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Weathering,Soil and Glacial Movement
Ch. 14,15
What Physical Properties Affect Rocks Near the Surface?
• There are many processes that break rocks apart and can change their – color– texture– composition – strength
• Called chemical and physical weathering
What is the role of joints in weathering
• Joints are fractures in rock that are not offset.
• Joints allow water and roots to penetrate the rocks to cause weathering.
Joints
• Closely spaced joints promote increased weathering.
• Exfoliation- when overlying rocks are unloaded the underlying rocks expand creating expansion joints.
Physical Weathering
• Heating and cooling- expansion of rock• Frost wedging• Mineral wedging- growth of minerals
weaken the rock around them
Physical Weathering cont
• Burrowing organisms
• Plant growth
Affect of Fracturing on Weathering
• As a rock fractures and breaks apart there is an increase in the rate of weathering.
• WHY??
• As a rock breaks into smaller pieces the surface area increases allowing more exposure to weathering.
Chemical Weathering
• Chemical factors that affect rocks at the earth’s surface
–Decreased temperature and pressure
–Increase O2
–Increased H2O
Dissolving Rocks
• Rocks are dissolved by weak acids formed in water that react with the minerals that make up rocks.
• Acids are abundant in H+ ions, the H+ ions enter minerals releasing other ions
• Ex- Limestone is rich in calcite which is soluble in weak acids. When Carbonic acid in rainwater reacts with the rock it dissolves.
Oxidizing Rocks
• Free O2 is common near the surface and reacts with minerals to cause rust.
• Most common in iron bearing minerals.
• Mafic minerals commonly contain iron and are most commonly affected by oxidation.
How Does Water React With Minerals?
• In some minerals water combines with the mineral causing formation of a new mineral, called hydrolysis.
• If exposed to wet conditions many rocks convert into clay minerals.
How Does Soil Form
• Soil consists of weathered rock, plus material from the atmosphere, decaying plants and microbes
• Soil is made up of different zones, or horizons.
Soil Horizons
• O Horizon- surface accumulation of organic debris.
• A Horizon- topsoil, dark gray or brown organic material mixed with minerals.
• E Horizon- light colored, leached zone• B Horizon- little organic material, red
color due to accumulation of iron oxide• C Horizon- weathered bedrock
Processes That Occur During Soil Formation
• Soil forms over thousands of years as a result of weathering.
• Involves the vertical movement of dissolved material up and down through the horizons.
How Materials Move Through Soil
• Materials move both up and down as carried by water, animals and gravity
• Zone of leaching- Upper part of soil loses material downward. Water leaches soluble materials and carries them downward.
• Zone of Accumulation- Chemical ions leached from above accumulate in the lower levels.
Glaciers
• Moving masses of ice.
• They form by the accumulation of snow and ice.–Average amount of incoming
snowfall > amount lost by melting and evaporation
–Blue color because there is less air.
Glacier Movement
• Glaciers move downhill because ice is not strong enough to support its own weight.
• Shearing force – the top of the glacier moves faster than the bottom because of friction.
Glacial movement continued
• As glaciers move internal stress causes the upper surface of ice to break forming fractures, called crevasses.
• When the glacier reaches water, it will float.
• When glaciers break off, in a process called calving, they float off as icebergs.
Glacial Rebound
• the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy.
• 2 phases– Initial uplift called elastic- rapid– Slow viscous flow- ~1cm/year