Rock and weathering, slope and plate tectonics, As geo 9696 paper 1 revision

1. Rock & weathering
As Geo Revision

2. Plate boundaries & landforms
Mid-Atlantic Ridge
Sea Floor Spreading
Earth’s ocean floors move like conveyor belts, carrying the continents
along with them. This movement begins at the mid~ocean ridge. The
ridge forms along a crack in the oceanic crust. At the mid-ocean ridge,
molten material rises from the mantle and erupts. The molten material
then spreads out, pushing older rock to both sides of the ridge.
As the molten material cools, it forms a strip of solid rock in the center
of the ridge. Then more molten material splits apart the strip of solid
rock that formed before, pushing it aside. This process, called sea-floor
spreading, continually adds new material to the ocean floor.
Convection currents in the asthenosphere along with the process of
slab pull at a subduction boundary act to pull the two sides of the
ocean floor apart.

3. Convergent plate boundaries
Nazca plate
S. American plate
Peru-Chile Trench
Philippines
plate
Pacific plate
MarianasTrench

4. Convergent plate-C & C
Eurasian
plate
Indian
plate
Himalayas Mts
Continent & Continent plates

5. Peltier Diagram
Factors affecting rate & types of
Weathering:
1. Climate
2. Rock Types
3. Vegetations

6. Climate
From Peltier diagram, it describe the relationship between climate
(temperature and rainfall) and weathering processes.
Chemical weathering tends to be most intense in wet and warm climates
where high temperatures promote chemical reactions, and heavy rainfall
provides the moisture necessary for the processes to operate.
Deep weathering profiles are common in area such as tropical climates and
the characteristic red soils reflect active oxidation.
Physical weathering active in cold climates where frost shattering dominates.
In desert environments physical weathering might be expected to dominate
due to absence of water, chemical weathering is important, particularly
oxidation.

7. Rock characteristics
Rate of chemical weathering is affected by chemical composition of a
rock. Some minerals are more prone to chemical change than others.
For example, in granite, both minerals feldspar and mica are chemically
vulnerable, whereas quartz weathers extremely slowly.
The presence of iron minerals and salts will affect oxidation and
hydration respectively, and rocks rich in calcium carbonate (i.e.
limestone) will be affected by carbonation.
Joints and bedding planes promote weathering as they enable water
to penetrate deep into rocks.

8. Vegetation
(C ) Vegetation:
Presence of vegetation also promotes weathering,
in that organic acids speed up hydrolysis, and plant
roots may prise, apart jointed rocks.
Some form of vegetation, such as moss, cling to
rock surfaces, holding water against them like a wet
sponge thereby encouraging chemical weathering.
However this same vegetation might, at the same
time, protect rock surface from temperature
extremes, reducing effect of physical weathering.

9. Slope system-factors affecting slope
processes

10. Slope as a system
It is an example of an open system because
there are inputs from outside such as heat and
precipitation and outputs e.g. water and
weathered rock into other systems.

11. (a) Rock type- the tougher the rock the more able it is to support a steep slope. Igneous and metamorphic rocks are
extremely strong and are capable of supporting near-vertical slopes, whereas sands and gravel can only support very
gentle slopes.
(b) Geological structure-Rock slabs may become detached along bedding planes or joints, promoting rockfalls and
landslides.
(c ) Permeability and porosity- An impermeable rock will be liable to surface water flow, and deep gullies may forms.
(d) Climate-The climate of an area will affect the type of weathering that operates on a slope. It will also depends on
presence or absent of water and vegetation. Most of mass movement such as mudflows and soil creep are dependent
on aspects of climate, particularly precipitations.
(e) Vegetation-If a slope is forested or covered with bushes and grass it is less likely to be active. This is because it will
protect a slope from direct rainfalls and help bind together particles of rock and soil.
(f) Weathering-Weathering affects the upper slopes, particularly bare rock outcrops. In general mechanical weathering,
particularly frost shattering will lead to a more jagged, angular, bare rock surface whereas chemical weathering with its
tendency to dissolve and produce fine clays, will produce more rounded slopes.
(g) Basal excavation- It can takes place in the form of river undercutting a slope or the sea cutting a notch in a cliffline.
Human activities such as road construction can have same effect. Basal excavation can lead to a steepening of a slope,
so making it unstable.
(h) Human activity- People are capable of altering slopes directly by mining and quarrying, construction roads and
housing estates, and terracing land for farming. Slope can also be altered indirectly when, for example, forests are cut
down for firewood or to make way for agriculture. This deforestation will encourage more surface runoff and soil
erosion may occur.
(i) Time- Time a slope exposed to weathering is also important controlling factor. Newly formed landscapes that are
steep and unvegetated are actively weathered and eroded until they assume a shape that is balance with their
environmental conditions. But if environment changes (global warming, for example) the balance may be upset and the
slope profile will be forced to adapt.

