Weathering

1. Weathering Objectives Describe the Rock Cycle. Briefly contrast weathering and erosion. Contrast physical, chemical and biological weathering. List and describe the types of mechanical weathering. List and describe the types of chemical weathering.

2. The Rock Cycle • The three major types of rocks: igneous, sedimentary and metamorphic. • Rocks are interrelated by a series of natural processes. • Igneous rocks (e.g. granite, basalt) form from the cooling and crystallization of hot molten lava and magma. Igneous rocks undergo weathering and erosion to form sediments. When Sediments are deposited they form Sedimentary rocks (e.g. sandstone, coal and chalk).

3. Sedimentary rock becomes buried by additional sedimentary deposition, and when they are deep within the Earth, they are subjected to intense heat and pressure which causes them to become Metamorphic rocks (e.g. marble and slate).

4. 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 (magma), which can be erupted onto the Earth's surface as lava, and cools and crystallizes to form volcanic igneous rock.

5. Complications within the rock cycle include: • Weathering of sedimentary and metamorphic rocks (in addition to igneous rocks) • Metamorphism of igneous rocks and repeated metamorphism of metamorphic rocks.

6. Definition of weathering • Weathering is the disintegration (break down) and decomposition (decay) of rocks in situ (in their place of origin) to form sediment. • Weathering (unlike erosion) need not involve the movement (transport) of material.

7. A) Physical or Mechanical weathering • Freeze / Thaw – water expands when it freezes Talus slope, Lost River, West Virginia Shale chips, West Virginia

8. Exfoliation or unloading - • 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. • Thermal expansion - • 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.

10. B) Chemical weathering Rock reacts with water, gases and solutions (maybe acidic). Thus chemical processes can add or remove elements from minerals. • Dissolution (or Solution, Carbonation) - • dissolving of calcium carbonate (limestone) in acidic rain or ground water. • Several common minerals dissolve in water (halite, calcite) • Limestone and marble contain calcite and are soluble in acidic water • Marble tombstones and carvings are particularly susceptible to chemical weathering by dissolution.

11. Photo taken in an above-ground cemetery in New Orleans

12. Caves and caverns typically form in limestone (karst environments) • speleothems are cave formations • speleothems are made of calcite • Stalactites: hang from ceiling • Stalagmites: on the ground

13. Karst topography forms on limestone terrain and is characterized by: • caves/caverns, sinkholes (dolines), disappearing streams, springs.

14. Sink-hole (doline) formation

16. Oxidation • Oxygen combines with iron-bearing silicate minerals causing "rusting"(olivine, pyroxene, amphibole,biotite). • Iron oxides are produced(limonite, hematite, goethite) • 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. Weathering Rind, Wilhite Formation, eastern Tennessee

17. Hydrolysis • Silicate minerals weather by hydrolysis to form CLAY. • Feldspar alters to clay (kaolinite) plus dissolved materials (ions) • ‘Feldspars’ are stable at high temperatures and pressures (but not at the temperatures and pressures of the Earth's surface) • Claysare 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.

18. Iron-bearing silicate minerals weather to form clays by hydrolysis (in addition to iron oxides) • Spheroidal weathering is caused by chemical weathering of jointed rocks. The jointed rocks weather to form roughly spherical shapes. Spheroidal weathering in jointed basalt, Culpeper Basin, Virginia

19. 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)

20. Roots of trees and plants Lichens