WEATHERING Big Rocks Into Small Rocks

2 Types – Physical/Mechanical & Chemical   Physical/Mechanical Weathering – The physical breakdown of rocks into smaller pieces (sediments) by the actions of weather.   Chemical Weathering – The breakdown of rock through a change in mineral or chemical composition.

Frost Wedging  Occurs in climates with freeze/thaw cycles.  Water occupies about 10% more volume when it freezes.  When water that has collected in cracks in rocks freezes, it wedges the rock apart.

Ouch! Roads are at the mercy of frost wedging. 1)Water seeps under pavement. 2) It freezes and expands. 3) With warming the ice melts and a cavity is left. 4) An auto’s weight crushes the road into the cavity

Abrasion  Water, wind and ice are capable of moving rock material.

Water  Water tumbles sand, pebbles and even boulders along streambeds.

Wind  Wind blows sand and pebbles across beaches and deserts.  It can wear away boulders.

Ice  Glaciers, which are moving masses of ice, move rock over long distances.

As water, wind and ice move sand, pebbles and larger rocks they grind and scrape against each other. These rock materials are worn down. This is called abrasion.

Biological – Plants & Animals Plants  Trees often take root in cracks in rocks.  With growth these roots widen the crack.

Root Wedging  Even small plants such as lichens and mosses break down rocks by rooting on rocks.  This is called root wedging.

Animals  Burrowing insects and animals dig holes and bring rock fragments to the surface exposing rock materials to weathering.  How much wood could a woodchuck chuck if a woodchuck could chuck wood?

More Burrowing Animals

Exfoliation  The scaling off or peeling of successive layers from the surface of rocks.  Reduction of pressure and expansion of surface allows large curved sheets to peel away.

Chemical Weathering  Occurs when rock is broken down by  chemical action resulting in a change in composition.  Agents of chemical weathering: –Oxygen –Rainwater –Carbon dioxide –Acids produced by decaying plants and animals

Oxidation  Occurs when oxygen unites chemically with minerals.

Hydration  Occurs when water unites chemically with minerals such as feldspar and hornblende (amphibole) and they crumble into clay particles.

Carbonation  Occurs when carbon dioxide unites chemically with minerals.  Rainwater contains dissolved CO 2 which forms carbonic acid.  This dissolves limestone bedrock forming:

Sinkholes

Caverns

Factors Affecting the Rate of Weathering  Climate is the single most important factor affecting rock weathering.  Climate is the average condition of the atmosphere over a long period of time.

Surface Exposure  When a rock is broken into smaller pieces, more surface area is exposed to weathering and it weathers faster.

Mineral Composition  Greatest importance in chemical weathering.  Rocks composed of minerals that react with acids or oxygen will weather most rapidly.  Quartz is almost unchanged by chemical weathering. And because it is hard (7) and does not have cleavage it resists physical weathering.

Acid Rain  The burning of fossil fuels has accelerated chemical weathering.  Nitric and sulfuric acid is produced in the atmosphere.

Natural Producer of Acid Rain  Volcanoes also add gasses to the atmosphere that produce acid rain.