Chp2 Less1
Weathering
Weathering and its Effects
Everything around you changes over time. Example=Brightly painted walls and signs slowly
fade. Weathering also changes Earth’s surface. Earths surface today is different from what it
was in the past and what it will be in the future. Weathering processes break, wear, abrade,
and chemically alter rocks and rock surfaces. Rocks broken down into smaller pieces by
weathering are known as sediment. Different sediments are given names based on their
size.Boulders are the largest and clay is microscopic.
Mechanical Weathering(Physical weathering)
Surface area
When something is broken into smaller pieces, the total surface area increases. Surface area is
the amount of space on the outside of an object. The rate of weathering depends on a rocks
surface area that is exposed to the environment.
Causes of Mechanical Weathering
1.
2.
3.
4.
Ice wedging= known as frost wedging. Water enters cracks in rock.
Abrasion=the grinding away of rock by friction or impact
Plants=crumbling rocks
Animals=create holes in the soil
Chemical Weathering
-Changes the materials that are part of a rock into new materials. If a piece of granite weathers
chemically, the composition and size of the granite changes.
Water and Chemical Weathering
Water is important in chemical weathering because most substances dissolve in water. The
minerals that make up most rocks dissolve very slowly in water. For a rock, the process of
dissolving happens when minerals in the rock break into smaller parts in solution.
Dissolving by Acids
An acid increases the rate of chemical weathering more than rain or water does. The action of
acids attracts atoms away from rock minerals and dissolves them in acid. Scientist use pH as a
property of solutions to measure acidity. Normal rain is slightly acidic, around Ph=5.6. Acid
forming chemicals enter the air from natural sources such as volcanoes. Pollutants in the air
also react with rain and make it acidic.
Oxidation
Oxidation is a process that causes chemical weathering. Most of the oxygen needed for
oxidation comes from the air. The addition of oxygen to a substance produces an oxide. Iron
oxide is a common oxide of Earths materials. The outside of a rock has the most contact with
oxyden in the air, therefore, the outer part oxidizes the most.
What affects weathering rates?
The environment in which weathering occurs helps to determine the rate of weathering. Both
types of weathering depend on water and temperature. Mechanical Weathering occurs the
fastest in locations that have frequent temperature changes. It requires cycles of either
wetting and drying or freezing and thawing. Chemical weathering is fastest in warm, wet
places( regions near equator). Rocks can be made of one mineral or many. The most easily
weathered mineral determines the rate that the entire rock weathers.