Weathering and Granite Landscapes : News Article

Weathering and Granite Landscapes

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Weathering and Granite Landscapes: Case Study - DARTMOOR

Characteristics & formation of Granite:

granite is formed by magmatic intrusion into the earth's crust (forming for example a batholith - e.g. Dartmoor / Isle of Arran)
Due to slow cooling, associated with the depth / pressure inside the earth's crust, large crystals of Quartz, Mica and Feldspar form;
Granite is a resistant and impermeable rock, but areas with excessive jointing are less resistant and permeable (granite is often characterised by both horizontal and vertical joints)
As the granite cooled and contracted, vertical joints formed;
Later denudation of the overlying layer of rocks led to pressure release, allow re-expansion of the granite and resulting in the further creation of horizontal and vertical joints.
It is these joints that make granite susceptible to weathering processes.

GRANITE (polished - blue granite)

Photo courtesey of Wikipedia

Weathering processes in Granite:

FROST SHATTERING (physical weathering) - results in granular disintegration (results from the cycles of freezing (and consequent expansion) and thawing of water in joints / crevices in the rock.
HYDROLYSIS (chemical weathering) - this involves the dissolving of feldspar minerals in the granite by hydrogen. The feldspar reacts with hydrogen in water producing Kaolin (china clay) in the processes of Kaolinisation - this occurs as water circulates through the granite. Granite weathered by hydrolysis becomes weakened as the quartz crystals remain as loose crystals, unaffected by the hydrolysis process.

Weathered Granite Landscape: DARTMOOR

Image produced from the Ordnance Survey Get-a-map service. Image reproduced with kind permission of Ordnance Survey and Ordnance Survey of Northern Ireland.

Most of Dartmoor consists of Granite. This granite was intruded around 280 million years ago. Dartmoor granite covers an area of 241 miles²and represents the largest area of exposed granite in the South of Britain. It is part of a much larger mass of granite hidden underground. It is this granite which underlies the contrast between Dartmoor and the surrounding countryside.

Photographs used by kind permission of Richard Knights.

The Granite Landscape:

Granite landscapes are usually upland due to the upthrust of intrusion during the formation of the granite itself and the resistance of the rock to erosion. Some of the main features of the granite landscapes are shown in the photograph and list below (thanks to geography pages)

'mor and tor' landscape (see photo below)
thin acidic soils
high rainfall (due to elevatoin)
poor drainage (due to largely impermeable nature of granite - led to peat formation / blanket bog in many areas)
absence of trees and the abundance of heather and gorse.

Tor Formation on Dartmoor: (named example: Hound Tor)

One of the most distinctive features of the Granite landscape are TORS - these are exposed granite outcrops formed by weathering processes (for example - Hound Tor - see photograph opposite)

Processes resulting in Tor formation ~280 million years ago as the granite forming Dartmoor cooled and solidifed;
The minerals making up the granite crystallised as closely interlocking grains forming the hard rock;

Photographs used with kind permission of Richard Knight http://www.richkni.co.uk/dartmoor/index.html

Theories of Tor Formation

There are a number of different theories on the formation of Tors, here is one possible formation theory:

1. Initial Formation of vertical joints in the granite

During cooling, contraction resulted in the formation of mainly vertical joints in the granite

2. Removal of overlying rock (2-3km thick) and creation of horizontal joints

The cover of rock above the granite is gradually worn away over time, resuting in the removal of pressure from the granite below (pressure release), resulting in expansion of the rock and the creation of horizontal joints and bedding planes in the granite, following the shape of the surrounding land.

3. Sub-surface chemical weathering widening the joints (Hydrolysis resulting in Kaolinisation)

Hydrolysis, results in a reaction between hydrogen in water and the feldspar minerals in the granite which are broken down into China clay, further weakening the rock.

4. Deep weathering of the Granite

Further weathering of the granite takes place, during a warm period, rainwater is able to enter the rock through joints, causing hydrolysis and carbonation. Organic acids from plants are also carried deeper into the granite, with water - resulting in further weathering. This deep weathering attacks and weakens minerals such as feldspar in the rocks and widens the joints.

5. Frost shattering and exposure of Granite by solifluction in periglacial times.

In periglacial times, frost action forces apart the weathered granite, exploiting the vertical and horizontal joints, breaking the granite into blocks. Loose material produced by freeze-thaw is then removed by the process of solifluction, exposing the granite outcrop known as tor.

The diagrams below show the granite before and after weathering. The joint spacing is important - the more widely spaced joints, formed large resistant core stones. Weathered debris accumulated around the tor forms blockfields (where there are large granite blocks) or clitter (smaller fragments of granite)

See also the following links / resources...

If you are stuck on any of these key concepts or want to develop your understanding further, try out some of the link from the Earth Systems Weblinks section.

Download this excellent factsheet on Tor Formation from the Dartmoor National Park Authority (external link - Dartmoor National Park Authority)

Excellent Photographs of the Dartmoor Tors (from Richard Knights - external link - many thanks to Richard for his kind permission to make use of his images of some of Dartmoor's Tors in compiling this page)

Early Theories of Tor Formation (a more detailed account of some of the theories of tor formation)

See the Granite links on the excellent Geography Pages.

For some more summary notes and some excellent weblinks see also the Weathering section on Geography Pages (with thanks to Alan Parkinson from King Edward VII School, Kings Lynn).