The internal structure of the Earth

The Earth's structure consists of distinct layers, each with unique properties that influence geological processes. Knowledge of these layers comes from studying seismic waves and volcanic materials, as direct observation is impossible.

Main layers of the Earth

• Core - Located at the Earth's centre, this layer generates intense heat through radioactive decay of elements like uranium. It divides into two parts:

  • Inner core - A solid sphere composed mainly of iron and nickel.
  • Outer core - A molten layer also containing iron and nickel.

• Mantle - Surrounding the core, this thick layer consists primarily of silicate rocks, which are rocks rich in silicon and oxygen. A geothermal gradient exists here, meaning temperatures decrease from the core outwards.

  • Lower mantle - Closer to the core, this section is rigid but exhibits plasticity, allowing rocks to deform under stress without breaking.
  • Upper mantle (asthenosphere) - Semi-molten and capable of slow flow, this cooler region is weaker and more ductile, allowing it to deform slowly rather than fracture. Earthquake foci often occur in the rigid lithosphere above due to its brittleness.

• Lithosphere - Comprising the uppermost mantle and the crust, this rigid outer layer forms the Earth's surface.

Types of crust in the lithosphere

The crust, the outermost part of the lithosphere, varies in thickness and composition, forming tectonic plates that float on the asthenosphere.

These differences affect how plates interact at their boundaries.

The movement of tectonic plates

Tectonic plates are large, rigid sections of the lithosphere that cover the Earth's surface and move slowly over the asthenosphere. This movement shapes continents and oceans over geological time.

Major tectonic plates

There are seven major tectonic plates: African, Antarctic, Eurasian, Indo-Australian, North American, Pacific, and South American. Numerous minor plates also exist, fitting together like a puzzle across the globe.

Continental drift and Pangaea

Continental drift describes the gradual movement of continents due to plate motion. Around 300 million years ago, all continents formed a single super-continent called Pangaea. Over time, plates have shifted, causing continents to separate and occasionally recombine.

This ongoing drift explains current continental positions and provides evidence for plate tectonics theory.

Causes of tectonic plate movement

Plate movement results from forces within the mantle. While early theories emphasised convection currents, modern understanding highlights slab pull and ridge push as primary drivers.

Convection currents in the mantle

Convection currents are circular flows of semi-molten rock in the asthenosphere, driven by heat differences.

How convection currents work:

1. Heat from radioactive decay warms the lower asthenosphere, making rock less dense so it rises slowly.
2. Near the top, the rock cools, becomes denser, and sinks.
3. This cycle creates drag on the base of tectonic plates, contributing to their movement.

Although important, convection currents are now seen as secondary to other forces.

Slab pull

Slab pull occurs at convergent plate boundaries where one plate sinks beneath another.

How slab pull works:

1. Convection currents heat and thin the crust near rising limbs, making distant sections cooler and denser.
2. The denser edge sinks into the mantle, pulling the entire plate along like a weight on a rope.

This force is particularly strong with dense oceanic plates.

Ridge push

Ridge push happens at divergent boundaries where new crust forms.

How ridge push works:

1. Rising magma heats and expands surrounding rocks, creating an elevated slope.
2. As the new crust cools and densifies, gravity pulls it downslope, pushing plates apart.

This mechanism complements slab pull in driving plate motion.

Evidence for seafloor spreading from palaeomagnetism

Seafloor spreading is the process where tectonic plates diverge, allowing magma to form new crust and widen ocean basins. Palaeomagnetism, the study of ancient magnetic fields preserved in rocks, provides key evidence.

Process of seafloor spreading

1. Plates move apart at divergent boundaries, creating gaps filled by rising magma.
2. The magma cools to form new crust, which is then pushed aside as more magma emerges.
3. Under oceans, this creates mid-ocean ridges, such as the Mid-Atlantic Ridge, where plates like the Eurasian and North American separate.
4. On land, it forms rift valleys, like the East African Rift System extending from Jordan to Mozambique.

Older rock lies at continental edges, while the youngest rock is at the ridges.

Role of palaeomagnetism

Magma contains iron-rich minerals that align with Earth's magnetic field upon cooling, recording polarity.

How palaeomagnetism provides evidence for seafloor spreading:

• Earth's magnetic field reverses every 200,000 years (last reversal 780,000 years ago).
• This creates alternating magnetic stripes on either side of mid-ocean ridges.
• Studying these stripes reveals formation dates and plate separation rates, confirming seafloor spreading.

The process of subduction and its seismic effects

Subduction occurs when denser crust sinks beneath less dense crust at convergent boundaries, recycling material into the mantle.

How subduction works

• Oceanic crust, being denser, typically subducts under continental crust.
• The sinking plate melts under intense heat and pressure.

Seismic activity in subduction zones

• The Wadati-Benioff Zone traces the subducting plate's path, marked by high seismicity (frequency and number of earthquakes).
• Friction builds along 'locked' faults, where plates stick together for years or centuries.
• When stress exceeds a threshold, the fault releases, causing an earthquake.

This process links subduction to volcanic and seismic hazards.