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
Affect 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.
–Average amount of incoming
snowfall > amount lost by melting
and evaporation
–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.
Glacial Rebound
• the rise of land masses that were
depressed by the huge weight of ice
sheets during the last glacial period,
through a process known as isostasy.
• 2 phases
– Initial uplift called elastic- rapid
– Slow viscous flow- ~1cm/year