1. Leaving Certificate Agricultural Science
Rock Types and Soil Formation
H. Jones, St. Columba’s College

2. Learning Objectives
Define the term soil and understand the factors that vary amongst different soils.
Describe the main components of soil.
Describe how soils are formed.
Distinguish between chemical, biological and physical weathering and give examples of both.
Describe the factors affecting soil formation.
Name the different types of peat bog in Ireland and describe how they are formed.
Describe the types of rock found on Earth’s surface and give examples of each type.

3. Introduction
Soil is the uppermost layer of the Earth’s crust and the medium for which plants grow.
The soil is essential for successful crop growth.
Soils vary from place to place, in terms of their composition (what they’re made of), their age, their fertility and physical features like colour, drainage or texture.
Soil consists of:
Broken up pieces of rock (MINERAL MATTER) – these vary in size and are often classified as sand, silt and clay.
Remains of dead animals and plants, referred to ORGANIC MATTER.
WATER & AIR – found amongst the soil pores (spaces between soil particles)
LIVING THINGS – like earthworms, plants, bacteria, fungi etc.

5. How a soil is formed
All soils form from rock or PARENT MATERIAL, broken down over a period of time by a process called WEATHERING. Weathering can be physical, chemical or biological.
Weathering creates an environment for simple living things to take hold on the surface of the rock e.g. Lichens
Lichens get minerals from rocks by secreting an acidic liquid into the rocks. This further breaks down the rocks and it decays slowly. When lichens die they also form the first layer of HUMUS (decaying organic matter).

6. Stages of Soil Formation

7. How a soil is formed
When the humus becomes a few inches thick, other plants (like grasses and mosses) can begin to grow on it. The roots of these plants then can break the rock further.
Layers begin to form in the soil called HORIZONS.
As the horizons get deeper, larger plants like trees and shrubs get established and further break the soil with their roots.

8. Learning Check What is soil?
How might soils differ from place to place?
What are the main components in soil?
Describe the stages in soil formation.

9. Phsyical Weathering
Physical or mechanical weathering breaks rock into smaller particles by actions of the sun, water freezing & cooling, the weather or glaciers.
Examples of physical weathering include:
Freeze – thaw action: water trapped in hair line cracks in rock expands when frozen, breaking up the rock.
Heating & Cooling: Repeated heating and cooling of rocks cause rock to break into smaller pieces.
Living things: Plant roots or burrowing animals can break large pieces of rock into smaller pieces.
Movement of glaciers: glaciers crushed rocks as they receded
Rivers, sea: coastal waves and moving rivers break down rock over time

10. Freeze thaw action

11. Chemical weathering
Minerals in rock can react with various other chemicals, breaking the mineral particles into smaller pieces.
Chemical weathering results in the formation of the smallest rock (or mineral) particles called CLAY PARTICLES.
There are various examples of chemical weathering and they include:
Solution: Water is an excellent solvent, dissolving the different minerals in the rock and breaking into very small particles.
Acid Rain: Rain is a mild carbonic acid (CO2 dissolves in rain as it falls) which reacts with and breaks down limestone easily.

12. Chemical weathering
Hydrolysis: Minerals react with rain water forming hydroxides, which help form clay particles and release minerals.
Oxidation / Reduction: In dry conditions, oxygen can bind to minerals and change their structure. In wet conditions, oxygen can be removed from minerals, again changing their structure.

13. Biological weathering
Living things can also break down rock. Some examples include:
Lichens produce acids to absorb minerals from rocks. These acids also change the chemical structure of rock.
Plant roots can break up the rock.
Burrowing animals can break rock into smaller pieces

14. Learning Check Describe two examples of physical weathering of rocks.
Name four types of chemical weathering of rocks.
Describe how living things can break up rock.

