Weathering and Erosion. Mechanical Weathering Chemical Weathering.
Ch 10 Weathering - MR. TILLMAN MR. ANDERSON EARTH … · 2.List and describe the types of...
Transcript of Ch 10 Weathering - MR. TILLMAN MR. ANDERSON EARTH … · 2.List and describe the types of...
Ch 10 – WeatheringObjectives
1.Contrast chemical and mechanical weathering.
2.List and describe the types of mechanical weathering.
3.List and describe the types of chemical weathering.
4.List the products resulting from the chemical
weathering of Igneous rocks.
5.List and discuss the factors that influence the type
and rate of rock weathering.
2
There are two primary types of weathering:
Chemical and Physical
1. Physical Weathering – the breakdown of rocks and minerals
into smaller pieces without a change in chemical composition.
Root/Plant Action
2. Chemical Weathering – the breakdown of rocks and minerals into smaller
pieces by chemical action. The rocks breaks down at the same time as it
changes chemical composition. The end result is different from the original
rock.
Example. Oxidation – oxygen combines with the elements in the rock and it
reacts. This the scientific name for rust.
Wind causes weathering
What evidence of
weathering do you see
in this picture?
Why wasn’t this mass
of land weathered
away?
Weathering
• Breakdown of rock
due to surface
processes
• 3 types of weathering
– Physical (Mechanical)
– Chemical
– Biological
http://www.geography.ndo.co.uk/animation
sweathering.htm#
Physical Weathering (Mechanical)
1. Abrasion – Means that the surface of the rock is weathered due to the action of wind, water & gravity
2. Frost wedging – Freezing–Thawing water expands when it freezes
3. Exfoliation or unloading –
– rock breaks off into leaves or sheetsalong joints which parallel the ground surface;
– caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial
Physical Weathering
(Mechanical)4. Thermal expansion -
– repeated daily heating and cooling of rock;
– heat causes expansion; cooling causes contraction.
– different minerals expand and contract at different rates causing stresses along mineral boundaries.
5. Plant Roots – break rocks apart as they grow
6. Animals – animals burrowing through the soil-examples ants, molds, coyotes, worms, mice
Chemical WeatheringRock reacts with water, gases and solutions
(may be acidic); will add or remove
elements from minerals.
1. Dissolution (or solution)
- also includes leaching
2. Oxidation
3. Hydrolysis
4. Biological Action/Acid in living thing
Dissolution• Dissolution occurs when rocks and/or
minerals are dissolved by water. The dissolved material is transported
away leaving a space in the rock. One consequence of this process is the formation of caves in limestone areas.
• Several common minerals dissolve in water
– halite
– calcite
• Limestone and marble contain calcite and are soluble in acidic water
• Marble tombstones and carvings are particularly susceptible to chemical weathering by dissolution.
Oxidation• Oxygen, the second most
common element in the air we breathe, reacts with iron in minerals to form iron oxide minerals, e.g. hematite (rust). As many minerals contain iron, it is not unusual to see red-colored rocks
• Oxygen combines with iron-bearing silicate minerals causing "rusting"
• Iron oxides are red, orange, or brown in color
Acid Precipitation
• 3. Acid Precipitation-Rain,
sleet, or snow, that contains a
high concentration of acids is
called acid precipitation.
Precipitation is naturally acidic.
However, acid precipitation
contains more acid than normal
precipitation.
Acid Rain
3. Acid Precipitation-Rain, sleet, or snow, that
contains a high concentration of acids is called acid
precipitation. Precipitation is naturally acidic. However,
acid precipitation contains more acid than normal
precipitation.
Acid in groundwater
Caves and caverns typically form in limestone
• speleothems - cave formations; made of calcite
• form a rock called travertine
– stalactites - from ceiling
– stalagmites - on ground
Karst topography forms on limestone
terrain and is characterized by:
• caves/caverns,
• sinkholes,
• disappearing streams,
• springs
More Dissolution
Hydrolysis – affected by H20• Hydrolysis occurs when minerals react
with water to form other products. Feldspar, the most common mineral in rocks on the earth's surface, reacts with water to form a secondary mineralsuch as kaolinite (a type of clay) and additional ions that are dissolved in water. The weaker clay is readily worn away by physical weathering.
• Feldspar alters to clay
– Feldspars = stable at high temperatures and pressures
– Clays are stable under conditions at the Earth's surface
• Quartz turns to sand
Biological Action
• Biological weathering is the actual molecular
breakdown of minerals. There are things called
lichens (combinations of fungi and algae)
which live on rocks. Lichens slowly eat away
at the surface of rocks.
• Lichens, fungi, and other micro-organisms
• Chemically and physically change rock
Mineral Content
Least stableOlivine
Ca plagioclase feldspar
Pyroxene
Amphibole
Biotite
Na plagioclase feldspar
Potassium feldspar
Muscovite
Quartz
Most stable
Factors Affecting Weathering*Surface Area
Texture
Temperature
Humidity
Topography
Time
Factors Affecting Weathering
Surface Area – Most
important
• Smaller particles,
more surface area
faster• Examples
– Crushed ice cools
faster
– Granulated sugar
dissolves faster
– More exposed rocks
weather more
Weathering and Elevation
• Rocks at higher elevations, as on a
mountain, are exposed to more wind,
rain, and ice than the rocks at lower
elevations are. This increase in wind, rain,
and ice at higher elevations causes the
peaks of mountains to weather faster.
Gravity
• Gravity affects weathering, too. The
steepness of mountain slopes
increases the effects of mechanical and
chemical weathering. Steep slopes cause
rainwater to quickly run off the sides of
mountains.
SummaryTypes of Weathering
• Mechanical or Physical - the breakdown of rock material into smaller and
smaller pieces with no change in the chemical composition of the
weathered material.
• Chemical - the breakdown of rocks by chemical agents. Obviously the chief
chemical agent is water which carries dissociated carbonic acid.
• Mechanical Weathering Expansion and Contraction - the thermal heating
and cooling of rocks causing expansion and contraction.
• Frost Action - Water freezes at night and expands because the solid
occupies greater volume. Action wedges the rocks apart. Requires
adequate supply of moisture; moisture must be able to enter rock or soil;
and temperature must move back and forth over freezing point.
• Exfoliation - process in which curved plates of rock are stripped from a
larger rock mass. Example Half Dome. Exact mechanism uncertain but
probably due to unloading.
• Other types - Cracking of rocks by plant roots and burrowing animals.
• Chemical Weathering Factors which effect the rate of chemical weathering
are: Particle size - Smaller the particle size the greater the surface area
and hence the more rapid the weathering
• Climate