The Earth is made up of four main layers, each with different properties:

The Earth's lithosphere (crust + upper mantle) is broken into large pieces called tectonic plates. These plates float on the semi-molten asthenosphere and move due to convection currents in the mantle.

Types of Plate Boundaries

Seafloor Spreading

At divergent boundaries, plates move apart. Magma rises from the mantle through the gap and solidifies to form new oceanic crust. This creates mid-ocean ridges — underwater mountain ranges that are the longest mountain chains on Earth.

Evidence for Seafloor Spreading

• Magnetic striping: Alternating magnetic patterns in rocks on either side of ridges
• Age of rocks: Rocks are youngest at the ridge and oldest furthest away
• Fossils: Similar fossils found on continents now separated by oceans

The ocean floor has distinct features formed by tectonic activity and sedimentation:

Earthquakes

Earthquakes occur when tectonic plates move suddenly at plate boundaries, releasing stored energy as seismic waves. They are most common at convergent and transform boundaries.

Tsunamis

A tsunami is a series of enormous ocean waves caused by:

• Underwater earthquakes (most common cause)
• Volcanic eruptions
• Underwater landslides

How a Tsunami Forms

1. An earthquake causes a sudden vertical displacement of the seafloor
2. This displaces a huge volume of water above it
3. Energy travels outward as waves — in deep water, waves are fast (~800 km/h) but low (< 1m)
4. As waves approach shallow coastlines, they slow down and height increases dramatically (up to 30m+)

Ocean currents are continuous movements of water. There are two main types:

Surface Currents

• Driven by wind in the upper ~400m
• Deflected by the Coriolis effect (Earth's rotation)
• Flow clockwise in Northern Hemisphere, anticlockwise in Southern Hemisphere
• Form large circular patterns called gyres

Thermohaline Circulation (The Global Conveyor Belt)

• Driven by differences in temperature and salinity (which affect density)
• Cold, salty water is very dense — it sinks at the poles
• Warm water at the surface flows towards the poles to replace it
• Creates a global circulation pattern that takes ~1,000 years for one complete cycle

Upwelling

Upwelling occurs when winds push surface water away from a coastline, causing cold, nutrient-rich deep water to rise to the surface. This makes upwelling zones extremely productive for marine life and important fishing grounds.

Tides are the regular rise and fall of sea level caused by the gravitational pull of the Moon (and to a lesser extent, the Sun) on Earth's oceans.

Key Facts About Tides

• Most coastlines experience two high tides and two low tides every ~24 hours 50 minutes
• The Moon has a greater tidal influence than the Sun because it is much closer to Earth
• The tidal range is the difference in height between high and low tide

Spring Tides vs Neap Tides

Wave Anatomy

Waves are caused by wind transferring energy to the ocean surface. Key parts:

• Crest — the highest point of a wave
• Trough — the lowest point of a wave
• Wavelength — the distance between two crests
• Wave height / Amplitude — the distance from trough to crest

Coastal Erosion Processes

Longshore Drift

Longshore drift is the movement of sediment along a coastline. Waves approach the shore at an angle (swash), carrying material up the beach diagonally. The backwash carries material straight back down due to gravity. This creates a zigzag movement of sediment along the coast.