#endogenetic

Endogenetic (geological, factors are related to the rock characteristics)

rock strength/hardness

generally, weaker rocks get weathered more easily than stronger rocks --> granite vs sandstone

however, this does not hold true all the time as even the hardest/strongest rocks can be betrayed by the presence of joints and lines of weaknesses which make them as/more vulnerable to weathering agents --> granite: vertical and horizontal joints, pseudo bedding planes and sheet joints 

mineral and chemical composition

determines the degree of susceptibility to weathering

determines the specific type of chemical weathering that are/is likely to operate on the rock

e.g. calcium carbonate in limestone and chalk --> carbonation-solution

feldspar in granite --> hydrolysis

iron compounds --> oxidation 

colour

affects insolation weathering in terms of the differential rates of expansion and contraction 

usually more stable minerals are light-coloured while less stable ones are dark-coloured --> darker-coloured rocks are more prone to chemical attacks than lighter-coloured ones in general 

heterogenous vs homogenous rocks

rock texture 

coarse-grained vs fine-grained 

coarse grained --> individual minerals can undergo selective chemical attack

fine grained --> crystals are more tightly bonded thus increasing their strength and coherence BUT crystal boundaries provide potential lines of weaknesses that may be exploited by weathering 

usually fine-grained rocks weather more quickly than coarse-grained ones 

rock structure

refers to the presence of joints and cavities

Provide agents of chemical and physical weathering access into rock masses

Exposes the subsurface portions of rock to agents of weathering --> allowing weathering to take place not just on the surface/near the surface, but deep underground as well as in the Deep Weathering Exhumation Theory 

may be inherent or may develop as time passes and rock is subjected to different types of stresses associated with different tectonic processes

massive vs densely jointed rocks 

Types of lines of weaknesses

Bedding planes --> horizontal joints or junctions separating different layers of sedimentary rock (limestone, sandstone, chalk etc.); provide routes for the movement of acidulated water --> formation of limestone caves and caverns as the water table migrates over time 

Lines of cleavage --> common in metamorphic rocks

Master joints --> very extensive and usually larger than normal joints 

formed as intrusive igneous rocks cool and contract 

their intersection with bedding planes and other joints give weathered/broken up rocks different shapes and patterns e.g. granite and gneiss usually have cuboidal jointing pattern --> block disintegration 

DENSITY of joints --> has the ability to affect the form and shape of landforms

dense jointing pattern --> agents of weathering have greater access to the rock --> more intense weathering

Exogenetic (factors other than rock characteristics; pertaining to the external environment)

Climate

Water (precipitation)

Needed for virtually every weathering process

Acids

Solution

Chemical weathering is most effective with the presence of abundant groundwater that is constantly renewed (to prevent neutralisation)

In climates where water is scarce (savanna, semi and hot deserts) physical weathering is more dominant 

Temperature 

influences the rate of weathering --> Van Hoff’s rule 

weathering in tropical areas usually 4 times faster 

temperature range is also important, particularly for physical weathering

large/wide temperature range --> insolation weathering/thermal expansion in desert regions

temperature that hover around freezing point of water --> frost action in periglacial regions

Vegetation (affected by and closely related to climate)

decaying vegetal matter produces organic acids for biochemical weathering 

tropical rainforest vs coniferous forests

tropical rainforest supply 100-200 tonnes of litter per hectare per year

coniferous forests supply 20-25 tonnes of litter per hectare per year 

Due to the stark difference in the amount of litter produced between tropical and coniferous forests, rates of weathering can differ with those in tropical rainforests being 40 times faster then those in coniferous forests

Relief/topography 

High relief/altitude --> lower temperatures--> frost action

steep slopes help transport weathered material away (mass movement), continually exposing new rock to weathering agents 

Steep slopes promote drainage (overland flow, throughflow), inhibits infiltration into the ground and therefore hampers/limits deep chemical weathering 

Gentle slopes hampers the removal of regolith and moisture but promotes/enhances deep chemical weathering as water is more likely to infiltrate and not removed so quickly

ASPECT (direction at which the slope is facing)

affects insolation receipts and hence temperature and evaporation rates --> responsible for the local variations in weathering rates 

Northern hemisphere: south-facing slopes receives more sunlight and hence have higher temperatures --> more melting and freeze-thaw cycles --> more physical weathering 

In cold but not frosty areas in the northern hemisphere: south-facing slopes receive more sunshine and have more vegetation --> biological and chemical weathering 

Time

rocks develop more lines of weaknesses over time --> increases their susceptibility to weathering 

climate change --> vegetation types may also change over time

however, the weathered products do not change with time --> at present times, we can still see evidence of previous climates through the remaining weathered products 

time also affects relief via tectonic uplift or erosion

Human activity

air pollution --> more acidic rainwater