Rock, Weathering, and Soils Day

Welcome to Rocks, Weathering, and Soils Day

Objectives:

  • To understand, through place-based exemplars, physical geography relating to:
    • rocks,
    • weathering,
    • soils.

To build on your knowledge of globes, maps and atlases and apply and develop this knowledge routinely in the classroom and in the field.

Session One - Erosion & Transport

Weathering is the physical breakdown and chemical alteration of Earth materials at or near the surface.

Parent material is rock altered by weathering, either broken into smaller pieces or some of its constituent minerals altered or dissolved.

Erosion

This refers to the wearing away of soil and rock by water, wind, or ice.

  • erosion degrades soils
  • eroded sediments degrade rivers and streams.
  • In their resistance to weathering, rocks are not homogeneous (happening in the same way and at the same time). One may weather faster or slower than another. Parts of the same rock may weather at different rates. This process, known as differential weathering, creates many picturesque and bizarre features, such as arches and honeycomb weathering.

Transport

This refers to the movement of eroded material from its place of origin to a new site where it is deposited as sediment.

Weathering, Erosion, and Deposition.

Erosion Activity

Plan and carry out the activity above. Take photos. It's that simple.

Session Two - Mechanical vs Chemical Weathering

Two types of weathering are recognized: mechanical and chemical. Both proceed at the same time, but at a given site one type will be predominant.

Mechanical weathering – physical breakdown of rocks without changing the composition.

Mechanical weathering occurs when physical forces break Earth materials into smaller pieces. Rocks can break apart into smaller pieces in many ways.

  • For example: granite can be mechanically weathered to produce smaller pieces of granite and individual grains of the minerals comprising the granite, quartz and feldspars.

How weathering temperature affects rocks.

Freeze-Thaw

  • When liquid water changes into solid ice, it increases in volume. You see this when you fill an ice cube tray with water and put it in the freezer. The ice cubes go to a higher level in the tray than the water. You also may have seen this if you put a can of soda into the freezer so that it cools down quickly.

Freezing Activity

Freeze a two bottles of water and leave no space for expansion in one.

Ice-Wedging

Ice wedging is common where water goes above and below its freezing point.

Ice wedging happens because water expands as it goes from liquid to solid. When the temperature is warm, water works its way into cracks in rock. When the temperature cools below freezing, the water turns to ice and expands. The ice takes up more space. Over time, this wedges the rock apart. Ice wedging is very effective at weathering. You can find large piles of broken rock at the base of a slope. These rocks were broken up by ice wedging. Once loose, they tumbled down the slope.

Mount Denali, Alaska

Ice-wedging Activity

Purpose: To build a model of ice wedging and see how it works.

Materials: plaster of Paris bandage, water balloon, water, syringe, paper cup, paint & brush, freezer.

Procedure:

1. Fill the balloon with water from the syringe and tie it. It should be about 1/2 the diameter of the cup in size.

3. Cover the balloon in plaster bandage until it is completely covered.

4. Leave to dry.

5. Paint it.

6. Leave to dry. (Take a photo at this point.)

7. Place it in the freezer.

Prediction:

  • What will happen?

Data:

  • Take a the frozen balloon out of the freezer (a few hours later). (Take another photo.)
  • What is the difference in the balloon model before and after freezing.

Analysis:

1. How was our model different from ice wedging in nature?

2. Why is ice wedging a form of weathering?

3. What did the control prove?

4. What does water do when it freezes?

5. Describe the steps of ice wedging that would occur is this crack. Be sure to mention temperatures and the phase water is in.

Conclusion:

  • Two things you learned.

Extension:

  • Allow it to defrost.
  • What do you observe of the original 'rock'?

Pressure Release

In deep, underground mines such as some of the diamond mines of South Africa, rock sometimes expands violently in response to the pressure released by removal of the mine rock. These rock bursts kill 20 miners/year. Excavations of only 7 or 8 m in the granite quarries have produced sheet joints with enough force to knock the tracks off of the heavy quarrying equipment.

 

Chemical weathering – breakdown of minerals by chemical reactions with air or water

Chemical weathering refers to the processes that decompose rocks and minerals.

  • In some instances, minerals are chemically altered such that new minerals are formed.
  • Other minerals may completely dissolve and their ions taken into solution

Atmospheric Gases

Atmospheric gases, especially oxygen, water, and organic acids produced by plants and decaying organic matter can cause chemical weathering.

Solution

Solution is a type of chemical weathering reaction wherein interaction of a solid with a liquid separates the ions of the solid such they come to reside in the liquid and the solid is thus dissolved.

Solution chemical weathering forms extensive cave systems in limestone and marble, such as Mammoth Cave and Carlsbad Caverns.

Check out these images of Mammoth Cave.

Solution Weathering Activity

Carry out the activity from the video. Simples.

Oxidation

  • Oxidation is a chemical weathering reaction whereby metal elements like iron combine with oxygen to form oxides.

Set up an oxidization experiment.

Session Three - Soils

Now we have established that rocks become soil. What do you think soil is?  What about sediment?

The main difference between sediment and soil is in the manner in which they are deposited. Think about where the soil comes from. There are mineral soils (inorganic) and there are organic soils (decomposed plant and animal matter). Most soils you will see are a combination.

Sediment is derived from mineral parent material (rock, stone).

Some people say that soil is just dirt or mud, but this isn’t accurate as soil is much more complex than that. Soil is vital to human existence.

Soils Activity - Rock Cycle

Explore this diagram:

... and draw your own version adding extra information from here.

Extension activity

Time to get out the microscope and find out what the soil is made of. 

  • Choose sand or soil.
  • Make slides.
  • Draw what you see.
  • Try and identify some of what you saw. This will help.