Every soil that we see now is originated from solid rock deep in the ground. Over a very long period, sunlight, rain, wind, and gravity slowly break that rock apart and eventually it becomes loose soil. There are many processes like weathering, erosion, and downhill movement of materials, mostly driven by the Sun. If you glance around you, the Earth may seem permanent, unchanging - and until a couple hundred years ago, most people thought that mountains, lakes, and deserts were permanent features. We can see now that the Earth is approximately 4.6 billion years old, and that the Earth's surface has always been in a state of flux. Mountains are slowly eroded, lakes gradually fill up with sediments or are drained by rivers, and deserts grow as the climate changes. we will see in detail the origin of soil and its formation.

Origin of Soil and its formation

Weathering

Weathering is the breaking down of rocks, soil, and minerals as well as wood and artificial materials through contact with the Earth's atmosphere, water, and biological organisms.

There are three types of weathering. Mechanical weathering, chemical weathering and biological weathering.

Mass wasting

Mass wasting is the movement of rocks and soil due to Gravity.

Erosion

Erosion is defined as removal and movement of earth materials by natural agents.

Formation of Soil

·       When a rock surface is exposed to sunlight the outer part of it expands.

·       At night that outer layer shrinks as temperature drops.

·       Once crack form, water enters them and dissolve minerals.

Factors acting together on rocks to form Soil

Climate-  Both temperature and rainfall are considered the powerful factor of forming soil.

   Temperature- Controls how fast the chemical reaction will occur.

   Rainfall- Rainfall effects the movement of soluble soil nutrients.

Types of weathering

Weathering goes on all around us, but it seems like such a slow and subtle process that it is easy to underestimate its importance. It is worth remembering that weathering is a basic part of the rock cycle and thus a key process in the Earth system.

Mechanical Weathering

In mechanical weathering rock is broken down into smaller pieces without changing its mineral composition.

Mechanical weathering is of different types includes loading/unloading, frost heaving and wedging, Plants roots, Friction and impact, Burrowing of animals and thermal expansion.

Loading / Unloading

• Due to pressure release the rock surface tends to expand, and this creates a great stress and results in creation of sheets on the rock surface

• aka exfoliation

Frost Wedging

• Frost wedging happens when water expends by about 9 % because of freezing. Cracks filled with water are forced further apart when it freezes.

Chemical weathering

Breaks rocks down chemically adding or removing chemical elements and changes them into other materials. The agents of chemical weathering are Water, Oxygen, Carbon dioxide, Living organisms & Acid rain.

Chemical weathering Processes

Solution. CO2 in soil profile making carbonic acid H2CO3 with percolating rainwater.

• Oxidation. Oxygen with mineral forms oxides

• Reduction. Oxygen leaves mineral

• Hydration. Absorption of water, expands clays, hastens the above processes

Water

In Rocks water dissolves minerals, slowly weakening and breaking it down.

Oxygen

• Iron combines with oxygen in the presence of water in a process called oxidation.

• The product of oxidation is rust

Living Organisms

• Lichens that grow on rocks produce weak acids that chemically weather rock.

 

Acid Rain

• Compounds from burning coal, oil and gas react chemically with water forming acids.

• Acid rain speeds up chemical weathering by reacting with minerals and breaking them down much faster than normal rainwater.