Earth's Structure

Layers of the Earth

Earth is not solid throughout — it has distinct layers with different compositions and properties. From the surface inward:

1. Crust (0–35 km deep)

The thinnest layer, forming Earth's solid surface. There are two types:

Type	Thickness	Density	Rock type
Oceanic crust	5–10 km	Denser (~3.0 g/cm3)	Basalt (dark, fine-grained)
Continental crust	30–35 km (up to 70 km under mountains)	Less dense (~2.7 g/cm3)	Granite (light-coloured, coarse-grained)

Because oceanic crust is denser, it sinks beneath continental crust when the two collide (subduction).

2. Mantle (35–2,890 km deep)

The thickest layer, making up about 84% of Earth's volume. The mantle is made of semi-solid (plastic) rock — solid but able to flow very slowly over millions of years.

• Convection currents in the mantle are driven by heat from the core
• Hot material rises, cools, then sinks — these slow currents drag the tectonic plates above them

3. Outer Core (2,890–5,150 km deep)

• Made of liquid iron and nickel at extreme temperature (4,000–5,000°C)
• The movement of this liquid metal generates Earth's magnetic field (the magnetosphere), which protects us from harmful solar radiation

4. Inner Core (5,150–6,371 km deep)

• Made of solid iron and nickel (despite temperatures of 5,000–6,000°C)
• It remains solid because of the enormous pressure — billions of atmospheres compress the iron into a solid state
• The hottest part of Earth

Tectonic Plates

The crust and uppermost mantle together form the lithosphere, which is broken into about 15 major sections called tectonic plates.

These plates:

• Float on the semi-solid mantle beneath (the asthenosphere)
• Move very slowly — a few centimetres per year (about the rate your fingernails grow)
• Are driven by convection currents in the mantle
• Carry continents and ocean floor

Alfred Wegener proposed the theory of continental drift in 1912, noting that:

• The continents appear to fit together like puzzle pieces
• Identical fossils are found on different continents
• Rock types and geological structures match across ocean gaps

This led to the modern theory of plate tectonics.

Plate Boundaries

Where plates meet, dramatic geological events occur:

Convergent Boundaries (Plates Moving Together)

Two possibilities:

• Oceanic-continental collision — the denser oceanic crust is forced under the continental crust (subduction), forming a deep ocean trench and volcanic mountains on the continent (e.g., Andes, Cascades)
• Continental-continental collision — neither plate subducts; both buckle and fold upward, forming fold mountains (e.g., Himalayas, Alps)

Divergent Boundaries (Plates Moving Apart)

• Mid-ocean ridges form as magma rises and creates new oceanic crust (e.g., Mid-Atlantic Ridge)
• On continents, rift valleys form (e.g., East African Rift)
• This process is called sea-floor spreading

Transform (Conservative) Boundaries (Plates Sliding Past Each Other)

• No crust is created or destroyed
• Friction causes earthquakes (e.g., San Andreas Fault in California)

Earthquakes

Earthquakes are sudden tremors caused by the release of energy stored in rocks as tectonic plates move.

Key terms:

• Focus (hypocentre) — the point underground where the earthquake originates
• Epicentre — the point on the surface directly above the focus
• Seismic waves — energy waves that travel through Earth; detected by seismometers
• Richter scale — measures earthquake magnitude (logarithmic; a magnitude 7 is 10 times more powerful than magnitude 6)

Earthquakes are most common at:

• Transform boundaries (sudden slip)
• Convergent boundaries (subduction)
• Divergent boundaries (less commonly)

Volcanoes

Volcanoes form where magma (molten rock) reaches the surface. When it erupts, it is called lava.

Volcanoes commonly form:

• At convergent boundaries — subducting oceanic crust melts; magma rises through the overlying plate
• At divergent boundaries — magma rises to fill the gap as plates separate
• At hotspots — plumes of hot mantle material rise through the middle of a plate (e.g., Hawaii)

A volcanic eruption may produce:

• Lava flows — molten rock on the surface
• Pyroclastic flows — fast-moving clouds of hot gas and rock fragments
• Ash clouds — can affect global climate and aviation
• Gases — including sulfur dioxide, CO2, water vapour

Evidence for Plate Tectonics

Evidence	How it supports plate tectonics
Matching coastlines	Continents fit together (e.g., South America and Africa)
Fossil evidence	Same species found on now-separated continents
Rock type matching	Identical rock formations across ocean gaps
Mid-ocean ridges	New ocean floor created at spreading centres; youngest rock nearest ridge
Magnetic striping	Alternating magnetic patterns in ocean floor rock record pole reversals
GPS measurements	Plates are moving at measurable speeds today

Key Terms

Term	Definition
Crust	Outermost solid layer of Earth
Mantle	Thick layer of semi-solid rock beneath the crust
Outer core	Liquid iron and nickel layer that generates Earth's magnetic field
Inner core	Solid iron and nickel at the centre of Earth
Tectonic plate	Large section of the lithosphere that moves over the mantle
Convection current	Circular flow of fluid driven by temperature differences
Subduction	Process where one tectonic plate is forced beneath another
Epicentre	Point on Earth's surface directly above an earthquake's focus
Magma	Molten rock beneath Earth's surface
Lava	Magma that has reached the surface

Worked Example

Q: Explain how the Himalayas were formed using plate tectonic theory.

The Himalayas formed at a convergent boundary between the Indian plate and the Eurasian plate.

About 50 million years ago, the Indian subcontinent (carried on the Indian plate) collided with Asia (on the Eurasian plate). Both plates carry continental crust, which is less dense — neither could subduct beneath the other.

Instead, the collision caused both plates to buckle and fold upward, pushing rock up to form the Himalayan fold mountains — the tallest mountain range on Earth. The plates are still converging today, so the Himalayas are still slowly rising (a few mm per year).

Common Mistakes

• Saying the mantle is liquid — the mantle is semi-solid (plastic); it can flow very slowly but is not truly liquid. The outer core is liquid.
• Confusing the focus and epicentre — the focus is underground where the earthquake starts; the epicentre is the surface point directly above it.
• Thinking volcanoes only occur at convergent boundaries — they also occur at divergent boundaries and hotspots.
• Confusing magma and lava — magma is underground; lava is what you call it once it reaches the surface.

Tips and Tricks

• Layers from outside in: Crust, Mantle, Outer core, Inner core — CMOI (or remember "Crust Makes Our Interior").
• Oceanic crust = basalt (dense, sinks); Continental crust = granite (less dense, floats).
• At convergent boundaries: if oceanic meets continental → subduction (ocean trench + volcanic mountain); if continental meets continental → fold mountains.
• The outer core is liquid; the inner core is solid — despite being hotter, the pressure keeps it solid.