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