Weathering, Mass Movement and Ground water:
The process of wearing away the earth causes a general lowering and
levelling out of the surface. It is known as Denudation and is carried out in
four phases.
1. Weathering the gradual disintegration of rocks by atmospheric or
weather forces.
2. Erosion the active wearing away of the earth’s surface by moving
agents like running water, wind, ice and waves.
3. Transportation: the removal of the eroded debris to new positions.
4. Deposition: the dumping of the debris in certain parts of the earth,
where it may accumulate to form new rocks.
Weathering:
The work of weathering in breaking up the rocks is of two kinds,
namely chemical and physical or mechanical weathering, but the
processes involved in each are closely interrelated.
1. Chemical Weathering
Chemical weathering is the basic process by which denudation
proceeds. It is extremely slow and gradual decomposition of rocks
due to exposure to air and water. Air and water contain chemical
elements, which though they may be in small quantities, are
sufficient to set up chemical reactions in the surface layers of
exposed rocks. Such reactions may weaken or entirely dissolve
certain constituents of the rock thus loosening the other crystals
and weakening the whole surface. When the surface of the rock is
weathered some of the material which is loosened is removed by
erosive agents such as wind or running water thus exposing the fresh surface to weathering, but much of the weathered material
or regolith (remains of the rocks stay in position forming the basis
of soil.
There are three major chemical weathering processes.
a) Solution. Many minerals are dissolved in water, especially
when, as with rain water, it contains enough carbon dioxide to
make it a weak acid. solution is the most potent weathering
process in limestone regions because the rain-water attacks and
dissolves the calcium carbonate of which the rock is chiefly
formed. The dissolved calcium carbonate carried away by the
water, joints and cracks in the rock are quickly widened and
whole systems of caves and passages are worn out.
b) Oxidation: Oxidation is the reaction of oxygen in air or water
with minerals in the rock. For example most rocks contain a
certain amount of iron, which when it comes in contact with air
is changed to iron oxide, familiar brownish crust or rust. Iron
oxide crumbles easily and is more easily eroded than the
original iron. It is thus removed, loosening the overall structure
of the rocks and weakening them.
c) Decompostion by Organic acids: Within the soil which covers
most rocks are bacteria which thrive on decaying plant or
animal material. These bacteria produces acids which, when
dissolved in water, help to speed the weathering of the
underlying rocks. In some cases micro-organisms and plants
like mosses or lichens can live on bare rock, so long as the
surface is damp. These absorb chemical elements from the
rocks as food and also produce organic acids. They are thus
agents of both chemical and mechanical weathering.
2. Physical or Mechanical Weathering:
Mechanical weathering is the physical disintegration of a rock by
the actual prising apart of separate particles. Mechanical
weathering takes place in several ways.
a) Repeated Temperature Changes: In deserts rocks are exposed
to the blazing sun during the day and are intensely heated. The
outer layers expand much faster than the cooler interior of the
rocks and tend to pull away from the rest. At nightfall the
temperature drops rapidly and the outer layers contract more
rapidly than the interior, setting up internal stresses. Such
stresses, repeated every day for months and years cause the
rocks to crack and split. Wellbedded and jointed rocks tend to
split along the joints and cracks, breaking up into rectangular
blocks. Shales and slates may split up into platy fragments
because of their platy structure. In crystalline rocks such as
granite the crystals of the various minerals (quartz, mica,
feldspar) will expand and contract at different rates, enhancing
the stresses and accelerating the disintegration of the rocks.
Stresses and pressures will naturally be greatest near the
surface and where there are sharp angles in the rocks.
Rectangular blocks are thus gradually rounded by the slitting
away of sharp corners. When the surface layers of rounded
boulders gradually split off the process is called Onion peeling,
because the various layers looks like the layers of an onion
peeling off one after another. The technical term for this
process is exfoliation.
b) Repeated wetting and drying: This takes place especially in
tropical regions like Malaysia, where short downpours
saturated the rocks and then the hot sun quickly dries them
again. Repeated wetting and drying also occurs at the coast,
where rocks may be rapidly dried by sun and wind between
tides. When rocks are wetted the outer layers absorb a certain amount of moisture and expand. When they dry this moisture
evaporates and the outer layer split off.