12. Physical and chemical weathering
• Weathering-decay or decomposition of rocks in
situ resulting from physical or chemical actions.
• Erosion-the picking up and transportation of rock
material by agents such as rivers, glaciers and
sea.
• Physical weathering-involves the disintegration
rocks into smaller fragments without any
chemical change taking place.
• Chemical weathering-involves chemical change,
causing rocks to decompose.

13. How slopes can be stabilised
(i) Plant vegetation to bind soil together and
intercept rainfall
(ii) Improve drainage to prevent the slope becoming
saturated and to stop lines of weakness, for
example bedding planes, becoming lubricated
(iii) use wire nets and metal stakes to hold a slope
together
(iv) reduce the gradient by adding material to the
base of a slope.

14. Physical weathering Chemical weathering
Insolation weathering-expand & contract
outer layers of rock due to diurnal change
temperature (rock is poor conductor of
heat-only outer layer expand & contract)
Outer layers peels off result granular
disintegration.
Solution-dissolving minerals in water. E.g.
rock salt (halite)
Calcium carbonate (limestone) easily
dissolve in rainwater that has absorbed
carbon dioxide from atmosphere to form
carbonic acid. This form of solution is
called carbonation.
Salt weathering-High temperature, high
evaporation rate & low rainfall, result
concentration salt lying on, below ground
surface, growth crystal acts frost
shattering causes stresses within rocks
and result break down rock.
Hydrolysis-Associated with process of
Hydration. Chemical change often occurs
when mineral absorbs water. Mineral
feldspar in granite vulnerable to
hydrolysis, weakly acidic water causes
feldspar to change into white clay called
kaolin.
Pressure release-released of overlying
rock by erosion, cause rocks to expand
and lead formation of cracks which later
exploited by weathering.
Oxidation- oxygen dissolves in water react
with minerals, such as iron converted to
iron oxide. (weaken the bond and more
vulnerable to other weathering processes.

15. Hydration-expansion of mineral or salts
resulting in absorption of water in rock
i.e. clay, causes stress within rocks and
break.
Chelation-Effects of organic acids in rock,
These acids derived either from
decomposition of humus (rotted
vegetation) in soil, or direct secretion
from organism such as lichen. Chelation
important in promoting the effects of
hydrolysis and carbonation, because
weathering of rock under soil seems to
be more active than where bare rock is
exposed to the elements.
Frost Shattering-Water enter joints, on
freezing expand volume, as temperature
rises above freezing, thawing takes place,
stresses released. Cycles of melting &
thawing of ice joint enlarged, result
shatter, angular rocks collect at foot called
scree.

16. Mass movement & effects of rocks
Rockfall Landslide

17. Rotational slip/slide Mudflow

18. Solifluction
Soil Creep

19. Granite
• Granite forms deep below the ground and is only
exposed on the surface after many millions of
years of erosion.
• A physically tough rock, which is resistant to
erosion, and commonly forms uplands, for
example Dartmoor and Bodmin in S.W England.
• One of the most common features associated
with granite is a barely rocky outcrop found on
hilltops called a tor.

20. Granite’s Landform (Tors)
A barely rocky outcrop found on hilltops called a tor.
Formed by weathering deep underground
before the granite became exposed on
the surface.

21. Formation of Tors
• Over a long period of time, the weaker parts of the rock may have
been weathered to greater depths than the more resistant parts. If
subsequent erosion, most likely by river action in the context of
Puebla, stripped away all the weathered rock, it would leave the
more resistant rock as upstanding craggy outcrops called tors.
(Theory by Linton, 1971)
• Despite being physically strong granite is very vulnerable to
chemical weathering. The feldspar readily reacts with acidic water
to form a clay, and this chemical processes is hydrolysis that
weakens the granite causing it to crumble apart.
• Granite is heavily jointed and the density of jointing is believed to
have been a critical factor in the formation of tors.

22. Limestone
• Carboniferous limestone is a common form of
limestone that was formed some 300 million
years ago during the Carboniferous geological
period. It outcrops throughout the UK, from the
Gower in S.Wales to Pennine Hills in Yorkshire.
• The limestone in these areas has resulted in a
characteristic landscape known as karst. (both
overground & underground features)

23. Karst
Water flow impermeable
Limestone Pavement: Bare rocky surface
criss-crossed by enlarged joints, separating
blocks of limestone. Enlarged joints are
grykes & blocks of limestone in between
them -clints Swallow hole
cavern
rocks
Sink Holes
Surface depressions
Surface weathering
Karst Scenery
Rainwater passes
Underground joints &
Along bedding planes,
Weathering & erosion
To form caverns.
Drips –long tapered feature
Down cavern roof-stalactite
Shorter, stubbier feature on
floor of cavern-stalagmite
Both join to form
Column or pillar