Weathering

3 laws of rock cycle history

3 laws of rock cycle history

• superposition

• original horizontality

• lateral continuity

superposition meaning

oldest layer of rock on the bottom

original horizontality meaning

processes move rocks from horizontal to angled

lateral continuity

layers of rock will continue on unless interrupted

Peridotite

• coarse grained ultra mafic igneous rock

• most dominant type of upper mantle

where do the first sedimentary rocks come from?

the weathering of an igneous protolith

physical weathering

mechanical and breaks rock into smaller pieces with no chemical change

Ice wedging

physical weathering, water entering a fracture then cools to form ice which expands ~10% and enlarges the fracture

insolation

physical weathering, caused by day and night cycles with strong temperature changes

• desert conditions, cold and dryp

unroofing

physical weathering, a buried body brought up due to:

• thermal contraction (cooling)

  • stress release (less and less confining pressure)

salt weathering

physical weathering, salty water forms “honeycomb” structures in rock

two effects of physical weathering

• exfoliation = homogeneous rocks fracture and are detached from the outcrop due to pressure release

  • spheroidal weathering = production of spheroidal boulders of unweathered rock, around which are to be observed layers or flakes of disintegrated material.

chemical weathering

rock composition is changed due to minerals being out of equilibrium with environment and in contact with water

dissolution

chemical weathering, NaCl→Na+Cl

most common

alteration

chemical weathering, mineral transforms into a new mineral and solutes with some remaining og mineral

• dehydration: gypsum→ anhydrite+water

• hydration: hematite+water→limonite

• redox rxns: oxidation loses electrons, reduc gains electrons

incongruent weathering

part of reactant is consumed

congruent weathering

all of reactant solid is entirely consumed

pH

hydrogen ion concentration, expressed as -log10[H+]

Eh

expresses potential for oxidation or redox to occur,

shown graphically with pH

• rain, rivers = 0.4

• seawater = 0.4

• soil = 0-0.2

Soil Profiles

• O-horizon/A-horizon = plant biosphere (living and dead)

• E = leached

• B = accumulation of altered minerals

• C = partially altered parent material

  • unweathered material (bedrock)

Laterization Soil

Laterites

• warm humid climates

• Fe and Al accumulations

• intense weathering

• little organic matter

Podzolization Soil

Podzols

• cool and moist climate

• well drained, leached, sandy soil

• lots of organic matter

calcification soil

Calcozols

• hot and dry environment

• evaporation > precipitation

• accumulation of excess CaCO3

• deserts

Chemical Index of Alteration

tells amount of chemical weathering that has occurred

• higher CIA = more chemical weathering due to the denominator weathering out while Al2O3 stays

• higher temperature = higher reaction rate

• higher rainfall = higher reaction rate

weathering processes

• thermal energy

• chemical energy

  • biological energy

transport processes

• wind

• flowing ice

• flowing water

• gravitational potential energy