Earth and its Habitats Chapter Notes | Year 4 Science IGCSE (Cambridge) - Class 4 PDF Download

The Structure of the Earth

Geological Dynamics

External Structure: The visible surface of the Earth is made up of rocks and seawater.

Internal Structure: The internal structure consists of layers beneath the Earth's surface that are not visible to us. Scientists use models to represent these layers because we cannot cut open the Earth to see them.

Role of Models: Models are essential in science to help us understand how things work or to visualize structures that we cannot see. For example, a model of the Earth's internal structure is like cutting open a peach to see its layers. These models are simplified versions that aid our understanding, not exact replicas.

Layers of the Earth: The Earth's internal structure comprises three main layers: the crust, mantle, and core.

Crust: The crust is the thin, outermost layer where humans reside. It is composed of rocks and varies in thickness, being about 5 km thick under the oceans and up to 70 km thick under land. The temperature ranges from 20 °C at the surface to 400 °C at its deepest parts.

Mantle: The mantle lies beneath the crust and is approximately 3000 km thick, making it much thicker than the crust. It consists of rocks that have melted due to high temperatures, forming a substance called magma. Magma behaves like a liquid and can move within the mantle. Temperatures in the mantle can reach up to 1000 °C.

Core: The core is about 3400 km thick and is divided into the outer core and inner core. It is primarily composed of the metals iron and nickel.

Outer Core: The outer core is made up of liquid metals due to high temperatures and lower pressure.

Inner Core: The inner core is solid because of the immense pressure from the layers above it. Initially estimated to be around 5000 °C, recent measurements in 2014 suggested that the inner core's temperature is about 6000 °C, comparable to the surface of the Sun.

Volcanoes

• How Volcanoes Form: Volcanoes are created when large cracks form in the Earth's crust. Magma from the mantle rises through these cracks and erupts onto the surface. Once it reaches the surface, magma is called lava.
• What Happens to Lava: Lava can reach temperatures of up to 1000 °C. When it's very hot, lava flows quickly over the Earth's surface. As it cools down, it hardens into rock, forming various geological features.
• Example of Lava Formation: The Hawaiian Islands were formed from layers of cooled and hardened lava, resulting in a flat surface known as a volcanic plateau.
• Materials Released by Volcanoes: In addition to lava, volcanoes can release rocks, volcanic ash, and gases. These materials travel through a central pipe called a vent. During an eruption, the vent widens at the surface, creating a crater.

Types of Volcanoes:

• Composite Volcano: This is the most common type of volcano, characterised by layers of ash and lava. The ash comes from burnt lava, and together with lava, it forms a cone-shaped mountain with smooth sides. Some magma escapes through cracks in the volcano's sides, creating smaller volcanoes known as secondary cones.
• Volcanic Plateau: This type of volcano is formed by layers of lava that flow, cool, and harden into flat surfaces.

Where Volcanoes Are Found: Volcanoes are more likely to be found in areas with many cracks in the Earth's crust, indicating a higher risk of eruption. Regions without such cracks have a low risk of volcanic activity. One significant volcanic zone is the Pacific Ring of Fire, which surrounds the Pacific Ocean and has many active volcanoes in countries like Indonesia, the Philippines, Japan, Hawaii, Mexico, Chile, and New Zealand. Indonesia, in particular, is prone to deadly eruptions, such as the 1812 Mount Tambora eruption that resulted in the deaths of 92,000 people.

Effects of Volcanoes:

• Negative Effects: Volcanoes can cause harm by releasing ash and lava that can injure or kill people. Lava can burn and bury crops and buildings, while ash can cover structures and crops, disrupting agriculture and infrastructure. Breathing in ash and toxic gases can also lead to health problems.
• Positive Effects: On the other hand, magma-heated underground water can be used for heating homes and generating electricity. Hot springs, warmed by magma, attract tourists. Additionally, lava creates fertile soil, and ash acts as a natural fertilizer, improving crop growth. Volcanic eruptions can also draw tourists, creating jobs in volcanic regions.

