Weathering, Soil, Sendimentary,
Metamorphic rocks
By
Doba Jackson, Ph.D.
Associate Professor of Chemistry
Huntingdon College
Mechanical and
Chemical Weathering
• Rocks and minerals are disintegrated and
decomposed by the processes of mechanical
and chemical weathering.
– The products of weathering include soluble salts,
ions in solution, and solid particles which can be
eroded and become sedimentary rock or modified
in place to become soils.
Weathering and Erosion
• How does weathering differ from erosion?
– Weathering is the mechanical and chemical alteration
of Earth materials at or near the surface
– Erosion involves removing weathered materials from
their place of origin-by running water or wind, for
example.
Mechanical Weathering –
Disaggregation of Earth Materials
•
Mechanical weathering
includes the processes of
– Frost action
– Pressure release
– Thermal expansion and
contraction
– Crystal growth
– Activities of organisms.
Products of Mechanical weathering is chemically
the same as it was prior to the weathering.
Mechanical Weathering –
Freeze-Thawing
- When water freezes in
cracks in rocks it
expands and then it
contracts when it
thaws, thus exerting
pressure and opening
the cracks wider.
- Repeated freezing and
thawing disaggregates
rocks into angular
pieces that may tumble
downslope and
accumulate as talus.
Mechanical Weathering –
Pressure Release
Rock formed underground at higher pressure can
undergo an expansion under lower pressure on the
earth’s surface.
– Sheet joints are fractures that more or less parallel
exposed rock surfaces, especially rocks now at the
surface that formed under great pressure at depth.
Mechanical Weathering –
Living Organisms, Plants, Trees,
• How do organisms contribute to mechanical and
chemical weathering?
– Any plant or tree roots
growing in cracks
contributes to mechanical
weathering by exposing the
cracks inside the rocks and
supplying water and other
chemicals to them
Chemical Weathering –
Decomposition of Earth Materials
•
Chemical
weathering
processes include
– Solution
– Oxidation
– Hydrolysis
•
Hot and wet
environments
accelerate chemical
weathering.
•
Chemical weathering occurs in all environments, except, possibly,
permanently frozen polar regions.
Chemical Weathering –
Dissolution or deposition in fluids
Dissolution of Carbonate rocks
(Limestone, Dolomite)
• Solution/Dissolution
– – rocks dissolve
• Carbonate Rocks
– Rocks such as
limestone (CaCO3)
are nearly insoluble
in neutral or alkaline
solutions, but they
rapidly dissolve in
acidic solutions
Oxidation of Iron, Magnesium
and other metals
• Oxidation
– – rocks rust
– Rocks such as
sandstone may contain
iron minerals that will
breakdown when
exposed to the
atmosphere
Fe
Iron in
any rock
+
O2
Oxygen
in the air
Fe2O3
Iron Oxide
(Rust)
Oxidation of Iron, Magnesium
and other metals
• Oxidation
– Iron Oxide (Fe2O3) can
further break down in
the presence of water to
Iron Hydroxide
Fe2O3 + H2O
Iron Oxide
Water
Fe(OH)3
Iron Hydroxide
Hydrolysis of Rock by water to
form Clays
• Hydrolysis
– breakdown to clays
– Potassium feldspar
• During hydrolysis hydrogen ions react with and
replace positive ions in potassium feldspar
• The result is clay minerals and substances in
solution such as potassium and silica.
KAlSi3O8
Orthoclase
+
CO2 + H2O
Al2Si2O5(OH)4 +
Clay (Kaolinite)
SiO2 + KHCO3
Silica
How does mechanical weathering
contribute to chemical weathering
Sandstone Arches at Arches
National Park, Utah
Soil and Its Origin
• Soils consist of weathered materials, air,
water, humus and also the plants which
they support.
Soil and Its Horizons
• Soil formation produces
horizons that are known
in descending order as
O, A, B, and C.
