Weathering and Soil Formation

What is weathering?

Weathering  The breaking down of rocks and other materials on the Earth’s surface  2 types:  Mechanical  Chemical

Mechanical weathering  Rocks are broken into smaller pieces and different shapes  Chemical makeup of rocks does not change  Beginning of process: rocks are sharp and angular  As process continues, rocks are smooth and have rounded edges

Mechanical Weathering  Caused by several agents:  Temperature  Frost action  Organic activity  Gravity  Abrasion

Temperature  During the day, sun heats outside of rock and it expands  During the night, the outside of the rock cools and contracts  Cycle of heating and cooling continues each day, and parts of rock crack or peel off  Causes exfoliation: rocks break off in curved sheets or slabs

Frost Action: see demonstration

Frost Action  Water gets into cracks of rocks, where it freezes  Freezing water expands inside crack of rock, and crack grows until it forces rock to break  Can you think of examples of this you have seen before?

Organic Activity  Roots of plants loosen rocks  Plant growing in a rock’s crack can make crack grow as roots grow and spread  Called root-pry: breaking apart of rocks caused by plant roots

Gravity  Gravity pulls loose rocks down cliffs of mountains: called a landslide  Rocks that fall collide with other rocks and break them into smaller pieces

Abrasion  Wearing away by solid particles carried in the wind and water  wind and water pick up particles that have been eroded  Sharp edges of sand and particles cut into exposed rocks

Chemical Weathering  Weathering that causes changes in the chemical makeup of rocks  Minerals can be added or removed from rocks  Substances react chemically with rocks and break them down

Causes of chemical weathering  Water  Oxidation  Carbonation  Sulfuric Acid  Plant Acids

Water  Can dissolve minerals that hold rocks together  Can form acids when it mixes with certain gases  These acids speed up rock decomposition  Can combine with a mineral to form a new mineral

Oxidation  Process in which oxygen combines with another substance  Forms a new substance  Example: iron and oxygen combine to form iron oxide (rust)  Indicated by its color

Carbonation  Carbonic acid reacts chemically with other substances  Carbonic acid: weak acid formed when carbon dioxide dissolves in rain

Sulfuric acid  Sulfur oxides are a byproduct of burning coal  When sulfur oxides dissolve in rainwater, they form sulfuric acid  Sulfuric acid is a strong acid that quickly wears away rocks and metals

Plant acids  Plans produce weak acids that dissolve certain minerals  Example: mosses, which grow in damp areas, produce weak acids that seep into rocks and dissolve certain minerals

Rate of weathering  Rate of weathering: how fast weathering takes place  Depends on several factors:  Type of rock  Stable rock: rock that can resist chemical weathering  Time  Size of exposed surface area

Assignment – pick one!  Draw a comic strip (at least 6 frames) that shows the difference between mechanical and chemical weathering and gives 2 examples of each.  Pretend you are a rock. Write a plea to nature asking it to stop weathering you so much. Your plea must show the difference between mechanical and chemical weathering and give 2 examples of each.  Due Friday, 2/26

  What do you think happens to the small pieces of rock that are broken off by weathering?

They form soil! Soil is formed when rocks are continuously broken down by weathering.

Why do we need soil?

Why we need soil   Very important for all living things   Plants need minerals and water found in soil to grow   Animals either eat these plants or eat other animals that eat the plants   Humans eat plants or eat the animals that eat the plants

Soil   Residual soil: soil that remains on top of the rock it came from   Transported soil: soil that moves away from its origin   Bedrock: layer of rock beneath the soil

Soil from organic material   Humus: part of the soil formed by decaying organic material   Comes from living things   Decay means to break down plants and animals into what they are made of   Humus is important for plant growth (fills soil with nutrients plants need)

What do you think soil is made of?

Ingredients of soil   2 main ingredients:   Humus (organic material)   Pieces of weathered rock (80% of soil)   Most abundant minerals: clay and quartz   Other ingredients:   Air   Water

Where is the air and water?   Pore spaces: space between soil particles   Fill with air and water   Plant roots get the oxygen they need from the air found in pore spaces   Plants use minerals that are dissolved in the water in pore spaces

Soil Composition Varies   Different types of rocks being broken down = different types of soil   Different types of weathering = different types of soil   Mechanical weathering: soil is similar to rock being weathered   Chemical weathering: soil is different from rock it came from

Soil Texture   Different size particles give soil different texture   Largest particles: gravel (2-64 mm in diameter)   Gravel breaks down into sand (less than 2 mm in diameter)   Silt is made of broken rock crystals (less than 1/16 mm in diameter)   Clay has smallest particles (less than 1/256 mm in diameter)

Soil has layers   Layers of soil are called horizons   You can take a cross section of soil to see the different horizons – called a soil profile   Soil with 3 layers: mature soil   Soil with 2 layers: immature soil

Uppermost layer   Called the A horizon   Soil here is topsoil   Contains mostly humus   Living organisms add minerals to it regularly   Many pore spaces   Fertile soil – good for planting

Middle layer   Called the B horizon   Soil here is called subsoil   Minerals are washed by water from topsoil down into B horizon: called leaching   Made of minerals from leaching, clay, and some humus

Bottom layer   Called the C horizon   Made of partly weathered rock   Continues until you reach the unweathered parent rock   Composition is similar to parent rock below the soil

Factors affecting soil formation   Type of weathering   Climate: How can rain and temperature change things?   Type of rock   What the region looks like