Granite versus limestone comparison notes
1) Characteristics (9m)
2) Weathering of the rocks (relates back to point 1 and causes point 3)
3) Landforms associated [16m]
Class:
formation
Limestone
Sedimentary
Non-clastic
- Calcium from decomposition of
marine organisms
- Gaps filled in with mud and clay
- Lithification: cementation and
compaction and drying
Chemical
>50% generally calcite / calcium
composition carbonate (carbonation and
solution): homogenous (little
regolith left behind) but pure
limestone should be >90%
50-90%: impure limestone
Calcite: natural cementing agent so
not all rocks with calcite are
limestones
Rock
*Only for carboniferous limestone
structure
High secondary permeability
- Bedding planes / joints / faults
(selective weathering – block
disintegration)
- Shrinkage joints: drying –
tensional (folding) / shear joints
- Older: probability of experiencing
tectonic forces
Physical
hardness
Low primary permeability
- Small pore spaces: water cannot
penetrate through easily
- Older: more compaction and
cementation – less pore spaces
Hard (not prone to physical
weathering) – compaction
Granite
Igneous (intrusive) (pressure
release – exfoliation)
- Crystallisation of magma
- Granitic (viscous) magma
- Rock texture – coarse-grained
(phaneritic) because more time for
crystals to grow before solidifies
into rock
Feldspar (hydrolysis – clay), quartz
(sand – granular disintegration)
(clay, sand and corestones due to
spherodial weathering = residual
debris): heterogenous (insolation
weathering – granular
disintegration) (many types of
elements)
Mica, biotite
High secondary permeability
- Joints / faults (selective
weathering – block disintegration):
sheet joints, shrinkage joints:
during formation stage of rock
cooling process during formation of
granite
Low primary permeability
- Crystals close together and
interlock during crystallization
Hard (not prone to physical
weathering) – feldspar and quartz –
hard minerals and interlock tightly
Words in red are how the characteristics link to weathering
Limestone landforms
Tropical
Temperate
Closed
Cockpit karst, tower karst
Dolines, uvalas
depression
Karren
Pinnacles (spitzkarren)
Limestone pavements
features
(need glacial / lateral erosion)
Caves
Less well-developed
More well-developed
(higher rainfall, but not much seeps (drizzle)
in to form caves / enter ground. Stalagtite (more spread out)
Most flow off to rivers)
Stalagmite (more concentrated)
Drainage
Shallow holes (resurgence stream) + dry valleys + blind valleys
*Cross question between geomo and hydro: drainage density falls
because stream length falls but area of drainage still the same
Cockpit karst:
Cone
Cockpit
Gullying /
surface erosion
Tower karst:
Cones
Cave
200m
plain
If water table reached,
then tower karsts
protruding from water
body
Granite landscape
Etchplanation
1) Pluvial period (1000s of years)
Wetter: higher rainfall
Granite: breaks down to quartz,
sand, clay, corestones =
residual debris
Slow regolith removal because
dense vegetation: infiltration +
percolation
Fast regolith formation because
rainfall amount high
Regolith formation > regolith removal: accumulation of regolith
Climate humid. Wet throughout whole year: no dry season. Similar to humid tropics.
2) Interpluvial period
Former surface
Etch plain
Ruware
Selective
weathering
where joints are
concentrated
Basal surface weathering: undulating
due to different depths of regolith:
high secondary + low primary
permeability
Regolith formation due to less rainfall, less chemical weathering < regolith removal
because less vegetation: net loss of regolith
Ruware: exposed part of basal surface of weathering
Savanna
Etch plain: isolated rock outcrop which rises abruptly from surrounding plain
3) Pluvial period + 4) Interpluvial period: repetition serves to increase height of ruware.
If not cannot explain tall inselbergs.
Etch plain
Former BSW
Pluvial period
Very little weathering at ruware, more weathering at sides
1) More joints
2) Shedding of water from ruware to sides
3 ) Regolith absorbs water vs solid rocks
Former surface
Tall inselbergs
Regolith
removed