Transcript Weathering, Soil and Unstable Slopes

Weathering,Soil and Glacial
Movement
Ch. 14,15
What Physical Properties Affect
Rocks Near the Surface?
• There are many processes that break
rocks apart and can change their
– color
– texture
– composition
– strength
• Called chemical and physical
weathering
What is the role of joints in
weathering
• Joints are fractures
in rock that are not
offset.
• Joints allow water
and roots to
penetrate the rocks
to cause weathering.
Joints
• Closely spaced joints promote increased
weathering.
• Exfoliation- when overlying rocks are
unloaded the underlying rocks expand
creating expansion joints.
Physical Weathering
• Heating and cooling- expansion of rock
• Frost wedging
• Mineral wedging- growth of minerals
weaken the rock around them
Physical Weathering cont
• Burrowing organisms
• Plant growth
Effect of Fracturing on Weathering
• As a rock fractures and breaks apart there
is an increase in the rate of weathering.
• WHY??
• As a rock breaks into smaller pieces the
surface area increases allowing more
exposure to weathering.
Chemical Weathering
• Chemical factors that affect rocks at
the earth’s surface
–Decreased temperature and
pressure
–Increase O2
–Increased H2O
Dissolving Rocks
• Rocks are dissolved by weak acids
formed in water that react with the
minerals that make up rocks.
• Acids are abundant in H+ ions, the H+
ions enter minerals releasing other ions
• Ex- Limestone is rich in calcite which is
soluble in weak acids. When Carbonic
acid in rainwater reacts with the rock it
dissolves.
Oxidizing Rocks
• Free O2 is common near the surface and
reacts with minerals to cause rust.
• Most common in iron bearing minerals.
• Mafic minerals commonly contain iron and
are most commonly affected by oxidation.
How Does Water React With
Minerals?
• In some minerals water combines with the
mineral causing formation of a new
mineral, called hydrolysis.
• If exposed to wet conditions many rocks
convert into clay minerals.
How Does Soil Form
• Soil consists of weathered rock,
plus material from the
atmosphere, decaying plants
and microbes
• Soil is made up of different
zones, or horizons.
Soil Horizons
• O Horizon- surface accumulation of
organic debris.
• A Horizon- topsoil, dark gray or brown
organic material mixed with minerals.
• E Horizon- light colored, leached zone
• B Horizon- little organic material, red
color due to accumulation of iron oxide
• C Horizon- weathered bedrock
Processes That Occur During Soil
Formation
• Soil forms over thousands of
years as a result of weathering.
• Involves the vertical movement of
dissolved material up and down
through the horizons.
How Materials Move Through Soil
• Materials move both up and down as
carried by water, animals and gravity
• Zone of leaching- Upper part of soil
loses material downward. Water
leaches soluble materials and carries
them downward.
• Zone of Accumulation- Chemical ions
leached from above accumulate in the
lower levels.
Glaciers
• Moving masses of ice.
• They form by the accumulation of
snow and ice.
–Snow falls by individual flakes and
then gets compressed until it
recrystallizes into solid ice.
–Blue color because there is less air.
Glacier Movement
• Glaciers move downhill because
ice is not strong enough to
support its own weight.
• Shearing force – the top of the
glacier moves faster than the
bottom because of friction.
Glacial movement continued
• As glaciers move internal stress causes
the upper surface of ice to break forming
fractures, called crevasses.
• When the glacier reaches water, it will
float.
• When glaciers break off, in a process
called calving, they float off as icebergs.