15. Factors affecting soil formation
Climate:
Rain, wind, high or low temperatures are all involved in physical and chemical weathering (see separate section on weathering).
The climate affects which plants grow in an area. Plants can cause biological weathering or soil pH.
High rainfall can cause a process called LEACHING, where minerals are removed from the soil.
Parent Material:
The type of rock (parent material) from which the soil forms types affect soil acidity (pH) and texture.
Texture affects soil fertility, drainage, water and mineral retention.
Soils formed from granite would be acidic (pH < 7).
Soils from Limestone would be basic. (Think of the use of lime)

16. Factors affecting soil formation
Living Organisms:
Earthworms affect soil aeration, drainage and mix organic matter.
Living things add organic matter when they die.
Plant roots cause physical weathering.
Time:
Time allows for greater levels of physical, chemical and biological weathering.
Irish soils are young soils, formed after the last ice age when glaciers removed most of the previous soil as they migrated.
The glaciers also transported huge amounts of rock, grit and minerals along their path.

17. Factors affecting soil formation
Topography:
Topography is slope of the land – whether it is hilly or flat.
Slope affects water movement and retention.
Some plants grow facing south, other north etc.
Soil become eroded from hillsides and can be shallow.
On the bottom of hills, the soils tend to be deeper, more fertile (due to leached clay). They can, however, become over wet and marshes can form.

18. Topography

19. Learning Check Name four factors that affect soil formation.
Describe how the type of rock influences the soil type.
Describe how the climate might affect the type of soil found in a particular area.
What is meant by topography? How does it affect the soil type?

20. Peat Bog Formation
A peat soil or bog is a soil high in organic matter, which for some reason has not decayed.
This is usually due to a lack of oxygen (caused by excess water).
The organic matter builds up over time and can form layers of peat (or turf) up to ten metres deep.
There are two main types of bog in Ireland – Basin (or Raised) Bogs and Blanket Bogs.

22. Types of Bog
Basin bogs form in a depression (usually a shallow lake) where organic matter builds up due to a lack of oxygen.
When basin bogs accumulate so much organic matter that it reaches a height above the surrounding the land, it can then be called a raised bog.
As the name suggests, blanket bogs form over a large area and are typically shallow (1-2 metres).
Blanket bogs form in areas of high rainfall (mountainous areas like the Wicklow mountains) where high levels of acid leaching occur.

23. Rock Types
Rocks are categorised by how they are formed. The three main rock types on the Earth’s surface are:
Igneous Rock – Formed from volcanic eruptions.
Sedimentary Rocks - Made from the sediments of rock or remains of living things, compressed over time.
Metamorphic Rocks - Sedimentary Rocks changed over time by heat and pressure.

24. Igneous Rocks
Rocks formed by cooling of molten lava are called Igneous Rocks and were the first ever rocks formed on the Earth 4.5 billion years ago.
Examples of Igneous rocks include Granite and Basalt.
Granite is made up of three main components: Feldspar, Mica, Quartz
Basalt and Granite are formed from the same material and differ only due to how they cooled down.
Granite cooled more slowly than Basalt and therefore gave rise to very large crystals.

25. Formation of Igneous Rocks

26. Granite

27. Basalt

28. The Giant’s Causeway

29. Sedimentary Rocks
Sedimentary rocks are formed by the fusion of various types of sediment from igneous rocks.
The material can either be another type of rock (Igneous) or shells on the bottom of the ocean.
Sandstone is formed by sediments of quartz, (broken by physical weathering).
Limestone is formed by the fusing of pieces of shells and other materials containing Calcium Carbonate (CaCO3)
Chalk, Iron stone and shale (the most common type) are other example of sedimentary rocks.
Sedimentary rocks form layers and can often contain fossils.

30. Formation of Sedimentary Rocks

31. Limestone

32. Sandstone

33. Shale

34. Sedimentary Rocks

35. Metamorphic Rocks
Metamorphic rocks arise from sedimentary rocks when they are exposed to pressure and heat over a very long period of time.
Over this time, the rocks compress and change their internal structures.
Examples are Slate (changed from shale) and Marble (from Limestone).

37. Marble

38. Slate

39. Learning Check Compare blanket and basin bogs.
Name two types of igneous rocks and describe how they are formed.
How are sedimentary rocks formed?
Name two types of sedimentary rocks.
Name the two main foctors than contribute to metamorphic rock formation.
Name two types of metamorphic rock and the rocks they are formed from.