Download chemical weathering - G

Adam C. Simon
Ph.D., University of Maryland, 2003
Research Associate
Department of Geology
University of Maryland
College Park, MD 20742
p: 301 405 0235
f: 301 314 9661
e-mail: [email protected]
Weathering and
Erosion
The Great Falls of the Potomac River
Rocks exposed at the Earth’s
surface are constantly being eliminated:
physically and chemically destroyed and transported elsewhere.
These are the processes of weathering and erosion.
Terminology
• weathering -- the chemical and physical breakdown of rock
exposed to air, moisture, and organic matter
• erosion -- the loosening and moving downhill of rock and soil
• regolith -- the loose layer of broken rock and mineral
fragments on the surface
The Rock Cycle
Revisited
The rock cycle traces the potential changes to rocks
resulting from Earth’s internal and surficial processes.
Physical Weathering
Physical weathering is when rocks break down as the result of
physical phenomena. This can be due to abrasion or processes
related to expansion and contraction.
Simple heating and cooling causes enough physical change to
promote physical weathering.
heat
spalling
Much weathering exploits pre-existing
fractures in rock -- joints: fractures along
which no movement has occurred.
Exfoliation
Exfoliation is the
natural formation of
fractures in rocks
(mainly igneous
plutons) due to the
release of pressure
upon reaching the
surface.
Peeling Away Granite
The classic Sierra Nevada (CA) granite mountains photographed
by Ansel Adams are examples of exfoliation domes.
Weathering in
Sheets
Exfoliation can operate
on very large scales.
It is most important in
areas of exposed
igneous rocks, even
though these rocks
generally lack a
layering or similar
planar features.
Freeze-Thaw (Frost Wedging)
The volume change as liquid
water transforms to ice is an
important physical weathering
force in temperate climates.
Root Wedging
The action of growing plants also promotes physical weathering.
Frost + Root
Wedging
A birch tree exploited a crack in this
boulder in Maine and, together with
freeze-thaw, has split it in half.
Chemical Weathering
Chemical weathering is the decomposition of rocks and
minerals as a result of chemical and biochemical reactions.
As in all reactions, unstable minerals react to form a set of
products more stable at near-surface conditions.
So, broadly speaking,
chemical weathering
is the process
whereby unstable
high temperature/
pressure minerals are
transformed into their
Earth-surface-stable
relatives.
Types of Chemical Weathering
hydrolysis: chemical reaction in which water is involved
Si also lost (~20%)
H2O + CO2 = H2CO3 (carbonic acid)
Hydrolysis involves pure water,
but most natural waters, from
reaction with atmospheric CO2,
are acidic (rain pH ~ 5.5).
These naturally acidic waters
are even more efficient at doing
chemical work.
electron microscope view of a
weathered feldspar crystal
Oxidation
Oxidation is the breakdown of minerals exposed to a
oxygen-rich environment. This is most obvious with minerals
rich in elements like iron.
Remember, iron in rocks is mainly Fe2+.
When rocks weather, the iron is oxidized to Fe3+, producing
red staining from minerals like hematite (Fe2O3).
Oxidation of
Iron-Rich
Silicate
Minerals
Iron in Hematite
How can you tell what the oxidation state of iron is in a
mineral like hematite, just from the formula?
Oxygen in nature has a - 2 charge. If hematite is Fe2O3, then
it carries (- 2) x 3 = - 6 charge from oxygen.
This must be balanced by the iron, so both Fe atoms in the
structure of hematite must be +3 charged.
A trickier problem: what is the oxidation
state of iron in the mineral magnetite:
Fe3O4
Dissolution
The dissolution of calcite (CaCO3) is chemically easy, since
calcite is much more soluble than silicate minerals.
In limestone caves we see the work of calcite
dissolution, as well as the results of calcite
precipitation (collectively called speleothems).
