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Weathering and Soil
Geology 101
Ted Brewster
The act of breaking down existing rock
Can be done by mechanical or chemical
Both methods serve to create sediment
from existing rock types
Mechanical Weathering
Physical Disintegration of the rock
The rock breaks into smaller pieces
More surface area exposed to chemical
weathering processes
The rock changes physically but not
Frost wedging, roots, pressure release, and
Change in Surface Area
Frost Wedging
Water fills into small cracks within the rock
When water freezes into ice it expands
This small expansion slowly makes the crack
larger, so more water can fill in
Over time and successive melting and
freezing cycles the cracks open up enough to
split the rock into pieces
Frost Wedging
Frost Wedging
Roots – Biological Weathering
The act of plant roots breaking rocks apart
The roots of the plant find cracks and slowly
grow downward into the crack
Over time the effect of the root growth
breaks the rock into pieces
Tree in
Tree Roots
Pressure Release
Most rocks are formed at depth and then
are moved to the surface
The pressure at depth is high, as the rocks
move to the surface they expand
This is similar to compressing a sponge, then
letting expand in water
The expansion causes the mineral grains to
break away from each other
Pressure Release - Sheeting
Spheroidal Weathering
Removing small pieces of rock over time by
wind or water
Like using sandpaper on the rock
Slowly the rock is just worn away
Causes the rounding of angular pieces
Abrasion – Death Valley, CA
Differential Weathering
Layers of rock are not of the same
resistance to weathering
Some layers are left intact while others are
weathered away almost completely
Most prominent in layers that have been
tilted onto their sides
Weathering Dike
Sorting and Rounding
As pieces of sediment move away from
their parent rock they change
Sorting – the ratio of different sizes of
material within the rock
More uniform size indicates farther from
Rounding – the shape of the edges of the
sediment pieces
More rounding indicates farther from source
Chemical Weathering
Rock decomposition that actually
dissolves the rock minerals and forms new
Most minerals form at depth with little
Surface has abundant water, so the
reaction can form new minerals
Some minerals are soluble in water and
Limestone, halite, gypsum, etc
Weathering of Granite
Chemical Weathering
Slate vs Limestone
Acid Weathering
Many carbonate minerals are susceptible to
If the water around them contains an acid,
then the rock is more likely to dissolve quickly
We see this in cities with acid rain
Buildings, Statues, Tombstones, etc
Cleopatra’s Needle
Salt Weathering - Greece
Climate plays a big role in determining the
type of weathering most common
Desert – mechanical abrasion
Tropical (warm and wet) – chemical
Arctic – mechanical frost wedging
Climate and Weathering
Weathered, unconsolidated materials on
top of bedrock
Capable of supporting plant growth
Soils form over long periods of time
Other material is called regolith
Exact time depends on the climate conditions
Soils have layers called horizons
Soil Horizons
O horizon
Dark colored layer just below the vegetation
 Contains decomposed plant material called
humus that forms organic acids
A horizon
Zone of leaching formed by percolation of rain
water through the O horizon
 Wet climates have larger A horizons than
Soil Horizons
B horizon
Accumulation of leaching products from horizon
 Tends to be clay rich and stained red by
hematite and limonite
C horizon
Incompletely weathered bedrock
 Transitional layer between bedrock and evolving
soil above
Soil Profile
by Climate
Soil Horizons - Hawaii
Soil Horizons in AZ
Sedimentary Structures
Ancient structures that can determine the
depositional environment of the rock
Graded Beds
Graded Beds – sorted layers of sediment
Ripple Marks
•Ripple marks – shallow
ripples that indicate a current
Cross Bedding
Cross bedding – angled sediment layers that indicate either
wind or water flow direction
Fossil Roots
Fossil plant roots – filled in holes made by roots
Mud Cracks
•Mud cracks – filled in cracks
from muddy and dry
Dinosaur Footprint
•Fossil footprints, tracks, or
burrows – any impression in the
sediment made by living creatures
that was then filled in