Chapter 5 Weathering, Soil, and Mass Movements

Section 5.1 Weathering Weathering – the breaking down of rock at or near the Earth’s surface. It is a basic part of the rock cycle usually involved in making sedimentary rock. Mechanical Weathering – occurs when physical forces break rock into smaller pieces without changing the rock’s mineral composition. Frost wedging is the most common type of mechanical weathering in mountainous regions.

Types of Mechanical Weathering Biological Activity – the activities of organisms, including plants, burrowing animals, and humans, can also cause mechanical weathering. Unloading – happens when pressure on previously buried rock is removed when it comes to the surface. Slabs of the outer rock begin to separate like the layers of an onion and break loose. This is called exfoliation.

Chemical Weathering Chemical Weathering – the transformation of rock into one or more new compounds. Water is the most important agent or cause of chemical weathering. Carbonation - as rain falls it combines with carbon dioxide to form carbonic acid. This weak acid will dissolve the sedimentary rock limestone. This chemical process forms the many caves, caverns, and sinkholes in our area. Oxidation – when iron in rocks reacts with oxygen and the rocks “rust”.

Factors that affect the Rate of Weathering Rock characteristics – the amount of cracks in a rock increases the surface area. The more cracks the more water can penetrate the rock, which increases both mechanical and chemical weathering. Climate – Mechanical weathering is the greatest in temperate climates, such as where we live. There is repeated freezing and thawing in a temperate climate which leads to frost (ice) wedging. Chemical weathering is greatest in wet, tropical climates.

Section 5.2 Soil Weathering produces a layer of rock and mineral fragments called regolith. Soil is part of the regolith that supports the growth of plants. Soil has four major components: mineral matter, or broken down rock; organic matter, or humus, which is the decayed remains of organisms; water; and air. Humus is found in topsoil. The most important factors in soil formation are parent material, time, climate, organisms, and slope.

The Soil Profile A horizon – known as topsoil, it contains the organic material called humus. It is teeming with insects, fungi, animals, and microorganisms. B horizon – contains leached minerals and fine clay. It is the lower limit for most plant roots and burrowing animals. C horizon – partially weathered parent rock. Bedrock – unweathered parent material.

Soil Types Pedalfer Soil – forms in temperate areas that receive more than 63 cm or rain each year, such as the Eastern United States. These soils have a brown to red-brown color. They contain large amounts of iron oxide and aluminum rich clays in their B horizon. Pedocal Soil – found in the drier Western United States. Contain much less clay than pedalfer soil, but are abundant in calcite and are typically light gray-brown. Laterites – form in hot, wet tropical areas. It is a distinctive orange or red color, due to the high presence of iron oxide. Very deep soil, but very little organic matter or humus.

Soil Erosion Water and Wind are two of the greatest agent of soil erosion. The rate of soil erosion depends on soil characteristics and on factors such as climate, slope, and type and amount of vegetation. Human activities that remove natural vegetation, such as farming, logging, and construction, have greatly accelerated erosion. Controlling erosion - replanting plants, planting rows of trees called windbreaks, terracing hillsides, contour plowing, and crop rotation are some of the ways we try to control erosion.

Section 5.3 Mass Movements Mass Movement – the transfer of rock and soil downslope due to gravity. Mass movements can be triggered by water, such as heavy rains, oversteepened slopes, removal of vegetation, and earthquakes. Geologists classify mass movements based on the kind of material that moves, how it moves, and the speed of the movement.

Types of Mass Movements Rockfalls – occurs when rocks or rock fragments fall freely through the air. Usually caused by frost wedging. Fastest mass movement. Rockslides – when segments of bedrock move suddenly along a flat inclined surface. They can be triggered by rain, melting snow, or glacial collapse and are very fast.

Types of Mass Movements Slumps – the downward movement of a block of a material along a curved surface. Usually not very fast. They leave a crescent shaped cliff right above the slump.

Types of Mass Movements Flows – mass movement of material containing a large amount of water, such as mudflows and earthflows. Creep – the slowest type of mass movement traveling downhill only a few millimeters of centimeters a year.