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ROCKS AND SOIL
The Rock Cycle
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Rock: Any naturally formed aggregate of mineral particles
Mineral: A naturally occurring inorganic substance usually
crystalline with a definite chemical composition
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E.g silica: SiO4-4
Olivine (Mg, Fe)2SiO4
Viewed over long time spans the rocks of the earth are
continually forming, changing and reforming
Rock cycle: The loop that involves the processes by which
one rock changes to another
Helps us understand the origin of igneous, sedimentary and
metamorphic rocks as well as how they relate to each other
Process driven by heat from the Earth's interior
Rock Types
• Igneous rocks
• Rocks that forms as lava or magma cools and crystalizes
• Sedimentary rocks
• Rocks that form due to the lithification of sediments
• Metamorphic rocks
• Rocks (either igneous or sedimentary) that have been altered due
to high temperature and/or pressure
Why is this relevant?
Different rock types weather in different ways and different rates. Since
weathering leads to the formation of certain landscapes, understanding the
nature of the rocks helps us understand the processes operating in the
landscape.
Igneous Rocks
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Two ways of formation
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Extrusive (a.k.a volcanic) – lava that crystalizes on the surface
Intrusive (a.k.a plutonic) - magma that crystalizes underground
Classified based on texture and mineral composition
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Texture: the size, shape and arrangement of the interlocking
crystals
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Extrusive: Rapid cooling and small crystals
Intrusive: slow cooling and large crystals
Mineral composition: refers to the types of minerals found within the
rock
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SiO4 by far the most abundant
 Mafic: rich in “dark” silicates associated with Fe and Mn
(They are termed dark since rocks composed of them are dark in color)
 Felsic: rich in “light” silicates with greater amounts of K, Na, Ca
(They are termed light since rocks composed of them are light in color)
Igneous Rock Examples
Basalt
Extrusive feature
Develops from
magma cooling
rapidly on the
surface
Small crystals
Mafic
Granite
Intrusive feature
Develops from
magma cooling
slowly beneath the
surface
Large crystals
Felsic
Scoria
Extrusive feature
Develops from
gaseous magma
cooling in flight
Vesiculated
Mafic
Igneous environments
Extrusive
Fast cooling, small crystals,
vesicular (Scoria)
Extrusive
Fast cooling, small crystals
(Basalt)
Intrusive
Slow cooling, large crystals
(Granite)
Sedimentary Rocks
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Classification of sedimentary rocks
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Clastic
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Chemical
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Rocks that are created from chemical precipitation (e.g limestone &
rock gypsum)
Organic
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Consists of discrete fragments and particles that are cemented and
compacted together (e.g sandstone & shale)
Are created from biological remnants (e.g coal)
Sedimentary environments
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An area of sediment deposition characterised by a particular
combination of climate conditions and physical, chemical or
biological processes
Sedimentary environments
Continental
Environments
Lake
Alluvial
Desert
Glacial
Colluvial
Transport Agent
Lake currents, waves
River currents
Wind
Ice, melt water
Gravity
Sediments
Sand, mud, saline
precipitates in arid
climates
Sand, mud,
gravel
Sand, dust
Sand, mud,
gravel
Sand, mud,
gravel, boulders
Wetland/ Bog
In situ accumulation
Organic deposits
Shoreline
Environments
Delta
Beach
Tidal Flats
Transport Agent
River currents, waves
Waves, tidal currents
Tidal currents
Sediments
Sand and mud
Sand and gravel
Sand and mud
Marine
Environments
Deep Sea
Continental Shelf
Organic Reefs
Continental
Margin/Slope
Transport Agent
Ocean currents,
Turbidity currents
Waves and tides
Waves and tides
Ocean currents
and waves
Sediments
Mud and sand
Sand and mud
Calcified
organisms
Mud and sand
Sedimentary Rocks Examples
Coal
Lithified organic
remains (part.
