Chapter 4 – Weathering Sedimentary rocks are composed of sediment. Sediment forms at or near earth’s surface through the processes of weathering, transportation, deposition, and lithification. Importance of sedimentary rocks include: Energy resources Petroleum Uranium Coal

Sedimentary Resources Importance of sedimentary rocks include: Mineral resources Limestone: cement Rock salt: salt Rock gypsum: plaster Importance of sedimentary rocks include: Environment of deposition Oceans and seas Lakes and swamps River valleys and flood plains Deltas Sand dunes

Every Rock Tells a Story Because sedimentary rocks form at earth’s surface, they tell the geologist what was happening on the surface when that rock was deposited. The rocks tell the story of Earth. For example... Limestone generally only grows in sea water. If a geologist finds limestone in Kansas, the geologist concludes that Kansas was once underwater.

Clastic Sediment Clastic sediment forms from the mechanical, physical and chemical breakdown of rock. The process of breaking rock down into smaller pieces aids further breakdown by chemical weathering.

Chemical Sediment Chemical sediment forms by the chemical dissolution and alteration of rocks at or near earth’s surface. Chemical sediment is transported by stream waters to lakes and the oceans.

Weathering Chapter 15

Mechanical Weathering Frost wedging Alternate freezing and thawing Temperate mountainous climates 9% expansion as water freezes Up to 80,000 pounds per square inch pressure

Talus Slope Generally form a large pile of rock at the base of mountains called talus slope.

Unloading As the weight of overlying rock is weathered and eroded away the rock expands and produces fractures (or joints) parallel to the topography. Exfoliation.

Joints Vertical joints (cracks without displacement) form by expansion and contraction.

Joints from Top with Striations Columnar joints with glacial striations. Devils Postpile National Monument.

Thermal Expansion Joints Results from the daily cycle of temperature changes producing daily expans- ion and contraction. Most prominent in hot deserts.

Biological Activity Plants and burrowing animals cause much joint expansion.

Biological Activity The greatest mechanical weathering is now produced by catepillars.

Biological Activity The greatest mechanical weathering is now produced by catepillars.

Mechanical Weathering Enhances Chemical Weathering As mechanical weathering progresses, it enhances chemical weathering by increasing the surface area to volume ratio.

Comparison of Surface Area to Volume Ratio The volume of a cube 2 cm on a side is: 2 x 2 x 2 = 8 cm 3 The surface area of one side is 2 x 2 = 4 cm There are six sides, so the total surface area is 4 cm x 6 = 24 cm 2

Comparison of Surface Area to Volume Ratio If the 2 cm cube is bisected by 3 joints how does this affect the surface area? Each face of the 1 cm cube has a surface area of 1 cm x 1 cm = 1 cm 2. There are six sides to each cube so the surface area would be 6 x 1 cm 2 = 6 cm 2. But there are a total of 8 little cubes so the total surface area is now 6 cm 2 x 8 = 48 cm 2 Compare this to the previous surface area of 24 cm 2. The surface area has doubled!

Chemical Weathering The most important chemical causing chemical weathering is H 2 O-the universal solvent. However H 2 O in contact with CO 2 gas in the atmosphere reacts to form carbonic acid. H 2 O + CO 2 = H 2 CO 3 with a pH of about 5.7; a weak acid.

Dissolution Weathering When water precipitates (rains) on halite it will dissolve. NaCl + H 2 O = Na + + Cl - + H 2 O

Cave Formation (Karst Topography) When carbonic acid infiltrates into limestone or marble rock joints it begins to dissolve the calcite. CaCO 3 + H 2 CO 3 = Ca + + HCO CO 2 Solid rock dissolves into calcium ions, bicarbonate ions and carbon dioxide gas. All dissolved in water which carries these products away leaving a cave. Cave roofs collapse to form cenotes or sinkholes.

Cave When.

Oregon Cave When.

Hydrolysis of Granite 1 The most abundant rock in earth’s crust is granite. The most abundant mineral in granite is feldspar. Feldspar hydrolyses to kaolinite clay + bicarbonate + soluble silica.

Hydrolysis of Granite 2 Feldspar + Carbonic acid = Kaolinite clay + Potassium + Sodium + Calcium + Soluble Silica (SiO 2 ) Soluble silica is important because it becomes the silica cement that makes sand into a sandstone.

White Sandstone sandstone.

Hydrolysis of Granite 3 This weathering of feldspar in granite then releases the quartz grains to form sand that will become sandstone. The kaolinite clay becomes the clay mineral that makes shale. Shale is the most abundant sedimentary rock.

Oxidation of Biotite Biotite mica in granite reacts with carbonic acid to make kaolinite clay and releases iron that can become the hematite cement that binds sand grains together to form sandstone.

Red Sandstone sandstone.

Factors that influence Weathering Minerals present Bowen’s Reaction Series Available liquid water Water is the most important weathering agent Tropics Antarctica Temperature As temperature rises rate of chemical reactions increases.

Bowen’s Reaction Series Minerals present Bowen’s Reaction Series Available liquid water Water is the most important weathering agent Tropics Antarctica Temperature As temperature rises rate of chemical reactions increases.

Liquid Water Minerals present Bowen’s Reaction Series Available liquid water Water is the most important weathering agent Tropics Antarctica Temperature As temperature rises rate of chemical reactions increases.

Cold and Dry Weathering Minerals present Bowen’s Reaction Series Available liquid water Water is the most important weathering agent Tropics Antarctica Temperature As temperature rises rate of chemical reactions increases.