GEOLOGY FOR ENGINEERS minerals (clays) and soluble salts.

These reactions
occur particularly when the water is slightly acidic.
What is Weathering?
Where does it occur?
- Weathering is the breakdown of rocks at the These chemical processes need water, and occur more
Earth’s surface, by the action of rainwater, rapidly at higher temperature, so warm, damp climates
extremes of temperature, and biological activity. It are best. Chemical weathering (especially hydrolysis
does not involve the removal of rock material. and oxidation) is the first stage in the production of
What is the difference of erosion and soils.
weathering?
- Erosion is the process by which soil and rock How does it occur?
particles are worn away and moved elsewhere by There are different types of chemical weathering, the
wind, water or ice. Weathering involves no most important are:
moving agent of transport.
Types of Chemical Weathering
There are three types of weathering,
physical, chemical and biological.
Solution - removal of rock in solution by acidic
rainwater. In particular, limestone is weathered by
rainwater containing dissolved CO2, (this process is
Physical Weathering - is caused by the effects of
sometimes called carbonation).
changing temperature on rocks, causing the rock to
break apart. The process is sometimes assisted by Hydrolysis - the breakdown of rock by acidic water to
water. produce clay and soluble salts.

Oxidation - the breakdown of rock by oxygen and

2 Main Types of Physical Weathering water, often giving iron-rich rocks a rusty-coloured
weathered surface.
 Freeze-thaw occurs when water continually
seeps into cracks, freezes and expands, Explanation:
eventually breaking the rock apart. Hydrolysis – Chemical reaction of anything with water.
 Exfoliation or onion skin weathering
occurs as cracks develop parallel to the land Example is water to coffee beans to coffee. When it
surface a consequence of the reduction in happens in nature, when it rains it will pour on the
pressure during uplift and erosion. ground and dissolve minerals.

Chemically change – what we had in the end, is not

Freeze-thaw what we have in the beginning.
Where does it occur?
In mountainous regions like the Alps or Hydrolysis takes place when acid rain reacts with rock-
Snowdonia. forming minerals such as feldspar to produce clay and
salts that are removed in solution.
How does it occur?
Rainwater or snow-melt collects in cracks in the feldspar, any of a group of aluminosilicate minerals
rocks. that contain calcium, sodium, or potassium.

The only common rock-forming mineral that is not

affected is quartz, which is a chemically resistant
At night the temperatures drop and the water mineral. This is why quartz and clay are the two of the
freezes and expands. most common minerals in sedimentary rocks.

Oxidation – “mars” Red planet Mars is known as the

Red Planet because iron minerals in the Martian soil
The increases in volume of the ice exerts oxidize, or rust, causing the soil and atmosphere to
pressure on the cracks in the rock, causing look red.
them to split further open. And here on earth, we have here Mafic rocks carry
magnesium and iron and if mixed with oxygen they
oxidize and rust.
2 types of Solution
During the day the ice melts and the water Carbonation – “carbonic acid” mixing of water and
seeps deeper into the cracks. carbon dioxide to produce carbonic acid.
Carbon dioxide in the air dissolves in rainwater and
becomes weakly acidic. This weak “carbonic acid” is
able to dissolve limestone as it seeps into cracks and
At night the water freezes again.etc. cavities. Over many years, solution of the rock can
form spectacular cave systems (see photo).
Where does it occur? Note: Stalagmites and stalactites form in caves as
Physical weathering happens especially in places water drips from the roof, depositing some of its
places where there is little soil and few plants grow, dissolved calcium carbonate, as carbon dioxide is
such as in mountain regions and hot deserts. released into the air.
Acid rain
How does it occur? Polluting gases, like sulphur dioxide and nitrogen oxide
Either through repeated melting and freezing of water dissolve in rainwater to make stronger acids. When this
(mountains and tundra) or through expansion and rainwater falls, we get acid rain. This acid attacks many
contraction of the surface layer of rocks that are baked rock types, both by solution and hydrolysis, seriously
by the sun (hot deserts). damaging buildings and monuments.
Biological Weathering
Chemical Weathering
2. Chemical weathering is caused by rain water Living organisms contribute to the weathering process in
reacting with the mineral grains in rocks to form new many ways:
Trees put down roots through joints or cracks in the rock in sediment is moved along – for example, pebbles rolled
order to find moisture. As the tree grows, the roots gradually along a river-bed or sea shore, sand grains whipped up
prize the rock apart. by the wind, salts carried in solution.
Even the tiniest bacteria, algae and lichens produce chemicals
EROSION by:
that help break down the rock on which they live, so they can
get the nutrients they need. 1. Gravity and Mass Wasting
Many animals, such as these Piddock shells, bore into rocks - Mass–wasting is the down-slope movement of
for protection either by scraping away the grains or secreting loose rock and soil due to gravity. Whilst
acid to dissolve the rock. processes such as soil creep are very slow,
landslides can be very sudden, and may cause
loss of life.