Chemical
Weathering in
Nature
solution of calcite by
mildly acidic rainfall
(hydrolysis)
over the past century
oxidation of
iron in
Hawaiian soils:
Fe2+ in rocks
converted to
Fe3+ in
sediment
Consequences of Chemical Weathering
One thing to consider during these chemical reactions is the
fate of the ions that are released.
In weathering feldspars, for instance, a lot of Si, K, and Ca
are shed. These start out as ions in aqueous (water-rich)
solutions (like groundwater), but where do they end up?
Weathering and Bowen’s Reaction Series
The order in Bowen’s reaction series of minerals to crystallize
from cooling magma is the same sequence when considering
mineral stability at Earth surface conditions:
calcite, halite
olivine
Ca-rich plagioclase feldspar
least stable at low
temperatures/pressures
pyroxene
amphibole
Na-rich plagioclase feldspar
biotite (mica)
K-feldspar
muscovite (mica)
clay minerals, Fe-oxides
quartz
most stable at low
temperatures/pressures
Weathering and Bowen’s Reaction Series
Rocks chemically weather
according to the stability
of their minerals, the
severity of their
environment and the time
available.
For example, a granite
(quartz, feldspars and
mica) typically chemically
weathers to a mixture of
quartz and clay minerals.
Spheroidal Weathering
When weathering begins along a
perpendicular network of
fractures, it will frequently
weather out resistant rock in
spherical shapes.
(see Fig 7.10 in
your text)
the Devil’s Marbles in central Australia
Surface Area / Volume
Breaking rocks up (physical weathering) increases the
ratio of surface area to volume.
Positive Feedback: Surface Area/Volume
If there is more exposed surface, then more chemical
weathering can take place, since it is on surfaces that
chemical weather concentrates.
We will see many
positive feedback
processes like this during
the semester. These are
mechanisms in which the
product works to
advance the process.
Climate and Weathering
Climate (combination of temperature and rainfall) strongly
influences weathering processes:
– physical weathering dominates in low rainfall, cold climate;
– chemical weathering dominates in high rainfall, warm climate.
Organisms and Weathering
Organisms are important agents of
weathering, physical and chemical.
Imagine how weathering on Earth would
be different without life.
Soil
The fine-grained portion of the regolith is called soil.
Soils characterize areas by type of bedrock and
climate, and tend to have distinct layered profiles.
weathered,
disaggregated bedrock
intact bedrock
Soil in Humid Regions
Water from
above
percolates
through the soil
column and
leaches the
upper surface of
soluble
components.
These
components
tend to
precipitate lower
in the column.
Fig 7.11
Soil in Humid Regions
Insoluble
materials remain
in the A horizon:
Fe-oxides,
Al-rich minerals,
and organic
compounds.
Fig 7.12
Soils in Dry
Regions
In arid climates, groundwater flow may
be principally from the bottom up, as
water is drawn to the surface and
evaporates.
This reverses the profile we see
in humid climates, such that
hard, calcite-cemented material
dominates the upper portion of
the profile.
Arid Soils
The hard calcite pavements
associated with arid soils
are called caliche or calcrete.
Soil Formation and Climate
North Pole
polar
temperate
low-latitude
desert
Equator
tropical
latitude
Why is weathering enhanced in the temperate regions,
even though temperature is lower than the tropics?
Factors Controlling Weathering Rates
Positive Feedback: Soil Formation
Why does soil cover aid chemical weathering?
Bury a nail for 10 years and compare it to
a nail in a 19th century house.
Chances are, the one in the soil is in far worse shape.
Soils have high contents of moisture, organic materials
and CO2, all of which enhance chemical weathering.
Credits
Some images in this presentation come from:
Hamblin and Christiansen, Earth’s Dynamic Systems, 8/e
AJ Kaufman (Univ. Maryland)
GS Solar (SUNY College Buffalo)
Brodo, Sharnoff and Sharnoff, Lichens of North America
Plummer, McGeary and Carlson, Physical Geology (8th ed.)
Carlsbad Cavern National Park
Press and Siever, Understanding Earth (3rd ed.)
Marshak, Earth: Portrait of a Planet (1st ed.)