plants)
Organic
sedimentary rock
Sandstone
Lithified sandy
deposits
Clastic sedimentary
rock
Limestone
Lithified calcium
carbonate deposits
Chemical
sedimentary rock
Metamorphic Rocks
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Metamorphic environments
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Regional metamorphism
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Most widespread of metamorphism
Takes place where both high temperatures and pressures are
imposed over large parts of the crust
Characteristic of convergent plate boundaries (e.g. volcanic mountain
belts such as the Andes and the cores of mountains in continentcontinent collisions)
High-pressure metamorphism
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Occurs at high pressures
Most form in subduction zones as sediments scraped from subducting
oceanic crust are plunged to depths of over 30km where the
experience high pressures
Metamorphic Rocks
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Metamorphic environments
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Contact metamorphism
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Heat from an igneous intrusion metamorphoses the rock immediately
surrounding it
Effect is local normally affecting only a thin zone of country rock along
the zone of contact
Seafloor metamorphism
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A form of metasomatism (change in the composition of a rock by fluid
transport of chemicals into or out of a rock)
Often associated with mid-ocean ridges
Hot basaltic lava increases temperature of infiltrating seawater which
circulates through the newly forming crust
The increased temperature promotes chemical interactions leading to
an alteration of the local basalt
Metamorphic Rocks
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Metamorphic environments
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Burial metamorphism
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Low-grade metamorphism caused by the progressive increase in
pressure created by growing layers of overlying
sediments/sedimentary rocks by the increase in heat associated with
depth of burial
Typically begins at depths of 6 -10km (temp at this depth range
between 100 and 200°C)
Shock metamorphism
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Occurs when a meteorite collides with the Earth
Energy present in meteorites mass and velocity (i.e. kinetic energy
Rare on Earth
Metamorphic environments
Burial
metamorphism
Contact
metamorphism
Regional
metamorphism
High-pressure
metamorphism
Seafloor
metamorphism
Shock
metamorphism
Metamorphic Rocks Examples
Anthracite
Pre-existing rock:
coal
Regional
metamorphosis
Serpentinite
Pre-existing rock:
Gabbro
Hydrothermal
metamorphosis
Slate
Pre-existing rock:
Shale
Contact
metamorphosis
The Rock Cycle
The Rock Cycle (Basic)
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Magma
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Molten material that forms
in certain environments in
Earth's interior where
temperature and pressure
are such that rocks melt
Migrates to lithosphere
Either cools and solidifies
beneath the surface or on
the surface
When flowing out on
surface it is known as lava
Leads to formation of
igneous rocks
The Rock Cycle (Basic)
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Igneous Rock
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Forms from either cooled
lava/magma
When exposed at surface
will undergo weathering
Weathered material
transported through a
variety of processes
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These transported
particles and deposited
particles are known as
sediments
The Rock Cycle
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Sedimentary Rock
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Lithification (conversion
into rock) of deposited
sediments
Either through compaction
(by overlying
rock/sediments) or
cementation
The Rock Cycle
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Metamorphic Rock
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Mineral/textural changes
due to high temperatures
or pressures
Solid state transformation
(i.e no melting occurs)
Why?
Rock Cycle (Alternative Paths)
Soil
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Soil is a combination of mineral and organic matter,
water and air.
Composition
25%
45%
25%
5%
Mineral Matter
Organic Matter
Water
Air
Controls on Soil Formation
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Parent Material
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Time
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Most influential control (esp. temp & precip)
Influences type & rate of weathering
Plants and Animals
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Affects the extent of soil development
Climate
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Source of weathered mineral matter
Affects rate of soil formation & soil fertility
Residual soil- derived from bedrock
Transported soil- derived from transported sediments
Source of organic material (e.g humus)
Slope
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The angle of the slope affects environmental processes such as the
movement of water, accumulation or removal of sediments etc. This leads
to changes in the soil characteristics at various points in the landscape
The Soil Profile
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Horizon
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Vertical differences in
texture, colour, or structure
Soil profile
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Vertical section through all
the soil horizons
Some Soil Groups Found in South Africa
Group
Organic
Characteristics
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Vertic
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Calcic
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Uses
Soils where the natural accumulation
of organic material has been
exceptional
Characteristic of wetlands
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Shrink and swell properties
sufficiently developed to give rise to
characteristics such as cracks
Alternates from extremely dry and
hard to wet and sticky
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Calcium (released during the
weathering process) remains behind
in soil as water evaporates.
Sub-surface soil particles cemented
together by CaCO3 (calcite)
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Restricted to grazing unless irrigation is
provided (value then depends on thickness
of overlying soil)
Mined as a stable road surfacing material
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Vital ecological niche (moderate stream flows
and act as natural filters for water passing
through)
Farming practices (leads to severe
degradation of this soil type)
One of the most problematic soils from a
management perspective
Soils used for extensive grazing or for
growing crops such as cotton
Oxidic
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Soils have a B-horizon that is
uniformly coloured with either red
and/or yellow iron oxides
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Highly productive when irrigated
Among the best soils to use for forestry
Lithic
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Soil is dominated by weathered
material (saprolite)
Soils generally very shallow (erosion
keeps pace with weathering)
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If better soils are available in region, lithic
soils avoided and left as unimproved veld
Used for growing grapes in regions such as
the western cape
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