Weathering and Soil Geology 101 Ted Brewster

Weathering The act of breaking down existing rock Can be done by mechanical or chemical processes Both methods serve to create sediment from existing rock types

Mechanical Weathering Physical Disintegration of the rock The rock breaks into smaller pieces More surface area exposed to chemical weathering processes The rock changes physically but not chemically Examples Frost wedging, roots, pressure release, and abrasion

Change in Surface Area

Frost Wedging Water fills into small cracks within the rock When water freezes into ice it expands This small expansion slowly makes the crack larger, so more water can fill in Over time and successive melting and freezing cycles the cracks open up enough to split the rock into pieces

Frost Wedging

Frost Wedging

Roots – Biological Weathering The act of plant roots breaking rocks apart The roots of the plant find cracks and slowly grow downward into the crack Over time the effect of the root growth breaks the rock into pieces

Tree in Limestone

Tree Roots

Pressure Release Most rocks are formed at depth and then are moved to the surface The pressure at depth is high, as the rocks move to the surface they expand This is similar to compressing a sponge, then letting expand in water The expansion causes the mineral grains to break away from each other

Pressure Release - Sheeting

Spheroidal Weathering

Abrasion Removing small pieces of rock over time by wind or water Like using sandpaper on the rock Slowly the rock is just worn away Causes the rounding of angular pieces

Abrasion – Death Valley, CA

Differential Weathering Layers of rock are not of the same resistance to weathering Some layers are left intact while others are weathered away almost completely Most prominent in layers that have been tilted onto their sides

Differential Weathering - Dike

Sorting and Rounding As pieces of sediment move away from their parent rock they change Sorting – the ratio of different sizes of material within the rock More uniform size indicates farther from source Rounding – the shape of the edges of the sediment pieces More rounding indicates farther from source

Sorting

Rounding

Chemical Weathering Rock decomposition that actually dissolves the rock minerals and forms new ones Most minerals form at depth with little water Surface has abundant water, so the reaction can form new minerals Some minerals are soluble in water and dissolve Limestone, halite, gypsum, etc

Weathering of Granite

Chemical Weathering

Slate vs Limestone

Acid Weathering Many carbonate minerals are susceptible to acids If the water around them contains an acid, then the rock is more likely to dissolve quickly We see this in cities with acid rain Buildings, Statues, Tombstones, etc

Cleopatra’s Needle

Salt Weathering - Greece

Climate Climate plays a big role in determining the type of weathering most common Desert – mechanical abrasion Tropical (warm and wet) – chemical Arctic – mechanical frost wedging

Climate and Weathering

Soils Weathered, unconsolidated materials on top of bedrock Capable of supporting plant growth Other material is called regolith Soils form over long periods of time Exact time depends on the climate conditions Soils have layers called horizons

Soil Horizons O horizon A horizon Dark colored layer just below the vegetation Contains decomposed plant material called humus that forms organic acids A horizon Zone of leaching formed by percolation of rain water through the O horizon Wet climates have larger A horizons than deserts

Soil Horizons B horizon C horizon Accumulation of leaching products from horizon A Tends to be clay rich and stained red by hematite and limonite C horizon Incompletely weathered bedrock Transitional layer between bedrock and evolving soil above

Soil Profile by Climate

Soil Horizons - Hawaii

Soil Horizons in AZ

Sedimentary Structures Ancient structures that can determine the depositional environment of the rock

Graded Beds Graded Beds – sorted layers of sediment

Ripple Marks Ripple marks – shallow ripples that indicate a current direction

Cross Bedding Cross bedding – angled sediment layers that indicate either wind or water flow direction

Fossil Roots Fossil plant roots – filled in holes made by roots

Mud Cracks Mud cracks – filled in cracks from muddy and dry environments

Dinosaur Footprint Fossil footprints, tracks, or burrows – any impression in the sediment made by living creatures that was then filled in