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examBoard: Cambridge
examType: IGCSE
lessonTitle: Earths Structure and Layers
Imagine the Earth as a giant peach. Just like a peach has different layers (the skin, the juicy part and the stone in the middle), our planet is made up of several distinct layers too. Understanding these layers helps us explain why earthquakes happen, how mountains form and why volcanoes erupt where they do.
Key Definitions:
The Earth is divided into four main layers:
Scientists discovered these layers by studying how seismic waves from earthquakes travel through the Earth at different speeds and directions.
Temperature: Increases with depth - from about 15°C at the surface to over 5,500°C at the inner core.
Pressure: Also increases with depth - at the centre, pressure is more than 3 million times that at the surface!
Density: The deeper you go, the denser the material. The inner core is almost as dense as pure iron.
State: From solid rock (crust) to semi-solid (mantle) to liquid metal (outer core) to solid metal (inner core).
The crust is the Earth's thinnest layer but the most important to us as it's where we live! It's not one continuous piece but is broken into sections called tectonic plates.
Thickness: 30-70km (thicker than oceanic crust)
Composition: Mainly granite rocks
Density: Less dense (about 2.7 g/cm³)
Age: Can be very old - up to 4 billion years!
Features: Forms continents and large landmasses
Thickness: 5-10km (thinner than continental crust)
Composition: Mainly basalt rocks
Density: More dense (about 3.0 g/cm³)
Age: Relatively young - less than 200 million years
Features: Forms the ocean floors
The difference in density between these two types of crust is crucial for understanding plate tectonics. When they meet, the denser oceanic crust usually sinks beneath the lighter continental crust in a process called subduction.
Beneath the crust lies the mantle, making up about 84% of Earth's volume. Although we often think of it as molten lava, most of the mantle is actually solid but can flow very slowly over long periods - like extremely thick treacle.
The mantle is divided into several parts:
Extends to about 410km depth. The uppermost part combines with the crust to form the lithosphere (the rigid outer shell of Earth).
A partially molten, weak layer in the upper mantle (100-350km depth) where rocks can flow. This is where tectonic plates "float" and move.
The deepest part of the mantle (660-2,900km depth), which is more rigid due to immense pressure despite high temperatures.
At the centre of our planet lies the core, divided into two parts: the outer core (liquid) and the inner core (solid). Both are made primarily of iron and nickel.
The outer core is a swirling mass of liquid metal about 2,200km thick. Its movement creates Earth's magnetic field, which protects us from harmful solar radiation and helps with navigation.
Temperature: 4,500-5,500°C (hotter than the surface of the Sun!)
Despite being even hotter than the outer core, the inner core is solid due to the enormous pressure. It's a solid ball of metal about 1,250km in radius.
Temperature: About 5,500°C
The inner core rotates slightly faster than the rest of the Earth, spinning eastward about 0.3-0.5 degrees per year faster than the surface.
The Earth's crust and uppermost mantle (lithosphere) are broken into large pieces called tectonic plates. These plates "float" on the semi-fluid asthenosphere below and move very slowly - about the same rate as your fingernails grow!
The movement of tectonic plates is driven by convection currents in the mantle:
Think of it like a pot of boiling water - the hot water rises to the top, spreads out, cools and then sinks again, creating a continuous cycle.
Scientists have never been able to drill more than about 12km into the Earth's crust, so how do we know what's inside?
Seismic Waves: When earthquakes happen, they send waves through the Earth. By measuring how these waves travel, bend and reflect, scientists can work out what materials they're passing through.
Meteorites: Some meteorites are fragments of planetary cores, giving us clues about Earth's core composition.
Volcanic Eruptions: Magma that reaches the surface comes from the mantle, providing samples of this otherwise inaccessible layer.
Laboratory Experiments: Scientists can recreate the extreme pressures and temperatures of Earth's interior to see how materials behave.
In 1909, Croatian seismologist Andrija Mohorovičić discovered a boundary between the crust and mantle (now called the "Moho") by noticing that seismic waves suddenly speed up at this depth.
There are seven major tectonic plates and numerous smaller ones. The boundaries where these plates meet are the most geologically active areas on Earth - where most earthquakes, volcanoes and mountain building occur.
Where plates move apart, creating rifts or ocean ridges. New crust forms as magma rises to fill the gap.
Example: Mid-Atlantic Ridge
Where plates push together. One plate may sink beneath the other (subduction), or both may crumple up to form mountains.
Example: Andes Mountains (oceanic Nazca Plate subducting under continental South American Plate)
Where plates slide past each other horizontally, creating fault lines where earthquakes are common.
Example: San Andreas Fault in California
Understanding Earth's structure and plate tectonics helps explain why tectonic hazards like earthquakes and volcanoes occur where they do. The theory of plate tectonics has revolutionised our understanding of Earth processes and is sometimes called the "unifying theory of geology" because it explains so many different Earth features and phenomena.
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