Rock Cycle and Soil Formation Notes

The Rock Cycle and Soil Formation

The rock cycle is the process by which rocks change from one type to another over a long period.
This cycle involves stages where rocks are formed, broken down, and transformed.
There are three main types of rocks: igneous, sedimentary, and metamorphic.
Each type of rock can change into another type through processes like melting, cooling, weathering, and pressure.

1. Igneous Rocks: Formed from Cooling Lava

Igneous rocks are formed when lava or magma from the Earth's interior cools and hardens.
This happens either beneath the ground or on the surface of the Earth after a volcano erupts.
Examples: Basalt and Granite.

2. Sedimentary Rocks: Created from Layers of Materials

Sedimentary rocks are formed when small pieces of other rocks, like sand, mud, and shells, settle at the bottom of lakes, rivers, and oceans.
Over time, these pieces get pressed together to form solid rock.
The rocks are built up in layers.
Examples: Sandstone and Limestone.

3. Metamorphic Rocks: Changed by Heat and Pressure

Metamorphic rocks are formed when sedimentary or igneous rocks are changed by heat and pressure deep beneath the Earth's surface.
The rocks melt a little, and their minerals rearrange, creating new rock types.
Examples: Marble (formed from limestone) and Slate (formed from shale).

The Rock Cycle in Action

The rock cycle doesn’t stop; rocks can be in any stage and go through the cycle many times.
An igneous rock might break down into small pieces and form a sedimentary rock, which could then be changed into a metamorphic rock by heat and pressure.
Weathering and Erosion:
- Rocks on the Earth’s surface break down over time due to forces like wind, water, and ice.
- These tiny pieces are carried away by erosion and can form sedimentary rocks when they settle and get pressed together.
Melting:
- When rocks are heated deep inside the Earth, they can melt and form magma.
- When magma cools, it forms igneous rocks.
Heat and Pressure:
- Rocks buried deep underground are exposed to heat and pressure.
- This causes them to change and form metamorphic rocks.

Key Terms

Igneous: Rocks formed by cooling magma or lava.
Sedimentary: Rocks formed by layers of material.
Metamorphic: Rocks that have been changed by heat and pressure.
Magma: Hot liquid rock beneath the Earth’s surface.
Lava: Hot liquid rock that reaches the Earth’s surface.
Weathering: The process of breaking down rocks into smaller pieces.
Erosion: The movement of rock pieces by wind, water, or ice.
Pressure: Force that can change the structure of rocks.
Minerals: Naturally occurring substances in rocks that contribute to soil fertility.
Fertility: The ability of soil to support plant growth.

1. Freeze-Thaw Weathering

What is it?
- Freeze-thaw weathering happens when water gets into cracks in rocks. When the temperature drops and the water freezes, it expands, putting pressure on the rock.
- Over time, this pressure causes the rock to crack and break apart.
How does it work?
- Water enters cracks in rocks.
- When temperatures drop below 0°C (freezing point), the water freezes.
- When water freezes it expands, ice takes up more space than water, so the ice pushes against the sides of the crack.
- As the ice thaws and the water melts, the crack left behind will be bigger than before.
- The freeze-thaw cycle repeats and makes the cracks grow larger and eventually break down the rock into pieces over time.
Where does it happen?
- Freeze-thaw weathering happens in cold climates, especially in places where temperatures go above and below freezing, like mountain ranges or areas near the poles.
Effects on rocks and landscape:
- Rocks break into smaller pieces or big chunks.
- It can create loose rock fragments called scree on the ground, which cover hillsides.
- The landscape may become jagged or broken apart.

2. Thermal Expansion and Contraction

What is it?
- Thermal expansion and contraction happen when rocks heat up during the day and cool down at night. As the rock heats up, it expands, and as it cools, it contracts.
- This constant change in temperature makes the rock crack and break over time.
How does it work?
- During the day, the sun heats up rocks, causing them to expand.
- At night, the temperature drops, and the rocks cool down and contract.
- Repeated heating and cooling causes the surface of the rock to crack and peel off in layers.
Where does it happen?
- This weathering process is most common in deserts, where there is a big difference between daytime and nighttime temperatures.
Effects on rocks and landscape:
- Rocks can break into thin layers or flakes.
- The landscape may become smooth or have rounded edges because of the breaking of rock layers.

