Rocks
and
Weathering
Chapter 8.1
Weathering- the breaking of rock
into smaller fragments
• Mechanical weathering – breaks rock
apart without changing the chemical
composition
Examples:
Rocks colliding with other rocks in rivers
Tree roots lifting rock
Ice wedging caused by freezing and thawing
• Chemical weathering – reaction of air,
water and/or other substances with the
minerals in rock
The composition of the
rock is changed
Water is the main cause of chemical
weathering
•
It dissolves away some minerals leaving a
different composition behind
•
Compounds in air combine with water and
react with minerals upon contact
•
Some metals react with oxygen and water
(oxidation)
Acids cause chemical weathering
• Water and carbon dioxide form
carbonic acids which dissolve
calcite and create caves
• Plant roots and decaying plants create acids that
dissolve minerals making rock weak
Climate effects the speed of weathering
- Chemical weathering is most rapid in
warm, moist areas.
- Mechanical weathering is most rapid where
freezing and thawing occur.
Chemical or Mechanical?
Colonies of lichens growing on the surface
of rocks.
Chemical
Chemical or Mechanical?
The wedging of tree roots along natural
joints in granite outcrops
Mechanical
Chemical or Mechanical?
Solution of limestone by acidic groundwater
Chemical
Chemical or Mechanical?
The oxidation of minerals which contain iron
Chemical
Chemical or Mechanical?
Animal burrows dug in rock which let in
water and air
Mechanical
Chemical or Mechanical?
Repeated freezing and thawing of water
cracks rock or joints
Mechanical
Chemical or Mechanical?
The action of water, salt and air on car
fenders and panels
Chemical or Mechanical?
The action of water, salt and air on car
fenders and panels
Chemical
Chemical or Mechanical?
The sun fading paint on houses and cars
Chemical
Chemical or Mechanical?
Formation of potholes in streets during
severe winters
Mechanical
Chemical or Mechanical?
Lifted sections of sidewalk along tree-lined
streets
Mechanical
How Soil Forms
Section 8.2
Soil Profile
Soil – a mixture of weathered rock and
organic matter
- can take hundreds of years to form
- varies in thickness
Humus – decaying plants and animals
that mix with weathered rock
Soil Profile - shows the layers of soil and
weathered rock
O-Horizon
Litter layer- leaves
A-Horizon
Top Soil – dark colored,
sediment, decayed material,
insects, worms
B-Horizon
Lighter color, little or no
humus, contains minerals
from leaching
C-Horizon
Some material leached from
B-horizon, partly weathered
rock, no humus
Solid rock below soil
profile