Rocks, The Rock Cycle and Soil Chapter Notes | Science Olympiad Class 6 PDF Download

Igneous Rocks

What Are Igneous Rocks?

• The term "igneous" comes from the Latin word for fire. Igneous rocks are formed from magma that has cooled and solidified. Magma is extremely hot, just like fire.
• When magma rises from deep within the Earth’s mantle and cools, it undergoes a process called solidification, turning into solid rock.

Intrusive Igneous Rocks

• Some magma cools and solidifies slowly inside the Earth’s crust. This process creates intrusive igneous rocks.
• Intrusive means these rocks form inside the Earth.
• An example of an intrusive igneous rock is granite. Over time, as the rocks above granite wear away, this granite can be exposed at the surface.

Extrusive Igneous Rocks

• Other magma reaches the Earth’s surface, where it is called lava. When lava cools, it solidifies into extrusive igneous rocks.
• Extrusive means these rocks form on the surface of the Earth.
• An example of an extrusive igneous rock is basalt, which forms when lava cools quickly on the surface.

What are igneous rocks made of?

• Igneous rocks are composed of minerals, which are made up of various substances. The most common substances in the Earth's crust are silica, oxygen, aluminium, magnesium, and iron. These substances combine in different ways to form minerals.
• A key feature of all igneous rocks is that they are crystalline, meaning they are made up of crystals formed when minerals cool down. For instance, quartz is a mineral made of silica and oxygen, and its crystals are visible when the mineral cools and forms solid crystals.
• When magma is deep beneath the Earth's surface, it cools slowly, allowing large crystals to form. These crystals are big enough to be seen with the naked eye, but because they are close together, they don’t form distinct shapes like individual crystals. This is why rocks like granite, which contains quartz, feldspar, and mica, have different colors due to the presence of different crystals.
• In contrast, when lava cools quickly, as on the Earth's surface, it forms extrusive igneous rocks with small, barely visible crystals, such as in basalt.

Sedimentary Rocks and Fossils

What are Sedimentary Rocks?

• Sedimentary rocks are formed from the accumulation and cementation of small particles of other rocks, known as sediments.

Where Do the Sediments Come From?

• Rocks are constantly being broken down into smaller pieces through a process called weathering. This can happen in several ways:
• Temperature Changes: Extreme temperatures can cause rocks to crack and break apart.
• Water: Rainwater can dissolve certain types of rocks, making them easier to break down.
• Plants: The roots of plants can penetrate and break the surface of rocks, contributing to their weathering.

Erosion and Transportation

• Once rocks are weakened by weathering, they are eroded by natural forces such as rivers, ice, and wind.
• Rivers play a significant role in this process by carrying the broken-down rocks, which are now in the form of sediments.

Sedimentation

• When river water reaches slower-moving bodies of water, like seas or lakes, it loses the energy to carry the sediments.
• As a result, these sediments are deposited, marking the beginning of the sedimentary rock formation process.

Formation of Sedimentary Rock

• Over millions of years, the layers of sediments at the bottom are compressed by the weight of the layers above them.
• Minerals present in the water help to bind these sediments together, forming solid sedimentary rock.

Characteristics of Sedimentary Rocks

• One of the key features of sedimentary rocks is that they always form in layers. This layering is a result of the gradual accumulation and compression of sediments over time.

Types of Sedimentary Rock

Sandstone

• Sandstone is a type of sedimentary rock formed from sand grains that are cemented together.
• Most of the sand in sandstone is made up of quartz, a very hard mineral that resists weathering.
• Sandstone can come in various colors, including red, orange, yellow, and grey.

Shale

• Shale is composed of very fine-grained sediments that are smaller and softer than sand grains.
• It is typically grey in color and can be scratched easily, leaving a mark.
• Shale is often soft enough to break with hand pressure and also shows distinct layering.

Limestone

• Limestone is formed from layers of shells that cover the remains of sea animals that died and sank to the ocean floor.
• It is usually white or grey in color, with chalk being a pure form of limestone.

Fossils

Fossils are another feature that can be found in some sedimentary rocks. They are the preserved remains of ancient animals and plants that have been buried in sediment over a long period of time.

How Fossils Form?

