Chapter'6: Weathering and Soils
Chapter 6: Weathering and Soils
Introduction: Weathering The breakdown of rock At the earth's surface, rocks are exposed to the effects of weathering: the chemical alteration and mechanical breakdown of rock, when exposed to air, moisture and organic matter Weathering is an integral part of the rock cycle Weathering converts rock to regolith
Introduction: Weathering— The Breakdown of Rock ◼ At the Earth’s surface, rocks are exposed to the effects of weathering: the chemical alteration and mechanical breakdown of rock, when exposed to air, moisture, and organic matter. ◼ Weathering is an integral part of the rock cycle. ◼ Weathering converts rock to regolith
Weathering profile
Weathering profile
Physical Weathering Rocks break at weak spots when they are twisted, squeezed, or stretched by tectonic forces Such forces form ioints Rocks adjust to removal of overlying rock by expanding upward Removal of the weight of overlying rocks releases stress on the buried rock and causes joints to open slightly, thereby allowing water, air, and microscopic life to enter
Physical Weathering ◼ Rocks break at weak spots when they are twisted, squeezed, or stretched by tectonic forces. ◼ Such forces form joints. ◼ Rocks adjust to removal of overlying rock by expanding upward. ◼ Removal of the weight of overlying rocks releases stress on the buried rock and causes joints to open slightly, thereby allowing water, air, and microscopic life to enter
Joints a Joints occur as a widespread set or sets of parallel fractures When dikes sills lava flows and welded tuffs cool they contract and form columnar joints joints that split igneous rocks into long prisms or columns)
Joints ◼ Joints occur as a widespread set or sets of parallel fractures. ◼ When dikes, sills, lava flows, and welded tuffs cool they contract and form columnar joints (joints that split igneous rocks into long prisms or columns)
Frost Wedging Wherever temperatures fluctuate about the freezing point, water in the ground periodically freezes and thaws As water freezes to form ice. its volume increases by about 9 percent. This process leads to a very effective type of physical weathering known as frost wedging frost wedging probably the most effective at temperatures of -5o to-150C
Frost Wedging ◼ Wherever temperatures fluctuate about the freezing point, water in the ground periodically freezes and thaws. ◼ As water freezes to form ice, its volume increases by about 9 percent. ◼ This process leads to a very effective type of physical weathering known as frost wedging. ◼ Frost wedging probably the most effective at temperatures of -5 o to -15oC
Frost Wedging
Frost Wedging
Daily Heating and Cooling Surface temperatures as high as 80C have been measured on exposed desert rocks Daily temperature variations of more than 400 have been recorded on rock surfaces u Despite a number of careful tests, no one has yet demonstrated that daily heating and cooling cycles have noticeable physical effects on rocks
Daily Heating and Cooling ◼ Surface temperatures as high as 80oC have been measured on exposed desert rocks. ◼ Daily temperature variations of more than 40o have been recorded on rock surfaces, ◼ Despite a number of careful tests, no one has yet demonstrated that daily heating and cooling cycles have noticeable physical effects on rocks
palling and Wedging by roots S Fire can be very effective in disrupting rocks。 Because rock is a relatively poor conductor of heat, only a thin outer shell expands and breaks away as a spall When plants grow they extend their roots into the cracks in rock where their growth can force the rock apart
Spalling and Wedging by Roots ◼ Fire can be very effective in disrupting rocks. ◼ Because rock is a relatively poor conductor of heat, only a thin outer shell expands and breaks away as a spall. ◼ When plants grow they extend their roots into the cracks in rock, where their growth can force the rock apart
Chemical Weathering Pathways In chemical weathering, chemical reactions transform rocks and minerals into new chemical combinations There are four different chemical pathways by which chemical weathering proceeds Dissolution Hydrolysis. Leaching. Oxidation
Chemical Weathering Pathways ◼ In chemical weathering, chemical reactions transform rocks and minerals into new chemical combinations. ◼ There are four different chemical pathways by which chemical weathering proceeds: ◼ Dissolution. ◼ Hydrolysis. ◼ Leaching. ◼ Oxidation
