- Weathering is breakdown of rock to form sediment.
The three major types of rocks, igneous, sedimentary, and metamorphic rocks are interrelated by a series of natural processes. Igneous rocks form from the cooling and crystallization of hot molten lava and magma. Igneous rocks undergo weathering and erosion to form sediments. Sediments are deposited and lithified by compaction and cementation to form sedimentary rocks. Sedimentary rock become buried by additional sedimentary deposition, and when they are deep within the Earth, they are subjected to heat and pressure which causes them to become metamorphic rocks. With further burial and heating, the metamorphic rocks begin to melt. Partially molten metamorphic rocks are known as migmatite. As melting proceeds with increasing temperatures and depths of burial, eventually the rock becomes molten and becomes magma, which cools and crystallizes to form plutonic igneous rock, or which is erupted onto the Earth’s surface as lava, and cools and crystallizes to form volcanic igneous rock.
Further complications within the rock cycle include
- weathering of sedimentary and metamorphic rocks (in addition to igneous rocks), and
- metamorphism of igneous rocks and repeated metamorphism of metamorphic rocks.
Types of Weathering
A. Physical or mechanical weathering
- rock breaks off into leaves or sheets along joints which parallel the ground surface;
- caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial;
example: Stone Mountain, Georgia.
- repeated daily heating and cooling of rock;
- heat causes expansion; cooling causes contraction.
- different minerals expand and contract at different rates causing stresses along mineral boundaries.
- Frost wedging – water expands when it freezes
- Exfoliation or unloading –
- Thermal expansion –
B. Chemical weathering
Rock reacts with water, gases and solutions (may be acidic); will add or remove elements from minerals.
- stalactites – hang from ceiling
- stalagmites – on the ground
- speleothems are cave formations
- speleothems are made of calcite
- form a rock called travertine
- disappearing streams,
- Several common minerals dissolve in water
- Limestone and marble contain calcite and are soluble in acidic water
- Karst topography forms on limestone terrain and is characterized by:
- Dissolution (or solution) –
- Oxygen combines with iron-bearing silicate minerals causing “rusting”
- Iron oxides are produced
- Iron oxides are red, orange, or brown in color
- Mafic rocks such as basalt (which may contain olivine, pyroxene, or amphibole) weather by oxidation to an orange color
- “Georgia Red Clay” derives its color from the oxidation of iron bearing minerals
- Silicate minerals weather by hydrolysis to form CLAY.
- Feldspars are stable at high temperatures and pressures (but not at the temperatures and pressures of the Earth’s surface)
- Clays are stable under conditions at the Earth’s surface
- Feldspars and clays are similar in composition.
- Feldspar readily alters to clay when in contact with acid and water.
- Feldspar alters to clay (kaolinite) plus dissolved materials (ions)
- Iron-bearing silicate minerals weather to form clays by hydrolysis (in addition to iron oxides)
Spheroidal weathering in jointed basalt, Culpeper Basin, Virginia
Spheroidal weathering is caused by chemical weathering of jointed rocks. The jointed rocks weather to form roughly spherical shapes.
C. Biological weathering
Organisms can assist in breaking down rock into sediment or soil.
- Roots of trees and other plants
- Lichens, fungi, and other micro-organisms
- Animals (including humans)
The Goldich Stability Series
This series describes the order in which silicate minerals weather.
Minerals which form at high temperatures and pressures are least stable, and weather most quickly because they are farther from their “zone of stability”, or the conditions under which they formed.
Minerals which form at lower temperatures and pressures are most stable.
The order of mineral stability in the weathering environment is the same order as Bowen’s Reaction Series.
Least stable (high temperature minerals) Olivine Ca plagioclase feldspar Pyroxene Amphibole Biotite Na plagioclase feldspar Potassium feldspar Muscovite Quartz Most stable (low temperature minerals)
What happens when granite is weathered?
- Na Plagioclase feldspar
- K feldspar
- Lesser amounts of biotite, amphibole, or muscovite
- First, unweathered granite contains these minerals:
- The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions
- The Na and K ions will be removed through leaching
- The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides.
- The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering.
- Weathered rock is called saprolite.
- Weathered rock fragments are one of the constituents of soil. (See section on SOIL).
- Quartz grains may be eroded, becoming sediment. The quartz in granite is sand- sized; it becomes quartz sand. The quartz sand will ultimately be transported to the sea (bed load), where it accumulates to form beaches.
Clays will ultimately be eroded and washed out to sea. Clay is fine-grained and remains suspended in the water column (suspended load); it may be deposited in quiet water.
Dissolved ions will be transported by rivers to the sea (dissolved load), and will become part of the salts in the sea.