Weathering

 Earth’s surface is always changing. There are internal processes like mountain building and volcanic activity.  There are also external processes that continually break down rock on the surface of Earth.  Weathering is the breaking down of rocks at or near Earth’s surface. Natural arches

Mechanical Weathering  Mechanical weathering occurs when physical forces break rocks into smaller pieces without changing the rock’s mineral composition.  Each piece has the same characteristics as the original rock, it is just broken into smaller pieces so the surface area is increased.

Mechanical Weathering  In nature, three physical processes are especially important causes of mechanical weathering: Frost wedging Unloading Biological activity

Frost Wedging  When water freezes, it expands. It enlarges the cracks in the rock.  After many cycles of this freezing and thawing, the rock will eventually split into pieces.  This process is called frost wedging. Frost wedging is most common in mountainous regions.  Rocks that break loose and tumble into large piles are called talus.

Unloading  Large masses of rock may be exposed over time.  Reduced pressure on the rock, called unloading, causes the outer rock layers to expand more than the rock layers beneath.  This causes the top rock layers to peel off like the layers of an onion in a process called exfoliation.

Unloading  Exfoliation is especially common in rock masses made of granite.  It often produces large, dome- shaped rock formations. Half Dome in Yosemite National Park, California

Biological Activity  The activities of organisms, including plants, burrowing animals, and humans, can cause mechanical weathering. Plant roots can grow into cracks in rocks and apply pressure over time, splitting rocks apart. Burrowing animals can move rocks to the surface where weathering occurs more rapidly. Decaying organisms can produce acids that cause chemical weathering.

Chemical Weathering  Chemical weathering is the transformation of rock into one or more new compounds.  In chemical weathering, the rock undergoes a transformation in its composition as a result of the process.

Water WWater is the most important agent of chemical weathering. WWater absorbs gases from the atmosphere and the ground. These dissolved substances then chemically react with various minerals.

 Oxygen dissolved in water reacts easily with certain minerals, forming oxides.  Iron oxide is the rust that forms when objects with iron are exposed to water.

Water  Water in the atmosphere absorbs carbon dioxide, sulfur and nitrogen.  Sulfur is released by the burning of coal. Water and pollutants cause acid rain.  Acid rain speeds up the chemical weathering of stone monuments and other structures.

Chemical Weathering of Granite  Chemical weathering affects rocks in different ways, depending on their composition.  Granite contains feldspar and quartz. During chemical weathering, the acid rain breaks down the feldspar into clay and the quartz grains into sand.

Spheroidal Weathering  Weathering can change the shape of rock as well as its size and chemical composition.  When water enters along the joints in a rock, it weathers the corners and edges most rapidly. The corners and edges break off and the rock becomes more rounded.  The rock takes on a spherical shape and this process is called spheroidal weathering.

Rate of Weathering  Mechanical weathering affects the rate of chemical weathering.  By breaking rock into smaller pieces, mechanical weathering accelerates chemical weathering. It increases the surface area of rock exposed to the chemical components.

Rate of Weathering  Two other factors affect the rate of weathering: Rock characteristics Climate

Rate of Weathering  Rock Characteristics:  Physical characteristics of rock, such as cracks, are important because water can get into the rock.  However, the rock’s mineral composition makes the most difference.

Rate of Weathering : Characteristics  Gravestones made of granite are relatively resistant to weathering.  In contrast, marble gravestones undergo much more rapid chemical weathering

Rate of Weathering: Climate  Temperature and moisture have a strong effect on the rate of weathering.  These factors control the freeze- thaw cycle and the amount of frost wedging.  Temperature and moisture affect the growth of plants.  Chemical processes happen faster at higher temperatures.

Differential Weathering  Different parts of rock weather at different rates. This process is called differential weathering.  Caused by differences in mineral composition.  More resistant rocks protrude as high peaks since they break down more slowly.