• For a fossil to form, the remains of an animal or plant must be buried quickly in a place with very little air. This is why fossils are typically found in sedimentary rocks that form under water, such as on the sea floor or lake bed.
• When animals that live in or near the sea die, their bodies are often washed into the water. The soft parts of the animal decay, and over time, minerals in the water replace the bones, turning them into rock. This process creates a fossil, like the dinosaur fossil shown in the photograph, which was found in limestone and is estimated to be about 150 million years old.
• Sometimes, only the imprint of an animal is left in the sediment, creating what is known as a mould. For example, a dinosaur footprint mould found in sandstone in Namibia is also around 150 million years old. Another example is a leaf mould found in shale that is 350 million years old.
• In some cases, a mould can be filled with minerals that harden, creating a cast of the animal. The photograph below shows two ammonite fossils in limestone, where the moulds were filled with minerals that hardened to form the shape of the original animal. Ammonites lived around 240 million years ago, making these fossils very old.

Metamorphic Rocks and the Rock Cycle

Metamorphic Formation

What are Metamorphic Rocks?

Metamorphic rocks are rocks that have undergone a process of change. The term 'metamorphosis' means 'change,' and it reflects how these rocks are transformed from their original state. There are different ways in which metamorphic rocks can be formed:

• Change by Heat: When rocks are subjected to high temperatures, all the minerals within the rock can melt and then re-form into new crystals. This process creates a metamorphic rock with a different mineral composition and structure.
• Change by Pressure: Sometimes, rocks are changed by immense pressure. This pressure can squeeze the rocks so tightly that the new rock ends up with many thin layers. The intense pressure alters the rock's structure and appearance.
• Change by Heat and Pressure Together: In some cases, rocks are changed by both heat and pressure at the same time. This combination results in a new rock with different minerals and a layered structure. The heat and pressure work together to create a unique metamorphic rock.

What leads to the formation of heat and pressure?

The illustration below depicts layers of sedimentary rocks buried beneath the Earth's surface. When magma rises from the mantle and forms an intrusive igneous rock mass, it exerts immense heat and pressure on the surrounding rocks. Imagine the scenario: a massive body of extremely hot liquid rock pushing solid rocks aside. This process generates heat and pressure, transforming the surrounding rocks into metamorphic rocks.                                                                                                                                                                                                                                                                                                        

Types of Metamorphic Rocks

Let’s explore what kinds of metamorphic rocks are formed when sandstone, limestone, shale, and granite are exposed to intense heat and pressure.

Quartzite

• When sandstone is heated, the sand grains melt and turn into new quartz crystals, forming the metamorphic rock known as quartzite. It appears similar to sandstone, but its crystals sparkle, and it’s much harder.
• Quartzite is so tough that it’s often used for making sculptures. An example is the statue of Ankhrekhu made from quartzite around 1850 BCE, nearly 4000 years ago.

Marble

• When limestone is heated, its minerals melt and grow into new crystals, producing marble. This rock is very hard, shiny, and can appear in colors like white, red, blue, or green, depending on the materials in the original limestone.
• Marble is valued for its beauty and durability. It is commonly used to carve statues, gravestones, and to decorate buildings and places of worship. In India, different colored marble has been used to beautifully decorate buildings.

Slate

• When shale is placed under great pressure, it transforms into slate, which forms in thin layers. This happens as layers of sediment build up on the seabed — the lower layers are buried by the upper ones. 
• If enough layers pile up, the pressure turns the shale into slate, which keeps its layered structure.
• Because slate is hard and durable, it’s often used for roof and floor tiles.

Gneiss

• When granite is subjected to heat, its minerals melt and form new crystals. The pressure arranges these crystals into bands or stripes, creating a rock called gneiss.
• Although the crystals in gneiss look somewhat like those in granite, they are arranged in distinct bands. Gneiss is a very hard rock and is usually crushed for use in road construction.

​The Rock Cycle

The rock cycle illustrates the continuous transformation and recycling of Earth’s rocks over an extended period, typically hundreds of millions of years.

Key Processes in the Rock Cycle

• Melting: Rocks can melt into magma.
• Cooling: When magma cools, it forms igneous rock.
• Weathering and Erosion: These processes break down rocks into smaller pieces, leading to the formation of sediments.
• Sedimentation: Sediments accumulate and compact over time, forming sedimentary rock.
• Burial, Heat, and Pressure: Sedimentary rock can be buried under layers of sediment, where heat and pressure transform it into metamorphic rock.