– These horizons differ
from one another in
texture, structure,
composition and
color.
Soil and Its Horizons
• Upper Layer (O) Humus
– Consists of decayed
organic matter
necessary for plant
life. The dark color
comes from Humus.
Soil and Its Horizons
• Topsoil (A)
– Less Humus but lots
of microbes, insects,
plant material. Also
has lots of water,
clays and quartz
Soil and Its Horizons
• Subsoil (B)
– Little to no Humus.
Mostly degraded
minerals
• Weathered Bedrock (C)
– Weathered rock.
Cannot grow and
living organisms.
What factors control soil
formation?
– Soil formation is
controlled by
• Climate
• Parent material
• Organic activity
• Relief and slope
• Time
Soil and Climate conditions
Introduction to Sedimentary Rocks
• What is sediment?
– The two primary types of sediment are Detrital
and Chemical.
– Detrital sediment
consists of solid
particles, products
of mechanical
weathering.
– Chemical
sediments consist
of minerals
precipitated from
solution by
inorganic
processes and by
the activities of
organisms.
Sediment Sources, Transport, and
Deposition
• Detrital sedimentary particles are classified
according to grain (particle) sizes, in
decreasing diameter:
– Gravel (> 2mm)
– Sand (1/16 to 2 mm)
– Silt (1/256 to 1/16 mm)
– Clay (less than 1/256 mm; as mud)
• During transport, abrasion effects reduce
particle sizes (rounding).
What is a depositional
Environment?
• Depositional environments are areas of sediment
deposition.
Lithification: Converting Sediment into
Sedimentary Rock
Lithification of sediment into sedimentary rock occurs by
compaction and cementation.
Cementation of Sedimentary Rocks
Cementation is a process that glues the
sediments together.
-Calcium Carbonate (CaCO3)
-Iron Oxide (Fe2O3)
- Iron Hydroxide (Fe(OH)3)
- Silica Dioxide (SiO2)
These materials are the products of chemical
 weathering of rocks!!!
Conglomerate and Sedimentary
Breccia
• Conglomerate- a sedimentary rock consisting of an
aggregate of gravel (> 2mm rocks) solidified
together with rounded edges.
• Sedimentary Breccia- same as a conglomerate
excepts it has sharp edges.
Sand and sandstone is the most
common sedimentary rock
• Why is quartz the most common mineral in sandstone?
• Quartz is a common mineral in many source rocks, and in addition it is
very stable chemically and quite durable mechanically.
Chemical Sedimentary Rocks
SeaShells
• Protective exoskeleton of Crabs,
Lobsters, worms, Sea Urchins, etc.
Other Carbonate Rocks:
Limestone & Dolostone
• Limestone is Calcium
Carbonate but
usually in a mixture
with other minerals.
• Dolostone forms
when magnesium
replaces calcium in
limestone.
Halites and Gypsum are formed
when water evaporates
• Bedded rock salt (halite)
and rock gypsum are
chemical evaporite
sediments formed by
precipitation of minerals
during the evaporation
of water.
Coal: Biochemical sedimentary
rock
• Coal is a biochemical sedimentary rock
composed largely of altered land plant
remains
Sedimentary Facies
• Sedimentary Facies- Bodies of sediment or
sedimentary rocks which are recognizably different
from adjacent sediment or sedimentary rocks and are
deposited in a different depositional (sub) environment
are known as sedimentary facies.
Marine Transgression
• A marine transgression
occurs when sea level
rises with respect to the
land, resulting in
offshore facies
overlying nearshore
facies.
• A marine regression,
caused when the land
rises relative to sea
level, results in
nearshore facies
overlying offshore
facies.
Marine Regression
•
A marine
regression, caused
when the land rises
relative to sea level,
results in nearshore
facies overlying
offshore facies.
• Notice the different
slant in the facies
between a
transgression and
regression.
Important Resources in Sediments
and Sedimentary Rocks
• What is oil shale?
