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examBoard: Cambridge
examType: IGCSE
lessonTitle: Earths Structure: Crust, Mantle and Core
Imagine the Earth as a giant peach - with distinct layers that get hotter and denser as you travel inward. Our planet isn't a uniform ball of rock, but rather a complex structure with layers that have different properties and compositions. Understanding these layers helps explain why earthquakes rumble and volcanoes erupt.
Key Definitions:
You might wonder how scientists know what's inside Earth when the deepest hole ever drilled (the Kola Superdeep Borehole) only reached about 12 km - just a tiny fraction of the way to the centre (6,371 km). The answer lies in studying seismic waves from earthquakes. These waves change speed and direction when they pass through different materials, giving scientists clues about what's inside our planet. It's like using sound waves to create a scan of Earth's interior!
Radius: 6,371 km
Mass: 5.97 × 10^24 kg
Average density: 5.51 g/cm³
Surface temperature: -88°C to 58°C
Core temperature: Up to 5,500°C (similar to the Sun's surface!)
Age: About 4.54 billion years
The crust is the thinnest layer of Earth - comparable to the skin of an apple relative to the whole fruit. Despite being so thin, it's where all human activity takes place and it's the source of most natural resources we use.
Thickness: 30-70 km (thickest under mountain ranges)
Composition: Mainly granite-like rocks rich in silicon and aluminium
Density: About 2.7 g/cm³
Age: Can be billions of years old
Features: Forms the continents and continental shelves
Thickness: Only 5-10 km
Composition: Mainly basalt-rich in iron and magnesium
Density: About 3.0 g/cm³ (denser than continental crust)
Age: Relatively young, less than 200 million years
Features: Forms the ocean floors
The difference in density between continental and oceanic crust is crucial for plate tectonics. When these crusts collide, the denser oceanic crust typically sinks beneath the lighter continental crust in a process called subduction, which can trigger earthquakes and volcanic activity.
Making up about 84% of Earth's volume, the mantle is a 2,900 km thick layer of hot, semi-solid rock. Though it's solid, it can flow very slowly over millions of years - similar to how silly putty behaves.
Extends to about 410 km depth. Contains the asthenosphere (100-350 km deep), a partially molten layer that allows tectonic plates to move. Temperature ranges from about 500°C to 900°C.
From 410-660 km depth. Contains minerals that change crystal structure due to increasing pressure, creating distinct boundaries that reflect seismic waves.
From 660 km to the core-mantle boundary at 2,900 km. Extremely hot (up to 4,000°C) but remains solid due to immense pressure. Makes up the bulk of the mantle.
The mantle's composition is mainly silicate rocks rich in magnesium and iron. The slow convection currents in the mantle are the main driving force behind plate tectonics - as hot material rises and cooler material sinks, it creates movement that pushes and pulls the tectonic plates above.
The mantle moves at about the same rate as your fingernails grow - just a few centimetres per year! Yet this incredibly slow movement is powerful enough to build mountain ranges, create ocean basins and shape Earth's surface over millions of years.
At Earth's centre lies the core - a ball of mostly iron and nickel divided into two distinct parts. The temperature here rivals the surface of the Sun and the pressure is millions of times greater than at Earth's surface.
State: Liquid metal
Thickness: About 2,200 km
Composition: Mainly iron and nickel with some lighter elements
Temperature: 4,500-5,500°C
Key feature: Its flowing metal generates Earth's magnetic field
State: Solid metal (despite higher temperature)
Radius: About 1,220 km
Composition: Nearly pure iron-nickel alloy
Temperature: Up to 5,500°C
Key feature: Solid due to extreme pressure (about 3.5 million times atmospheric pressure)
The liquid outer core is crucial for life on Earth. As our planet rotates, the movement of liquid metal creates electrical currents that generate Earth's magnetic field. This magnetic field acts like a shield, protecting us from harmful solar radiation and preventing our atmosphere from being stripped away by the solar wind.
Earth's magnetic field is created by a process called the geodynamo. As the solid inner core grows (at about 1mm per year), it releases heat that helps drive convection in the liquid outer core. Combined with Earth's rotation, this creates a self-sustaining magnetic field. Evidence from ancient rocks shows that this magnetic field has occasionally reversed polarity throughout Earth's history - with the north and south magnetic poles switching places! The last reversal happened about 780,000 years ago and some scientists think we may be due for another one.
Earth's layered structure directly influences the occurrence of earthquakes and volcanoes. The movement of tectonic plates, which float on the semi-fluid asthenosphere, creates stress that can be suddenly released as earthquakes. When plates collide or separate, it can also create pathways for magma from the mantle to reach the surface as volcanoes.
Most earthquakes occur in the crust or upper mantle. The rigid lithosphere breaks under stress, while the plastic asthenosphere allows for movement. Deep earthquakes (up to 700 km) happen in subduction zones where cold, brittle slabs sink into the mantle.
Magma forms when mantle rock melts, often due to changes in pressure or the addition of water. This less dense magma rises through cracks in the crust. Different types of volcanoes form depending on the magma composition, which varies based on which part of the mantle it comes from.
This horseshoe-shaped zone around the Pacific Ocean contains 75% of Earth's volcanoes and experiences 90% of earthquakes. It exists because the Pacific Plate is being subducted beneath surrounding plates, creating conditions for both earthquakes and volcanoes.
Understanding Earth's internal structure helps scientists predict where earthquakes and volcanoes are likely to occur, which is vital for hazard assessment and disaster preparedness. It also helps explain why some regions experience more tectonic activity than others.
Knowledge of Earth's structure is essential for environmental management. It helps us understand natural resource distribution (like minerals and fossil fuels), identify geothermal energy potential, assess geological hazards and even plan for climate change adaptation. For example, understanding how magma forms in the mantle helps geologists locate potential geothermal energy sites, which can provide renewable energy with lower environmental impacts than fossil fuels.
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