Weathering and Erosion. Weathering Weathering is the chemical alteration and physical breakdown of rocks and minerals at or near the Earth's surface.

Weathering and Erosion

Weathering

• Weathering is the chemical alteration and physical breakdown of rocks and minerals at or near the Earth's surface.

Erosion

• Erosion involves the transportation - movement of weathered particles.

The Rock Cycle

• The three major types of rocks, igneous, sedimentary, and metamorphic rocks are interrelated by a series of natural processes.

Igneous rocks

• Igneous rocks form from the cooling and crystallization of hot molten lava and magma. Igneous rocks undergo weathering and erosion to form sediments

Sedimentary Rocks

• Sediments are deposited and by compaction and cementation form sedimentary rocks.

Metamorphic Rocks

• Sedimentary rock become buried by additional sedimentary deposition, and when they are deep within the Earth, they are subjected to heat and pressure which causes them to become metamorphic rocks.

Magma

• With further burial and heating, the metamorphic rocks begin to melt. As melting proceeds with increasing temperatures and depths of burial, eventually the rock becomes molten and becomes magma.

Back to Igneous Rock

• Magma cools and crystallizes to form igneous rock, or which is erupted onto the Earth's surface as lava, and cools and crystallizes to form volcanic igneous rock.

Types of Weathering

• Physical or mechanical weathering

• Chemical weathering

Physical Weathering

• Reactions at the Earth's surface tend to make large particles smaller. A decrease in particle size increases the total surface area of the particles.

Consider a cube of rock that is 1" on a side. The total surface area is 6 square inches. If this cube is broken down into 8 cubes each

0.5 " on a side the total surface area increases to 24 square inches.

Continued reduction in size leads to an increasing total surface area. This is important because chemical reactions generally begin at the surface of an object. The greater the surface area the greater the reactivity of the particles.

The Wind -- Mechanical Weathering in Action!

So what causes physical weathering?

• A number of processes are capable of reducing particle size. A particularly effective process involves the repeated freezing and thawing of water in cracks in rocks.

• When water freezes there is a 9% increase in volume. The expansion exerts a force on the size of the crack which may cause the crack to widen.

Ice wedging - water expands when it freezes, which increase the size of the cracks and causes crumbling of

the rock.

Exfoliation or unloading -

o rock breaks off into leaves or sheets along joints which are to parallel the ground surface;

o caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial;

• Eventually large sheets of rock may "slide" off the rock creating a rounded upper surface. These exfoliation domes are quite common.

Enchanted Rock State Park - Texas

Thermal expansion -

o repeated daily heating and cooling of rock;

o heat causes expansion; cooling causes contraction.

o different minerals expand and contract at different rates causing stresses along mineral boundaries.

Sediment

• The loose material released by physical weathering is termed sediment.

• The size of sediment is described using the following terms:

gravel - > 2 mm in diameter

sand - 1/16 mm to 2 mm in diameter

silt - 1/256 mm to 1/16 mm in diameter

clay - < 1/256 mm in diameter

    Chemical weathering• The reactive components in the Earth's

atmosphere include:

o Oxygen

o Carbon Dioxide - CO2

o Water - H2O

• Nitrogen, the most abundant consistent in the atmosphere has little effect on the chemical weathering of rocks and minerals.

Dissolution (or solution) -

o Several common minerals dissolve in water

o halite

o calcite

o Limestone and marble contain calcite and are soluble in acidic water

oMarble tombstones and carvings are particularly susceptible to chemical weathering by dissolution.

Oxidation

• Oxygen combines with iron-bearing silicate minerals causing "rusting"

• Iron oxides are produced

• Iron oxides are red, orange, or brown in color

Hydrolysis

• Silicate minerals weather by hydrolysis to form CLAY.

• Feldspar alters to clay (kaolinite) plus dissolved materials (ions)

Clay

Biological weathering

Organisms can assist in breaking down rock into sediment.

What organisms?

• Roots of trees and other plants

• Lichens, fungi, and other micro-organisms

• Animals (including humans)

Roots of Trees

Lichens on a rock

Soils

• Loose rock material at the surface of the Earth is referred to as regolith. Regolith becomes soil when a surface layer that can support plant life has developed due to a combination of climatic and weathering factors.

Soil consists of:

Soil consists of:

• minerals and weathered rock fragments

• organic matter

• gases

• water

• living organisms

There are five factors influencing soil formation:

• Climate

• Parent rock

• Organisms

• Relief (slope, topography)

• Time

What is a soil profile?

• A soil profile consists of several soil horizons.

• They are numbered roughly alphabetically, beginning at the ground surface, going downward:

O horizon

o This is the layer of humus on the ground surface.

A horizon

o Top soil.

• Rich in organic matter. Typically has dark color.

• B horizon

o Subsoil.

o May contain soluble minerals such as calcite

• C horizon

o Weathered bedrock or saprolite (rotten rock).

• Bedrock lies below the soil profile.

Soil erosion

• Soil may be eroded due to the action of water (runoff) or wind.

• Soil is transported by streams, and most will eventually be deposited on a floodplain or at the mouth of the river in a delta. (Today, much eroded soil will be trapped because of dams across rivers, filling reservoirs. Loss of sediment transport to the coast also depletes beaches of sand, and can lead to accelerated beach erosion

• The actions of wind and water change the contour of the land.

After Katrina

Before Katrina

Wind Erosion

• The Dust Bowl of the 1930's was due to wind erosion of soil following extended drought and over-tilling of the soil.

How can we slow soil erosion?

• Contour plowing

• Terraces

• Planting groundcovers; roots hold the soil.

• Windbreaks

Contour plowing

Terraces

Planting Groundcovers

Windbreak

Properties of Water

• Water is a remarkable compound. Near the surface of the Earth water can exist in three phases - ice (solid), water (liquid), and vapor (steam).

Specific Heat

• Specific heat is a physical property of matter.

• The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C which is higher than any other common substance.

• This means that the temperature of water changes slowly.

• What does this high specific heat of water effect the weather on Earth?

Specific Heat

• Water has a high Specific Heat. Imagine a shoreline. The land and the water surfaces receive equal thermal energy from the sun but the land heats up quicker than the water; in general, rocks have lower heat capacities than water. Thus, water may remain quite cool even in the summer.

• What would happen if the Earth had no water?