Weathering & Soils Mr. Manzo

Erosion Features of Bryce Canyon Nat. Park in Utah

Definition of Weathering: Weathering = the breakdown of rock due to physical and chemical changes, to produce smaller pieces. (called fragments or sediments)

Physical Weathering: Physical (mechanical) weathering = changes to the size and/or shape of rock (usually into smaller particles of the same composition)

Types of physical weathering: Ice wedging – water seeps into cracks, freezes, expands and wedges the rock apart! Water and ice are more powerful than rock over time!

Ice Wedging (Frost Action) Potholes in the road are examples of the power of ice to wedge rock (or pavement) apart…

Ice Wedging at Whistler Mountain, B.C., Canada

Types of physical weathering: Root action – tree roots grow into small fissures or wedges and expand rock

Root Action…tree roots are stronger than rock Root Action…tree roots are stronger than rock!!! They physically break rock apart as they grow!

Some trees need virtually no soil to grow…and can live on bare rock!

Types of physical weathering: Animal activity – digging of burrowing animals can break down rock Abrasion – collision of rocks with one another (especially in streams) breaks them down NOTE: abrasion rounds off particles!! Exfoliation – the peeling of rock due to release of pressure experienced when rock was formed

Exfoliation (release of pressure) allows rock to expand when exposed to Earth’s surface!

Chemical weathering Chemical Weathering = changes in the chemical composition of rocks due to chemical processes and reactions

Types of chemical weathering: Hydrolysis (hydration) – minerals react with and absorb water Example: feldspar reacts with water to form kaolin (clay)

Types of chemical weathering: Carbonation – carbon dioxide from air reacts with water to create carbonic acid which breaks down rocks (especially limestone, marble, etc.) NOTE: chemical weathering typically results in a rounded appearance of the landscape…just like here in the Catskills!

Types of chemical weathering: Oxidation – rocks with metallic elements combine with oxygen (in air and water) and rust away (rust stains the rocks REDDISH BROWN)

Oxidation (water reacts with Iron)

Types of chemical weathering: Acid Rain: rain is naturally slightly acidic, but industry has made it more so. Acids break down rocks, especially carbonate rocks such as limestone. Carbonate rocks = any rock containing calcium carbonate, which is the mineral calcite (CaCO3) Carbonate rocks react quickly with acids, and so they will chemically weather away at a much faster rate than other rocks IF plenty of acidic water is available!

Effect of Acid Rain on Carbonate Rocks: Cleopatra’s Needle while in Egypt Cleopatra’s Needle now in NY City

Effect of Acid Rain on Carbonate Rocks

Types of chemical weathering: Plant acids – some plants naturally produce acids (humic acids) that seep into rock and cause chemical weathering For example, pine needles can be very acidic and weaken the rocks beneath them

Rate of Weathering Rate of Weathering – how fast rocks break down due to physical or chemical processes

Factors affecting rates of weathering:

Factors affecting rates of weathering: Rock composition: Sedimentary rocks are generally soft and weather fast. Limestone is easily dissolved by acids. Igneous and metamorphic rocks are usually dense, hard, and more resistant to weathering. Any rock containing quartz (hardness of 7) will generally be resistant to weathering (conglomerate, quartzite, granite, etc.)

Factors affecting rates of weathering… Exposure to surface: rocks exposed at surface weather much more quickly than buried rock. Rock Outcrops are places where rock layers actually stick out above ground…where they are quickly exposed to lichens, moss, trees, rainfall, ice, and animals!

Factors affecting rates of weathering: Climate: the amount of precipitation and temperature are the most important factors.

Factors affecting rates of weathering: Surface Area – as rocks begin to break down into smaller pieces, they weather faster as the total surface area of the rock is increased

Surface Area & Rate of Weathering:

Factors affecting rates of weathering: Topography – higher elevations tend to be colder and experience more freeze-thaw cycles

Soil Soil is the result of weathering of rock, which produces smaller pieces called fragments, which combine with organic matter, air, and water to comprise soil. Soil = rock fragments, humus (decayed plant and animal material), air and water

Composition of Soil….typical analysis

Transported & Residual Soils Residual Soil = soil that has been left in place after bedrock weathers into smaller pieces…so it has the same composition as the underlying bedrock. Transported soils = soil that has been brought to a location by wind, glaciers, or rivers, so that the soil composition is different from the underlying bedrock. Most of NYS soils are transported here by glaciers moving south from Canada…so the soils are all mixed sizes of sediment (pebbles, sand, silt, clay) and can be called glacial till.

Glacial Till = mixed soil transported by glaciers during the last ice age (12,000 yrs. Ago)

Soil develops slowly over time from bedrock.

Soil Horizons Soil separates over time into distinct layers as follows: A horizon (Topsoil): the upper layer of dark brown soil containing humus and plenty of animal activity (bugs, worms, etc.) B horizon: also called “subsoil”, containing many more minerals leached down by water infiltrating through the A horizon, so often stained red by oxidation of iron minerals C horizon: made of weathered bedrock (parent material) Usually contains larger particles and rock fragments Bedrock: the underlying rock that is still mostly un-weathered

Soil Horizons = layering of soils over time

Soil Texture Soil texture = the relative amounts of SAND, SILT, and CLAY contained in a given soil. Usually determined by percentages of each sediment size.

Place the following points on the texture triangle. What type of soil texture will the following soil compositions produce? Place the following points on the texture triangle. C) 15% silt, 30% sand, 55% clay D) 70% silt, 30% sand, 0% clay 2) 80% silt, 10% sand, 10% clay = _________________ A) 30% silt, 10% sand, 45% clay B) 0% silt, 20% sand, 80% clay 7) 60% silt, 20% sand, 20% clay = ______________________ 6) 10% silt, 60% sand, 30% clay = _______________________ 1) 20% silt, 20% sand, 60% clay = ______________________ 3) 5% silt, 85% sand, 10% clay = __________________________ 4) 40% silt, 20% sand, ______% clay = ____________________ 5) 45% silt, 10% sand, 45% clay = ________________________