Weathering

Sedimentary Cover + Sedimentary rocks form a veneer over much older igneous and metamorphic “basement” rocks. + This veneer varies in thickness from 0 to 20 km

Types of Weathering Weathering + Physical – Mechanical breakage, disintegration. + Chemical *- Decomposition by reaction with water. Produces dissolved matter and an insoluble residue. + Biological – Action of organisms often combines physical and chemical weathering processes. + Coffee metaphor for weathering: ! Physical – Grinding coffee beans. ! Chemical – Leaching coffee with hot water 8 Generates dissolved material (coffee), and 8 Depleted residue (coffee grounds).

Rock Disintegration + Weathering – Processes that break-up solid rock. ! Weathered Granite ! Fresh Granite

Rock disintegrates to form sediments which can form sedimentary rocks.

Physical Weathering aka mechanical weathering -Mechanical breakup with no change in mineral composition. -Often occurs with chemical weathering, except at climate extremes. -Physical weathering dominates in polar and desert regions. -Everywhere else, chemical weathering dominates. + Types of Physical Weathering -Jointing (release of overburden pressure) -Frost Wedging -Root Wedging -Thermal Weathering -Alternate wet / dry

Jointing Rocks expand when rock erodes off of the top of a formation from reduction in pressure. + Rocks develop cracks called joints. + Igneous rocks crack in parallel layers,

Frost Wedging + Water enters a rock fracture, freezes in fractures. + Ice expansion wedges the fracture open. + During thaw, water infiltrates deeper into crack. + Repetition results in rock disintegration. + Dominant process in mountainous regions.

Frost wedging -Rocks wedged loose by freeze-thaw accumulate as talus at the base of steep slopes. - Talus (broken rock) accumulate at the bottom of the slope

Chemical weathering -Breaks minerals into chemical components. -Requires water “the universal solvent”. -Significant in humid (water-rich) landscapes. -Chemical weathering virtually absent in deserts. - 3 major processes 1. Dissolution 2. Oxidation 3. Hydrolysis

Types of Chemical Weathering Dissolution- -Some minerals (halite, gypsum, calcite) dissolve. -Acidity (i.e. acid rain) enhances this effect. Oxidation- - A reaction whereby a metal loses electrons. -Important in decomposing minerals. -Rusting is a familiar example of oxidation Hydrolysis- -Breaking (lysis) by water (hydro). -Water breaks apart elements that hold silicates together. Alteration minerals (residues) such as -Clay minerals -Iron oxides (rust)

Biological weathering Mosses and plants release acids during decomposition that cause rock to deteriorate

Effects of weathering Unstable minerals decompose. Rock volume is reduced. Corners and edges attacked rapidly = spheroidal weathering.

Chemical Weathering in Granite + Biotite and feldspar weather to clay minerals. + These minerals are removed by running water. + Mineral removal causes the rock to crumble. + Quartz, resistant to weathering, is left over.

Weathering Rates -Temperature and wetness extremes Hot and Wet – Chemical weathering at a maximum Cold and Wet – Physical weathering at a maximum Hot and Dry – Physical weathering dominates Cold and Dry – All weathering at a minimum -Biological effects – Active biota accelerates weathering. -Geologic past – Rates of weathering have varied with atmospheric and biotic changes.

Differential weathering - Rock masses do not weather uniformly. -This results in many unusual and spectacular rock formations, landforms and landscapes.