Mass-wasting is often aided by water, so

landslides and mudflows frequently occur after
heavy rain.
Examples of Erosion by Gravity
Soil creep is slow – a few cm per year. Fences & walls
lean over but trees bend as they grow. Grassland
develops terracettes.
Rock falls, like this one at Beachy Head, may occur
without warning after heavy rain.
Land Slips Repairs to the road at Mam Tor,
Derbyshire, were abandoned in the 1970s after
repeated landslides in the weak mudstones.
Mudflows take place after heavy rain has weakened
The Rock Cycle Processes soft clay or mudstone. Mudflows of volcanic ash are
1. Weathering called Lahars.
2. Erosion and Transport 2. Wind
3. Deposition of Sediment - Wind erosion and transport is a serious
4. Burial and Compaction environmental problem in the driest parts of the
5. Deformation and Metamorphism world, removing soil from farmland and covering
6. Melting whole towns with sand and dust.
7. Crystallization of Magma A strong breeze (wind speed 20 km/h) can easily
8. Uplift pick up dry dust and fine sand. The lightest
particles are swirled up into the air and carried in
suspension for hundreds of kilometres, whilst sand
QUIZ #2 grains bounce along the surface by saltation.
Small pebbles can be moved along with the sand,
1. Water has the power to split rocks because when it freezes, a process called creep.
3. Water
it:
Around the world, moving water picks up and
Contracts transports millions of tonnes of sediment every day,
along rivers, coasts, and even in the deep oceans.
Evaporates
Expands
2. Which of the following sedimentary rocks is commonly
weathered by solution?
Limestone
Sandstone
Mudstone
3. Chemical weathering is most likely to take place in
environments that are:
Cold
Sediment, whether picked up by flowing water or by
Wet waves, is moved along in one of four ways:
Dry
4. In which type of environment would you be most likely to 1. Traction is the rolling or dragging of large
observe Biological weathering? grains along a river bed or shore, aided by the
A hot desert push of the smaller grains (below).
2. Saltation is the bouncing of sand grains as
A high mountain they are picked up, carried along, and dropped
A rocky sea-shore repeatedly by flowing water.
5. A common mineral group produced by chemical 3. Fine particles (silt and clay) are carried in
(hydrolysis) weathering is: Suspension in the water – they will only settle
out if the water is still.
Clay
4. Soluble salts are carried in Solution in the
Mica water – the sea is obviously salty, but rivers
contain dissolved salts, too.
Feldspar

The bed load of a river (pebbles and sand grains) helps

What is Erosion? to erode the river bed and banks by abrasion,
Erosion especially when the river is in flood. In a similar way,
waves cause cliffs to erode especially during storms. As
Erosion is the process by which soil and rock particles the pebbles and sand grind away the river bed or sea
are worn away and moved elsewhere by gravity, or by shore, they also become smaller and more rounded, a
a moving transport agent – wind, water or ice. process called attrition.
Transport refers to the processes by which the
 Glaciers erode rapidly downward, forming deep, U-
shaped valleys.

1. Mass-wasting is a process that produces:

In the rivers –

Upland streams form narrow V-shaped valleys. The Sand dunes

river erodes downward. The river bed has large angular Glaciers
grains in transport.
Landslips
Mature streams form broader, more gentle valleys.
Sediment grains get smaller and smoother

Lowland rivers meander in wide floodplains. Erosion on 2. U-shaped valleys, like the one in the photo, are
outside of bends is balanced by deposition on inside.
formed by:
Sediment mainly sand & mud.

As a river enters the sea it deposits most of its Wind

sediment forming tidal sand and mud-flats. Rivers may Water
split into many channels (photo).
Ice
In the sea –

Erosion by the sea occurs mainly on exposed

shorelines where waves constantly wear away the cliffs 3. Which of the following is not a way that rivers
and foreshore. Storm waves hurl shingle and rocks at
the cliff foot, and drag them over the foreshore, slowly transport sediment?
wearing it away to form a wave-cut platform. This
Saltation
action breaks up and rounds the broken rock
fragments to form a beach. Organisms living on the Subduction
rocks of the foreshore may help this process.
Suspension
Waves and tidal currents transport sediment along the
shoreline, a process called longshore drift. Tidal
currents can also move sediment out into deeper 4. Most erosion by the sea results from the action of:
water.
Waves
Turbidity currents are density flows that transport
sediment for hundreds of miles - from the continental Wind
shelves out into the deep ocean. (See animation). Tidal currents

4. Ice
- A glacier is a river of ice formed from compacted 5. In which type of environment would you be most
snow. They are most common in the Polar regions
(e.g.Antarctica) and in mountain regions like the likely to observe wind erosion?
Alps and Himalayas. Glaciers move slowly down
valleys (at speeds up to a few metres per day), but A high mountain
have enormous erosive power. Glaciers erode the A hot desert
surface of the Earth in two ways:
A rocky sea shore
Plucking

Plucking

Melt water at the base of the glacier enters cracks in

the bedrock and re-freezes, causing the rock to break
apart (see also Weathering pages)

As the glacier continues to move the broken rock gets

stuck in the ice and carried away.

Abra s
As the

glacier moves downhill, rock fragments embedded in

the ice at the base of the glacier scrape and grind
against rock beneath the ice.