Permeability is the rate at which water maybe stored within a rock, allowing the free passage and the interfusion of fluids. The primary permeability depends upon the size, shape, and texture of the rock and the arrangement of its mineral particles. The air spaces, which are the areas between the mineral particles, are called pore spaces, and there size and setting can decide how much water can be absorbed by the rock i.e. large pore spaces allow rapid water movement as there is less resistance. Porosity is lowest in those that are fine-grained, such as granite.
Secondary permeability occurs in rocks that have joints and fissures along which water can flow, such as carboniferous limestone. Impermeable rocks such as granite, neither store water nor allow it to pass through it. The crystals in granite fit together more closely than rounded grams, limiting the amount of water held and inhibiting the motion of moisture. Granite therefore has a higher drainage density than carboniferous limestone. Granite has fewer joints and bedding planes, which means that it is more resistant to weathering and erosion. However carboniferous limestone is more jointed, less compact and softer, resulting in it being more vulnerable to weathering. Because it of these features it produces jagged karst scenery as it also has a low porosity.
Carboniferous limestone not only has thick bedding planes and joints it is pervious (capable of being passed through). But calcium carbonate is also soluble and carbonic acid in the rainwater together with humic acid from moorland plants dissolves the limestone and widens the existing cracks and weaknesses in the rock. Frost shattering is also able to produce scree at the foot of steep cliffs as there is minimum surface damage and the vegetation cover tends to be thin or even absent.
Granite was formed when magma was intruded into the earths crust. As it was formed at the depth and under pressure the rate of cooling was slow, enabling large crystals to be formed. As it continued to cool, it contracted and a series of cracks originated. These cracks have been further enlarged by pressure release.
Although it is hard rock, granite is subject to both physical and chemical weathering (the break down of rock surfaces and weathering that involves reactions on the rock). Frost shattering widens the joints, which hold water- (the colder temperatures of the north mean that the water can freeze into the cracks, and as it cools it expands forcing the cracks to become larger).
While granular disintegration is caused by the rates of expansion and cooling of minerals.
The spacing in the joints is crucial in tor formation, core stones have been left where they were closely packed and weathering was more active. The rounded nature of these cores stones especially in tropical regions is caused by spheroidal weathering, which is a form of exfoliation.