animations/ch2.htm#9
Transform Boundaries

Fault
– Crack in earth’s surface
where two plates can slide
past one another
– When plates make a
sudden, violent shift,
earthquake!
http://www.uky.edu/AS/Geology/howell/go
odies/elearning/module04swf.swf
http://www.wwnorton.com/college/geo/egeo/a
nimations/ch2.htm#9
External Forces of
Change
Weathering
 Definition:
The breaking
down of rocks
on the earth’s
surface into
smaller pieces.
 Different from
erosion!
 No movement
Physical Weathering
 Caused
by heat, water, ice, or
pressure
Frost Wedging
Frost Heaving
Plant Roots
Burrowing of Animals
Temperature Changes
Chemical Weathering
•Caused by
chemical
reactions.
•Can be
caused by
water, carbon
dioxide,
oxygen, acid
rain
1908
to
1969
Acid rain has eaten away this limestone statue.
Water
 Water
weathers rock by dissolving it
Oxygen
 Iron
combines
with oxygen in the
presence of water
in a processes
called oxidation
 The product of
oxidation is rust
Acid Rain
 Burning
coal, oil and gas react
chemically with water forming acids.
 Acid rain causes very rapid chemical
weathering
animation
Erosion
 Definition:
The
process by
which water,
ice, wind or
gravity moves
pieces of rock
and soil
 Different
from
weathering!
 Movement
Water Erosion

Caused by fastmoving water such
as rain, rivers,
streams and
oceans. Can
eventually form a
canyon or cliff
(ocean).
– Ex. Grand
Canyon
Water Erosion
Wind Erosion
 Caused
by the
movement of
dust, sand and
soil. Can create
loess, a fertile
soil carried by
wind.
–Ex. Great
Plains
Wind Erosion
Glacial (Ice) Erosion

Caused by large
pieces of ice
moving across the
earth’s surface.
Can leave behind
moraines (large
piles of rock and
debris).
– Ex. Rocky
Mountains
Glacial Erosion
http://www.wwnorton.com/college/geo/oceansci/animations.asp
Water Cycle
 Constant
movement of water on
earth.
 Evaporation, condensation,
precipitation
 Amount of water on earth really
never changes!
http://earthguide.ucsd.edu/earthguide/diagrams/watercycle/index.html