c) Frost Action: In temperature latitudes frost is a potent rock
breaker. All rocks contain cracks and joints or pore spaces, and
after a shower water or snow collects in such places. When the
temperature drops at night or during the winter , this water
freeze .When water freezes it expands by one-tenth its volume
and exerts a bursting pressure of almost 140 kg per square cm
(2,000 lb. To the square inch). Repeated freezing of this kind
will deepen and widens the original cracks and crevices and
break the rock into angular fragments.
d) Biotic Factors: Small fragments of rock loosened by either
chemical or mechanical weathering lodge in cracks and crevices
in the rock and plants may sprout in such crevices. As they
grow their roots penetrate the rocks below, usually along joints
and other lines of weakness, prising them apart.
the actual prising apart of separate particles. Mechanical
weathering takes place in several ways.
a) Repeated Temperature Changes: In deserts rocks are exposed
to the blazing sun during the day and are intensely heated. The
outer layers expand much faster than the cooler interior of the
rocks and tend to pull away from the rest. At nightfall the
temperature drops rapidly and the outer layers contract more
rapidly than the interior, setting up internal stresses. Such
stresses, repeated every day for months and years cause the
rocks to crack and split. Wellbedded and jointed rocks tend to
split along the joints and cracks, breaking up into rectangular
blocks. Shales and slates may split up into platy fragments
because of their platy structure. In crystalline rocks such as
granite the crystals of the various minerals (quartz, mica,
feldspar) will expand and contract at different rates, enhancing
the stresses and accelerating the disintegration of the rocks.
Stresses and pressures will naturally be greatest near the
surface and where there are sharp angles in the rocks.
Rectangular blocks are thus gradually rounded by the slitting
away of sharp corners. When the surface layers of rounded
boulders gradually split off the process is called Onion peeling,
because the various layers looks like the layers of an onion
peeling off one after another. The technical term for this
process is exfoliation.
b) Repeated wetting and drying: This takes place especially in
tropical regions like Malaysia, where short downpours
saturated the rocks and then the hot sun quickly dries them
again. Repeated wetting and drying also occurs at the coast,
where rocks may be rapidly dried by sun and wind between
tides. When rocks are wetted the outer layers absorb a certain amount of moisture and expand. When they dry this moisture
evaporates and the outer layer split off.
c) Frost Action: In temperature latitudes frost is a potent rock
breaker. All rocks contain cracks and joints or pore spaces, and
after a shower water or snow collects in such places. When the
temperature drops at night or during the winter , this water
freeze .When water freezes it expands by one-tenth its volume
and exerts a bursting pressure of almost 140 kg per square cm
(2,000 lb. To the square inch). Repeated freezing of this kind
will deepen and widens the original cracks and crevices and
break the rock into angular fragments.
d) Biotic Factors: Small fragments of rock loosened by either
chemical or mechanical weathering lodge in cracks and crevices
in the rock and plants may sprout in such crevices. As they
grow their roots penetrate the rocks below, usually along joints
and other lines of weakness, prising them apart.
Mass Movement:
Mass movement is the movement of weathered materials down
a slope due to gravitational forces. The movement may be
gradual or sudden, depending on the gradient of the slope, the
weight of the weathered debris and whether there is any
lubricating moisture supplied by rain-water. Several kinds of
mass movement are distinguished.
1. Soil Creep:
This is is slow gradual but more or less continuous
movement of soil down hillslopes. Soil creep is most
common in damp soils where the water acts as a lubricant so
that individual soil particles move over each other and over
the underlying rock. It is also found where continuous
trampling by animals grazing on the slopes sets up
vibrations which loosen the soil and cause it to move.
Though the movement is slow and cannot readily be seen in
action, the gradual movement tilts trees, fences posts and so
on which are rooted in the soil.
2. Soil Flow ( Solifluction)
When the soil is completely saturated with water the
individual particles are almost suspended in the water and
move easily over one another and over the underlying rock.
The soil acts like a liquid and a soil-flow or mud-flow occurs.
In Ireland such flows are known as ‘Bog-Bursts’.
3. Landslides ( Slumping or Sliding)
Landslides may be caused because of steep slope is undercut
by a river or the sea so that it falls by gravity. Earthquakes or
volcanic disturbances may loosen rocks and start off a
landslide. Man-made steepening both undercuts the slope
and set up vibrations which may loosen rocks or soil. But
often landslides are caused by the lubricating action of
rainwater. Water may collect in joints or bedding planes in
rocks so that one layer slides over another, especially in
areas of tilted strata. Slumping is particularly common
where permeable debris or rock layers overlie impermeable
strata such as clay. Water sinking through the permeable
material is halted by the clay. The damp clay provides a
smooth slippery surface over which the upper layers easily
slide.
action, the gradual movement tilts trees, fences posts and so
on which are rooted in the soil.