Case Studies of Volcanoes:

• Mount Etna (Sicily): Mount Etna is a composite volcano known for producing large clouds of ash and jets of lava that can reach heights of 100 to 200 meters. The lava flows from this volcano can cause destruction by damaging buildings and power lines, forcing thousands of residents to evacuate.
• Goma Volcano (Democratic Republic of Congo): In Goma, a 500-meter-long crack in the ground allowed lava to flow, contaminating lake water with ash and gases. This volcanic event resulted in 45 deaths due to poisonous gases and displaced around 500,000 people. The area eventually formed a volcanic plateau due to the extensive layers of lava that covered the ground.

Earthquakes

Understanding Earthquakes: An earthquake is the shaking of the Earth caused by sudden movements in its crust. These events can lead to significant damage, such as buildings collapsing, infrastructure like railway lines being harmed, and landslides that can bury towns and agricultural land.

Real-Life Examples: The 2015 Nepal earthquake resulted in over 9,000 deaths and 23,000 injuries, while the 2017 Mexico earthquake caused damage to railway lines. Worldwide, about 500,000 earthquakes occur each year, with around 100 being strong enough to cause serious harm.

How Earthquakes Occur: Earthquakes happen when rocks in the Earth's crust, which is made up of large, flat pieces, suddenly move. At the point where two rock pieces meet, they rub against each other, generating a lot of energy. This energy transforms into waves that travel through the crust to the surface, where we feel them as an earthquake. The focus is the point inside the crust where the earthquake begins, and the epicentre is the point on the surface directly above the focus, experiencing the most severe impact.

Areas Prone to Earthquakes: Areas that are prone to earthquakes often overlap with regions of high volcanic activity, particularly where the Earth's crust is fractured and in motion. The Pacific Ring of Fire is a notable earthquake-prone area due to the presence of many crustal faults.

What is a Tsunami? 
A tsunami is a massive sea wave triggered by an underwater earthquake or volcanic eruption. The energy from the seismic event transfers to the sea, creating large waves. As these waves approach shallower coastal areas, they can increase in height (up to 50 meters), leading to flooding.

Example of Tsunami Impact: The 2011 Japan tsunami resulted in extensive coastal damage, with waves pushing objects like boats onto buildings.

Case Study: Chile Earthquake: Chile is highly susceptible to earthquakes due to its location in the Pacific Ring of Fire, where crustal rocks are actively shifting. Earthquakes in Chile can also trigger tsunamis, affecting distant regions like California and Japan as the energy travels through ocean waves. The consequences of these earthquakes include building destruction, power outages, and loss of life, with impacts felt in neighbouring countries such as Peru.

Different Habitats

Adaptations of Animals in Different Habitats: Animals are adapted to their habitats based on their needs, with physical features that enhance their survival.

Fish: Fish have gills for breathing underwater and fins for swimming, which help them thrive in aquatic environments. Different species of fish are suited for either warm or cold waters. Fish can withstand small, gradual temperature changes caused by climate change. In artificial environments like aquariums, fish can survive with proper care, including maintaining suitable water conditions.

Birds: Birds adapt to various habitats with wing shapes that facilitate flight and beak shapes that are adapted to their diet.  Habitat-Specific Diets:  Birds have different diets depending on their habitat:

• Birds in river or sea habitats eat fish (e.g., kingfishers).
• Birds in grassland habitats consume seeds or insects (e.g., pigeons, starlings).
• Birds in tree-rich habitats eat fruits, seeds, and small animals.

Beak Adaptations: Pigeon: Small, pointed beak for cracking open seeds. Starling: Longer, pointed beak for digging insects and worms from the ground. Kingfisher: Large, sharp-pointed beak for catching fish in rivers. Flamingo: Long, flat beak to strain small plants and animals from water.

Birds can also thrive in urban habitats, such as parks and gardens, by exploiting various food sources. For example, seagulls, typically coastal birds, can survive inland by finding alternative food sources like scraps or insects.  Tigers:  Tigers are adapted to live in forested or grassy areas, with features like camouflaged coats that help them hunt and strong limbs for capturing prey. In zoos, tigers can survive with provided food and controlled environments that mimic their natural conditions.  Plants in Non-Natural Habitats:  Plants can thrive outside their natural environments with human support. Orchids: Orchids are naturally suited to hot, wet forest habitats but can be grown indoors in pots with regular watering and humidity control. Bamboo: Bamboo naturally thrives in warm, wet Southeast Asian climates but is cultivated worldwide in gardens with sufficient water and care. Garden plants can adapt to non-native environments through resources provided by gardeners, such as water, nutrients, and protection from extreme conditions.