Types of Rocks

• Igneous Rock: Formed from the cooling of magma.
• Sedimentary Rock: Formed from the accumulation and compression of sediments.
• Metamorphic Rock: Formed from the transformation of existing rock through heat and pressure.

The rock cycle is a slow but constant process that shows how Earth’s materials are reused and transformed over time.

Soil

What does soil consist of?

Soil is made up of tiny pieces of rocks that have been worn down over time. This process of breaking rocks into smaller bits is called weathering. Just like how different ingredients can make different dishes, the type of rock affects what kind of soil is formed. For instance:

• Sandstone and quartzite break down into sandy soil.
• Shale turns into clay soil.
• Igneous and metamorphic rocks also create sandy and clay soils.

Soil is not just made of rock particles; it also contains organic matter, which comes from decayed plants and animals. This organic material is important because it provides nutrients for plants. Soils with a lot of organic matter are dark in color and are called loam.

Types of Soil

Sandy Soil

• Drainage: Water drains through sandy soil easily because the particles are large and there are big spaces between them.
• Texture: The texture is rough and crumbly due to the large particles.
• Air and Water: The large spaces between particles contain air and water, which means sandy soil has plenty of both.

Clay Soil

• Particle Size: Clay soil is made up of very small particles that are packed tightly together.
• Texture: The texture is smooth and sticky because the particles are so small.
• Air Spaces: The spaces between the particles are very small, so there isn’t much air in clay soil.
• Water Retention: Clay soil absorbs and holds water well, but it doesn’t drain easily. This can lead to waterlogging.
• Drying: In hot, dry weather, clay soil can dry out and crack.

Loam Soil

• Composition: Loam soil is a mix of sand and clay, so it has particles of different sizes.
• Particle Packing: The particles are neither too tightly packed nor too loose, allowing for good air circulation.
• Organic Matter: The organic matter in loam soil helps it retain water and provides plenty of nutrients.
• Nutrients: Loam soil is rich in nutrients, making it ideal for growing plants.

Modifying Soil Composition

• Different plants thrive in different types of soil. Loam is considered the ideal soil for plant growth due to its balanced composition. However, if a plant requires soil that drains very well, a farmer or gardener might mix sand into the soil to create sandy loam.
• Some plants have a specific need for lime in the soil to grow effectively. In such cases, the farmer or gardener will add lime to the soil to meet this requirement.
• Soil also needs to contain air to maintain its composition. This is one of the reasons why farmers and gardeners dig or plough the soil, as these actions help mix air into it.
• Additionally, soil requires water. If there is insufficient rainfall, farmers and gardeners need to water the soil to ensure it has enough moisture for plant growth.

Compost: Nourishing the Soil

Importance of Organic Matter

• All plants require organic matter for healthy growth. However, plants deplete the nutrients present in this organic matter over time.
• To replenish these nutrients, it is essential for farmers and gardeners to regularly add compost or artificial fertilizers to the soil.

What is Compost?

• Compost is composed of dead organic matter.
• It is beneficial for the soil because it not only provides essential nutrients but also improves the soil's texture.

Making Your Own Compost

• You can create compost at home using food scraps such as vegetable peels, eggshells, overripe fruits, and bones.
• Additionally, garden waste like grass clippings, leaves, and weeds can be added.
• Keep the mixture moist and place it in a warm location.
• In a few weeks, you will have nutrient-rich compost!

Artificial Fertilizers

• Artificial fertilizers provide the same nutrients as organic matter but do not enhance the soil's texture like compost does.

Damaging Soil

Soil composition is crucial for the healthy growth of plants. However, sometimes farmers and gardeners neglect the soil, leading to changes in its composition.

• When farmers plant the same crop every year, it depletes the soil of its nutrients. As a result, the soil becomes dry and less suitable for growing plants.
• The use of pesticides by farmers to eliminate weeds and insects can harm the organic matter in the soil as well. Pesticides are intended to kill harmful organisms, but they can also disrupt the delicate balance of life in the soil.
• Artificial fertilizers, while providing nutrients to plants, can adversely affect the soil's texture and moisture levels. These fertilizers may alter the soil structure and lead to increased dryness.
• Regularly digging and turning over the soil is essential to mix air and organic matter throughout it. If farmers do not do this often enough, the soil can become compacted, and its quality may decline.