Earthquakes and volcanoes Revision Notes

The Earth is composed of three primary layers, each with distinct properties and roles in geological processes. The outermost layer is the crust, a thin, solid shell that forms the continents and ocean floors. Beneath the crust lies the mantle, a thick layer of semi-solid rock that behaves plastically over long periods, allowing slow convection currents. At the centre is the core, divided into a liquid outer core and a solid inner core, primarily composed of iron and nickel. The core generates Earth's magnetic field through its dynamic movements.

The crust is fragmented into large pieces called tectonic plates, which float on the more ductile upper mantle (the asthenosphere). These plates move slowly due to mantle convection, slab pull, and ridge push forces. Understanding these layers is essential to grasp how geological phenomena like earthquakes and volcanoes occur.

• Crust: thin, solid outer layer forming continents and ocean floors.
• Mantle: thick, semi-solid layer beneath the crust with convection currents.
• Core: inner part with liquid outer core and solid inner core, generates magnetic field.
• Tectonic plates are fragments of the crust moving on the mantle.

Tectonic plates interact at boundaries where their movements cause geological activity. There are three main types of plate boundaries, each with characteristic movements and associated hazards.

Constructive boundaries occur where plates move apart. Magma rises from the mantle to fill the gap, creating new crust. This process forms mid-ocean ridges and volcanic activity, such as the Mid-Atlantic Ridge.

Destructive boundaries happen when an oceanic plate collides with a continental plate or another oceanic plate. The denser oceanic plate subducts beneath the lighter plate, melting and forming magma that can lead to volcanic eruptions. This collision also causes intense earthquakes due to friction and pressure build-up, as seen along the Pacific Ring of Fire around the western coasts of the Americas and eastern Asia. The collision of two continental plates can create mountain ranges, like the Himalayas, but typically fewer volcanoes occur here due to lack of subduction of oceanic crust.  Conservative boundaries involve plates sliding past each other horizontally. The friction between plates prevents smooth movement, causing stress to accumulate until it is released as an earthquake. The San Andreas Fault in California is a classic example. Volcanoes are generally not formed at conservative boundaries because there is no magma generation.

• Constructive: plates move apart, magma rises, new crust forms, volcanoes common.
• Destructive: plates collide, oceanic plate subducts, earthquakes and volcanoes occur.
• Conservative: plates slide past each other, earthquakes occur, no volcanoes.

Earthquakes and volcanoes are primarily caused by the movement of tectonic plates at their boundaries. At constructive boundaries, the creation of new crust and magma movement can cause volcanic eruptions and minor earthquakes.

At destructive boundaries, subduction leads to melting of the oceanic plate, generating magma that rises to form volcanoes. The collision and subduction process also causes powerful earthquakes due to the sudden release of built-up stress.

At conservative boundaries, the plates grind past each other, and the sudden slip after stress accumulation causes earthquakes. Since there is no magma generation, volcanoes are absent here.

• Plate movements cause stress and friction, leading to earthquakes.
• Magma generation at constructive and destructive boundaries causes volcanoes.
• Subduction zones are hotspots for powerful earthquakes and volcanic eruptions.
• Conservative boundaries cause earthquakes but no volcanoes.

The strength of an earthquake is measured by its magnitude, which quantifies the energy released during the event. Magnitude is a logarithmic measure, meaning that each whole number increase represents a tenfold increase in the amplitude of seismic waves recorded by seismographs.

For example, an earthquake of magnitude 6 releases approximately 31.6 times more energy than one of magnitude 5. This logarithmic scale helps compare earthquakes of vastly different sizes in a manageable way.

Magnitude is distinct from intensity, which describes the effects of an earthquake at specific locations. Magnitude is a single value representing the earthquake's overall size.

• Magnitude measures energy released by an earthquake.
• Logarithmic scale: each increase by 1 means 10 times greater wave amplitude.
• Energy release increases roughly 31.6 times per magnitude unit.
• Magnitude is a single value; intensity varies by location.

The Richter scale is a numerical scale developed to quantify earthquake magnitude based on the amplitude of seismic waves recorded by seismographs. It is logarithmic, so a magnitude 7 earthquake produces seismic waves ten times larger than a magnitude 6.

The scale has no upper limit, but most recorded earthquakes fall between magnitudes 0 and 9. The Richter scale allows scientists and emergency services to assess the potential damage and respond accordingly.

For example, a magnitude 4 earthquake might be felt but rarely causes damage, while a magnitude 7 earthquake can cause severe destruction over a wide area.

• Richter scale measures earthquake magnitude from seismograph data.
• Logarithmic: each step up means 10 times larger wave amplitude.
• No fixed upper limit; typical earthquakes range 0–9.
• Helps estimate potential damage and emergency response needs.