3. Pressure Release (Unloading)

What is it?
- Pressure release weathering happens when rocks that formed deep underground are exposed at the Earth's surface. This is commonly igneous rocks formed by magma but can be all types of rocks.
- As the heavy weight above them is removed, the rock expands and cracks, causing the outer layers to peel away.
How does it work?
- Rocks form deep underground under high pressure from the weight of layers of rock and earth above.
- When erosion (breaking down of rocks) removes the layers above, the pressure is reduced.
- The rock expands and develops cracks parallel to the surface.
- Over time, these cracks cause thin layers of rock to break away, like peeling an onion.
Where does it happen?
- Pressure release weathering is common in places where deep rocks are pushed up to or eventually exposed to the surface, such as:
  - Mountain ranges, where tectonic uplifting brings buried rocks upward.
  - Old landscapes, where erosion has removed thick layers of rock over millions of years.
Effects on rocks and landscape:
- Rocks may have smooth, rounded surfaces.
- Large rock formations may split into sheets or break into boulders.
- The landscape may have exposed rock faces with curved or dome-shaped surfaces.

4. Chemical Weathering - Oxidation

What is it?
- Oxidation is a type of chemical weathering that happens when rocks react with oxygen and water. If a rock contains iron, it can rust, just like a metal object left outside in the rain.
- Over time, this weakens the rock and causes it to break down.
How does it work?
1. Some rocks contain iron minerals.
2. When these rocks are exposed to oxygen in the air or water, a chemical reaction takes place.
3. The iron reacts with oxygen and forms iron oxide (rust), which is weaker than the original rock.
4. The rock becomes soft, crumbly, and changes color—often turning reddish-brown or orange.
5. Over time, the rock breaks apart, leaving behind rust-colored soil or dust.
Where does it happen?
- Oxidation is common in warm and wet places because water speeds up the reaction.
- It often happens in tropical rainforests, deserts, and coastal areas.
- It can also occur underground if water moves through soil and rock layers containing iron-rich minerals.
Effects on rocks and landscape:
- Rocks turn red, orange, or brown as they rust.
- The surface becomes weaker and crumbly.
- In extreme cases, rocks can completely break down into soft, powdery material.
- Large rock formations in deserts and mountains may develop reddish stains or layers.

5. Carbonation

What is it?
- Chemical weathering happens when rocks break down because of chemical reactions. These reactions change the minerals in the rock, weakening it and causing it to crumble or dissolve over time.
How does it work?
- Rainwater absorbs carbon dioxide from the air, making it slightly acidic.
- When this water lands on certain rocks, it reacts with minerals and breaks them down.
- Some minerals dissolve in water, while others change into softer substances that crumble more easily.
- Water can also move into the soil and rock layers underground, continuing to react with minerals beneath the surface.
- Over time, these changes weaken the rock, causing it to wear away.
Where does it happen?
- Chemical weathering can happen on the surface and underground, especially in:
  - Warm and wet climates, where heat and moisture speed up reactions.
  - Limestone landscapes, where acidic water creates caves and sinkholes.
  - Coastal areas, where salty water reacts with rocks.
  - Underground, where water seeps through soil and rock, slowly dissolving minerals and forming underground rivers or caves.
Effects on rocks and landscape:
- Limestone and chalk dissolve, creating caves, sinkholes, and underground rivers.
- Rocks may become soft and crumbly as minerals change into weaker substances.
- Surfaces may look pitted or rough, with holes and grooves forming over time.
- Some rocks turn red or orange due to iron reacting with oxygen (rusting).

6. Biological Weathering

What is it?
- Biological weathering occurs when plants, animals, or microorganisms cause rocks to break down through physical actions.
- This type of weathering doesn't change the chemical composition of the rock but instead weakens it by physically breaking it apart.
How does it work?
- Plant Roots:
  - Plant roots often grow into cracks and small gaps in rocks as they search for water and nutrients.
  - Over time, the roots grow thicker and stronger, applying pressure to the rock.
  - This pressure causes the rock to crack, break, and eventually split into smaller pieces.
- Animals:
  - Certain animals, such as rodents or burrowing insects, dig into the soil or directly into rock surfaces to create tunnels or burrows.
  - As they dig, they loosen and break the rock into smaller pieces.
- Microorganisms:
  - Tiny organisms like lichens and mosses can grow on rock surfaces.
  - These organisms release acids that gradually break down the minerals in the rock, contributing to its weakening and disintegration.
Where does it happen?
- Biological weathering can occur in almost any environment where plants, animals, or microorganisms live. Some common places include:
  - Forests, where plant roots and animals like squirrels and rabbits burrow into the soil and rocks.
  - Grasslands, where plants and burrowing animals like moles break up rocks in the soil.
  - Urban areas, where plants grow in cracks in pavement and buildings.
  - Coastal areas, where sea birds and other organisms can break down rocks on the shore.