• Oil shale is a fine-grained sedimentary rock that
contains kerogen from which liquid oil and combustible
gases can be derived.
• None is mined at present
in the United States
because oil and gas from
conventional sources are
cheaper. Oil shale and tar
sands are increasingly
important petroleum
reserves.
Metamorphic Rocks
Introduction to Metamorphism
• What is metamorphism?
• The transformation of rocks, usually beneath Earth's surface,
as the result of heat, pressure, and/or fluid activity, produces
metamorphic rocks
• Why is metamorphism worth studying?
• Metamorphism is an important process that is closely related to
plate tectonics, the growth of continents, and even climate
change.
• It is also responsible for producing a number of economically
valuable materials.
• Though largely hidden from view, metamorphism is part of the
world around you. The world would be a much different and
less interesting place without it.
Equilibrium and the Causes of
Metamorphism
• Intrusive magmas or
deep burial provide
heat which causes
metamorphism.
• Pressure is produced
by overlying rocks
(lithostatic) or is
differential pressure
produced by various
stresses.
Equilibrium changes
• Equilibrium: A system in which all competing forces are
balanced.
• Minerals are no longer stable (no longer “in equilibrium”) as
conditions change. Minerals can react with one another to
create more stable kinds of minerals (heterogeneous
metamorphic reactions) or undergo rearrangements in their
atoms to reduce the strain that builds up within them (crystal
lattice reorientation and polymorphic transformation).
The Main Types of
Metamorphism
• Principal types of metamorphism –
»Contact
»Dynamic
»Regional metamorphism
Contact Metamorphism
• Contact metamorphism
– Contact metamorphism is when a body of
magma alters surrounding rock. This provides
intense heat which drives changes within the
rock.
Contact
metamorphism
Increased (or Differential)
Pressure
Lithostatic pressure is
a uniform field of
pressure
experienced by
most rocks beneath
Earth’s surface.
Like the hydrostatic
pressure
experienced by
divers underwater,
the pressure acting
on a rock
embedded in the
crust “feels” the
same from all
directions.
The Main Types of
Metamorphism
• Dynamic metamorphism
– Dynamic metamorphism is associated with faults
and areas where lots of pressure builds up in the
crust, but the temperature is not very great, such
as in the accretionary wedges at convergent plate
boundaries.
The Main Types of
Metamorphism
– Shock metamorphism is a type of dynamic
(pressure-dominated) metamorphism
associated with meteorite impacts. The
pressure is extremely high relative to other
natural processes that operate on Earth.
Regional Metamorphism
– Regional metamorphism is the most common type of
metamorphism. As the name implies, regional metamorphism has a
broad range. Temperature and pressure both act as driving forces for
metamorphic reactions in regional metamorphism.
Classification of Metamorphic
Rocks
• Metamorphic rocks are classified principally according
to texture. The texture is
» Foliated - Foliated texture is produced by the
preferred orientation of platy minerals.
» Nonfoliated - Nonfoliated textures do not exhibit
preferred orientation of minerals.
Foliated Metamorphic Rocks
• What is foliated texture, and what are some examples
of foliated rocks?
– Foliated texture is produced by the preferred
orientation of platy minerals.
– Amphibolite is another fairly common coarse grained
foliated metamorphic rock.
Schist and Gneiss
Non-Foliated Metamorphic
Rocks
• What is a common nonfoliated metamorphic
texture, and what are some examples of
nonfoliated metamorphic rocks?
– Nonfoliated textures do not exhibit preferred
orientation of minerals.
– Common nonfoliated metamorphic rocks are
marble, quartzite, greenstone, and homfels.
Metamorphic Zones
• What are isograds and metamorphic zones?
– Metamorphic rocks often can be arranged in
metamorphic zones which reflect the pressure
and temperature conditions of metamorphism
Some Economic Uses of
Metamorphic Materials
Marble