granite scenery lesson 7

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