2. Soil Flow ( Solifluction)
When the soil is completely saturated with water the
individual particles are almost suspended in the water and
move easily over one another and over the underlying rock.
The soil acts like a liquid and a soil-flow or mud-flow occurs.
In Ireland such flows are known as ‘Bog-Bursts’.
3. Landslides ( Slumping or Sliding)
Landslides may be caused because of steep slope is undercut
by a river or the sea so that it falls by gravity. Earthquakes or
volcanic disturbances may loosen rocks and start off a
landslide. Man-made steepening both undercuts the slope
and set up vibrations which may loosen rocks or soil. But
often landslides are caused by the lubricating action of
rainwater. Water may collect in joints or bedding planes in
rocks so that one layer slides over another, especially in
areas of tilted strata. Slumping is particularly common
where permeable debris or rock layers overlie impermeable
strata such as clay. Water sinking through the permeable
material is halted by the clay. The damp clay provides a
smooth slippery surface over which the upper layers easily
slide.
Groundwater:
The whole process of the circulation of water between the land, sea and atmosphere is known as the hydrological cycle. When rain falls on the earth it is disturbed in various ways some is immediately evaporated and these returns to the atmosphere as water vapour. Some is absorbed by plants
and gradually return to the atmosphere by transpiration from the leaves of plants. Much of it flows directly off slopes to join streams and rivers, eventually reaching the seas and oceans. This is known as run off.
A considerable proportion of the water received from rain or snow however, percolates downwards into the soil and rocks filling up joints and porespaces and forming what is known as ground water.
Ground water plays an important part in weathering and mass movement and is also important as means of natural water storage. It re-enters the hydrological cycle by way of springs. Porous rocks are those, like sandstone, which have many pore spaces between the grains. Water is easily absorbed by
such rocks and may be stored in the pore spaces.
such rocks and may be stored in the pore spaces.
Permeable or pervious rocks are those which allow water to pass through them easily. Thus most porous rocks are impermeable.
Water which seeps through the ground moves downward under the focus of gravity until it reaches an impermeable layer of rock through which it cannot pass. If there is no ready outlet for the groundwater in the form of a spring, the water accumulates above the impermeable layer and saturates the rock. The permeable rock in which the water is stored is known as the aquifer. The surface of the saturated area is called the water table.
Springs:
The ground water stored in the rock is released onto the surface at points where the water-table reaches the surface.
The ground water stored in the rock is released onto the surface at points where the water-table reaches the surface.
A spring is simply an outlet for such water.
Springs are of several kinds due to the nature of the rocks and the position of the water table. The main types of springs are:
a. In areas of tilted strata, where permeable and impermeable rocks alternate, water emerges at the base of the permeable layers.
b. In well jointed rocks water may percolate downwards until it reaches a joint which emerges at the surface. The water may come to the surface through the joint.
c. Where a dyke or sill of impermeable rock is intruded, through permeable rocks, it causes the water-table to reach the surface and the water issues as a spring.
d. In limestone or chalk escarpments, where the permeable rock lies between impermeable strata, water issues at the foot of the scarp as a scarp foot spring, or near the foot of the dip slope as a dip-slope spring.
e. In karst regions rivers often disappear underground.
b. In well jointed rocks water may percolate downwards until it reaches a joint which emerges at the surface. The water may come to the surface through the joint.
c. Where a dyke or sill of impermeable rock is intruded, through permeable rocks, it causes the water-table to reach the surface and the water issues as a spring.
d. In limestone or chalk escarpments, where the permeable rock lies between impermeable strata, water issues at the foot of the scarp as a scarp foot spring, or near the foot of the dip slope as a dip-slope spring.
e. In karst regions rivers often disappear underground.
Then they flow through passages worn in the rock by solution, and may re-emerge when limestone gives place to some impermeable rock. This kind of spring is sometimes called a vauclusian spring but is referred to as
a resurgence.
a resurgence.
Springs are the natural emergence points of ground water, but Man can
make use of stored water below ground by sinking wells. A hole is bored
through the earth until the water table is reached. Wells are particularly
important in arid areas where there is little surface water but where the
underlying rocks contain ground water.
Artesian wells are important type of well.
(Source: Goh Cheng Leong's Certificate physical and Human Geography)