Constructive & Destructive
Forces on Landforms
5-3.1
Two Types of Forces
Destructive Forces: processes that destroy
landforms.
– 2 types: Slow (weathering) and Fast (Erosion)
– Ex. landslides, volcanic eruptions, earthquakes,
floods
Constructive forces: forces that build up an
existing landform or create a new one.
– Caused by: water, gravity, wind and glaciers.
– Ex: deposition, landslides, volcanic eruptions, floods
Weathering
Weathering Pictures
Weathering: a slow, destructive force that
breaks rocks into smaller pieces called
sediments. Can by physical (mechanical)
or chemical.
Keywords:
wear down, break
apart
Mechanical weathering: the
breaking of rock into smaller
pieces by forces due to gravity,
ice, plant roots, or other physical
forces.
Mechanical Weathering:
Rock is physically broken into
smaller pieces
Small pieces have same
composition as the original rock
Very slow
Mechanical Weathering:
Caused by…(Agents)
– Freezing and Thawing
– Release of Pressure
– Growth of Plants
– Animals
– Abrasion – Grinding away of rock by
other rock particles that are carried
by water, ice, wind, or gravity
Chemical Weathering: the changing of materials in
a rock by chemical processes. Rock particles have
different mineral make-up than original rock
Chemical Weathering:
Caused by…(Agents)
– H2O  MOST important
– Oxygen (O2)
– Carbon Dioxide (CO2)
– Living organisms
– Acid Rain  very rapid
Agent
Type
Description
Mechanical Rock particles wear away rock
Chemical
Forms from coal, oil, and gas
burning
Freezing/Thawing
Breaks rock by ice wedging
Carbon Dioxide
Forms carbonic acid in water
Chemical
Weathers marble and limestone
Mechanical Burrowing in the ground breaks rock
Plant growth
Roots pry apart cracks in rock
Living Organisms
Produce weak acid that weathers
rock
Oxygen
Causes rust on some rock
Mechanical Sun or forest fire causes rock to
crack
Rates of Weathering:
Type of rock
Minerals that make up the rock
determine how fast it weathers
Climate
Occurs faster in wet climates & at
high temperatures
Grand Canyon Video
Erosion
Erosion: the destructive movement of
materials away from one place by wind,
water, ice and gravity.
Wave Erosion- caused by water
Wind Erosion (Dust Storm)
Landslides
Landslides: occur when gravity quickly pulls rock
and dirt downhill.
Floods
Floods: a great flow of water over an area
that is usually dry land.
Erosion & Deposition
Deposition: the placing of materials
in a new place (constructive force).
Ex. Sandbars
Water Erosion & Deposition
River DeltaDeposits of sediment
at the mouth of the
Mississippi River
creating new land
called a delta.
Wind Erosion & Deposition
Sand Dunes- sand is moved by wind
Desert Sand Dunes
Ocean Sand Dunes
Mass Movement:
Caused by gravity
Process that moves sediment downhill
4 types
– Landslide
– Mudslide
- Slump
- Creep
Landslide:
Most destructive
When rock & soil slide quickly down a
steep slope
Mudflow:
Rapid downhill movement of H2O,
rock, and soil
Often occurs after heavy rains
Slump:
A mass of rock & soil suddenly slips
down a slope (in a large mass/clump)
Creep:
Very slow downhill movement of rock
& soil
Volcanic Eruptions
Volcano: an opening
in the Earth’s crust
through which steam,
lava and ashes erupt.
Cause both
destructive and
constructive changes
to landforms.
Mount St. Helens
Mount St. Helens Video
Saint Helen’s Before
Saint Helen’s After
Volcanoes can be constructive, but also
destructive…
Earthquakes
Earthquakes: the
shaking of Earth’s
surface caused by the
release of energy
along a fault.
San Andreas Fault, California
Effects of Weathering on Mountain Ranges
Moraine Lake,
Rocky Mountains
(younger mts.)
Appalachian
Mountains
Older mountains
Effects of weathering
Rounding, softening
of edges