granite scenery lesson 7
TRANSCRIPT
GRANITE
SCENERY
Lesson 7
CharacteristicsForm under the surface of the earth,
cooling very slowly to form large batholith
Exposed after millions of years of erosion
Rock has low porosity and permeability but water easily pass through joints
Joints are cracks formed during cooling and shrinking of the magma
Cracks are further enlarged by pressure release
Three main minerals: Quartz Feldspar Mica
Main processes that affect granite:
Freeze thaw action occurring in joints of highland areas
Feldspars in granite are easily weathered by hydrolysis
Quartz is tough mineral which will not be weathered and will be left behind
The minerals are made of different colours, – they are susceptible to insolation weathering and resulted in granular disintegration.
GRANITE SCENERY: TOR FORMATION ON DARTMOOR
Theories of Tor formation:
1. Initial formation of vertical joints in the granite
2. Removal of overlying rock (2-3m thick) and creation
of horizontal joints
3. Sub-surface chemical weathering widening the joints
( Hydrolysis resulting in kaolinisation)
4. Deep weathering of granite
5. Frost shattering and exposure of granite by solifluction in periglacial times
2 critical factors for the formation of tor
Taught to have been formed by weathering deep underground before granite became expose on the surface
1. Despite being very strong, granite is very vulnerable
to chemical weathering, Feldspar readily reacts with
acidic water to form kaolin and this hydrolysis process
weaken the granite causing it to crumble apart
2. Granite is heavily jointed and the density of
jointing is believed to have been a critical factor
LIMESTONE SCENERY
Characteristic of carboniferous limestone
Formed from the remain of organic matter, usually seashells and plants
Formed under the sea, over 200 millions ago
Limestone rock has no pore spaces because the rock is so old.
Layers of limestone become very compressed and cemented under the weight of overlying sediments
Very strong and resistant to erosion
Able to form steep slopes without collapsing
Has large numbers of joints and bedding planes
These lines of weaknesses allow water to pass through the rock to produce Karst scenery
Composed mainly of mineral calcium carbonate which is insoluble
The main processes, which affect it are carbonation and solution
Characteristic of carboniferous limestone
Limestone Landforms1. Surface features caused by solution:
Limestone pavement
-large area of bare exposed rock-When overlying rock was eroded, the pressure release on limestone below caused it to crack even more.
-Characterise by large gaps between the rock called grikes.
-The remaining blocks are called clints .
LIMESTONE PAVEMENT
CLINTGRIKE
Limestone Landform
2. Drainage Features:
Swallow holes and sink holes are where river flow down into the rock.
Sink holes are relatively small while
Swallow holes are larger
Both have been formed by constant chemical attack of water on joints or by the collapse of a cavern below
Swallow hole
Sink hole
Limestone Landform3. Surface features resulting from underground drainage
Poljes - in the tropics, the landscape may be composed of either cone shaped hills and polygonal depressions
E.g. ‘cockpit country’ (Jamaica) Tall isolated towers rising from wide plains -
(Guilin, China)
Dry valleys – a river valley without a river. a common feature on chalk and limestone
Cockpit country- Jamaica
Guilin - China
Dry valley
Limestone Landform4. Underground features and underground depositional features
Cavern - underground caves that have been hallowed out by the action of underground streams and by carbonation and solution.
Three distinctive features:
Stalactites – hang from the roof of the cavern, and basically lime deposits
Stalagmites – grow from the floor and also lime deposits
Pillars – where stalactites and stalagmites have joined
Stalactites
Pillars