WEATHERING. Turning Rock to Sediment and Solutions 10/22/2012

Transcription

1 WEATHERING Turning Rock to Sediment and Solutions Igneous rocks form at high temperatures; at the Earth s surface they are chemically unstable and will begin to disintegrate and decompose in a process known as weathering. The products of weathering are particles known as sediment (sand grains, gravel, etc.) and rock constituents dissolved in water. 1

2 Weathering of Rocks to Form Sediment Earth is a very dynamic environment: Volcanic & other mountain building processes elevate portions of the Earth's surface, while opposing forces move material from high elevation to lower elevation. Weathering The physical breakdown and chemical disintegration of rocks at the earth s surface Erosion -- The physical removal of material by mobile agents such as water, wind or ice. Weathering Two Types: Mechanical Weathering - physical forces that break rock into smaller and smaller pieces without changing the rock's mineral composition (same minerals). Chemical Weathering - involves a chemical transformation of rock into one or more new compounds. Weathering is the response of rocks to a changing environment: Plutonic rocks form under high pressures and temperatures. At the Earth's surface they are not as stable as the conditions under which they formed. In response they gradually change: weathering. Although we will look at them separately, mechanical and chemical weathering work simultaneously and aid one another. 2

3 Mechanical Weathering Mechanical weathering increases the surface area available for chemical weathering. Physical processes lead to mechanical weathering: 1. Frost Wedging, Crystal Wedging, Thermal Expansion 2. Unloading/Mechanical Exfoliation 3. Biologic Activity Erosional agents (water, ice) also result in "breaking rocks /abrasion. Jointed Rock, Sierra Nevada 3

4 Mechanical Weathering (cont.) 1. Frost Wedging - caused by repeated cycles of freezing and thawing. H 2 O has the unique property of expanding (~9%) when it freezes. H 2 O works its way into cracks in rock, and upon freezing, expands and enlarges these openings. After many freeze-thaw cycles, the rock is broken into angular fragments. Frost wedging is most prevalent in mountainous regions where there is a daily freeze-thaw cycle. It is sometimes frost wedging that causes potholes in roads during the winter. 4

5 Wave Erosion with Salt Wedging Salt water gets into cracks, evaporates, and the resulting growth of salt crystals expands the cracks and mechanically breaks the rock apart. Erosion sweeps the rock fragments away. Thermal Expansion seems to require water to effectively break apart rock. 5

6 Exfoliation and Domes 6

7 Mechanical Weathering (cont.) 3. Biological Activity Mechanical weathering can be accomplished by organisms such as trees and burrowing animals. Plant roots grow into fractures, and as they grow larger, they wedge the rock apart. 7

8 Mechanical and Chemical Weathering by Plants Chemical Weathering Chemical Weathering is a chemical process that breaks down minerals. 1. Dissolution 2. Oxidation 3. Hydrolysis Products of Chemical Weathering Caves Clay Minerals Metal Ores Aluminum Rounded Boulders -Spheroidal Weathering 8

9 Dissolution Pure water acts as a solvent, however the presence of even a small amount of acid in water dramatically increases the corrosive force of water. Carbonic acid is produced by atmospheric CO 2 dissolving in rainwater and surface waters. Other sources of acid: organic acid from the decay of organic matter sulfuric acid from the weathering of sulfide minerals like pyrite. The mineral calcite is particularly susceptible to dissolution by acid. CaCO 3 + 2H + Ca 2+ + CO 2 + H 2 O In this process, calcite is dissolved. This is important for the formation of caves in limestone. 9

10 H 2 CO 3 = Carbonic Acid = H 2 O + CO 2 Water + Carbon Dioxide (Gas) Blanchard Springs Cave, Arkansas 10

11 Hydrolysis Hydrolysis is the major chemical process that produces clay minerals. Feldspar disintegrates through hydrolysis, the addition of water to its Lattice structure. 2KAlSi 3 O 8 + 2H 2 CO 3 + H 2 O AlSi 2 O 5 + 4SiO 2 +2K + + 2HCO 3 Feldspar acid water Clay silica K ion carbonate When feldspars add water to their lattice structure, the expand. This expansion breaks apart the rock and releases quartz as sand or silt. 11

12 Hydrolysis-The most abundant product of the chemical weathering of rocks is the formation of clay minerals - clays are stable at the Earth s surface and are a dominant component of soils. Clay minerals are major portion of many sedimentary rocks such as shale. Boulders? Anza Borrego Desert, 12

13 Granite Boulders and Outcrop Weathering of Granite to Clay 13

14 Spheroidal Weathering Oxidation 14

15 Chemical Weathering (cont.) 2. Oxidation oxidation is a chemical process that is responsible for rust forming from iron. 4Fe + 3O 2 2Fe 2 O 3 The oxidation of iron is greatly accelerated in water. Igneous ferromagnesian minerals such as olivine, pyroxene and amphibole oxidize in the presence of water at surface conditions. Another important oxidation reaction occurs when sulfide minerals such as pyrite (FeS 2 ) oxidize to form iron oxide minerals. Grand Canyon of Kawai - Oxidation 15

16 Types Types Description Description of of process process and and results results Dissolution Water molecules gang up on ions on outside Dissolution Water molecules gang up on ions on outside of Dissolution Water of mineral molecules lattice gang (surface) up on ions and on break outside the of mineral lattice (surface) and break the mineral mineral lattice bonds, (surface) releasing and break the ions the mineral into bonds, releasing the ions into solution. Water bonds, solution. releasing Water the carries ions into ions solution. away. Water carries ions away. carries ions away. Hydrolysis Water molecules enter mineral formula, Hydrolysis Water molecules enter mineral formula, replacing replacing other components and changing other components and changing mineral to a new Hydrolysis mineral to a new one: a clay mineral. one: a clay mineral. Example: Example: 2KAlSi 3 O 8 + 2H + + 9H 2 O = 2KAlSi 2 2 O 3 O 5 (OH) H 4 4H + 4 SiO + 9H O 2K = + Al 2 Si 2 OK-feldspar 5 (OH) + 4 4H + water 4 SiO 4 = + 2K + Kaolinite clay K-feldspar + silicic acid + water and potassium = ions Kaolinite clay + silicic acid and potassium Oxidation ions Oxidation Oxidation Oxygen bonds with Fe ions on outside of mineral lattice (surface) removing Fe from mineral and producing Hematite (rust). Weathering Activity Na, Ca, K, Mg, other cat ions, AND excess silica (silica not in quartz or not associated with Al and O) Dissolves and is removed by water. Al, Si, O Hydrolyzes: combines with water to form clays (only if all three available in mineral). Fe Oxides: combines with O to form insoluble iron oxides, giving red to yellow soil cover. Quartz Stays in place; doesn t break down chemically. 16

17 What s Left? Unlike other silicate minerals, quartz is particularly resistant to chemical weathering - that is why it tends to accumulate on beaches and sand dunes since everything else has weathered away. 17

18 Group Activity: List all the characteristics of a rock outcrop in the mountains that would make it weather fastest include information about the type of rock as well as the environment and local activity. Differential Weathering same cemetery Granite Marble (1870) 18

19 Differential Weathering Soft rock weathers rapidly and easily Harder rock protrudes and forms ridges Differential Weathering Harder sandstone forms ridges, softer shale forms valleys 19