Key definitions related to volcanoes and earthquakes

Below are the fundamental terms and their meanings, covering both volcanic structures and seismic events.

Volcanic terminology

• Crater - A depression at the top of a volcano following a volcanic eruption. It may contain a lake.
• Lava - Molten magma that has reached the Earth's surface. It may be liquid or may have solidified.
• Magma - Molten rock within the Earth. When magma reaches the surface it is called lava.
• Magma chamber - The reservoir of magma located deep inside the volcano.
• Vent - The channel through which volcanic material is ejected.
• Ash - Very fine-grained volcanic material.
• Cinders - Small-sized rocks and coarse volcanic materials.
• Pyroclastic flow - Superhot (700°C) flows of ash, pumice (volcanic rocks) and steam at speeds of over 500km per hour.
• Dormant volcano - Volcanoes which have not erupted for a very long time but could erupt again.
• Active volcano - A volcano currently showing signs of activity.
• Extinct volcano - A volcano which has shown no signs of volcanic activity in historic times.

Earthquake terminology

• Epicentre - The point on the Earth's surface directly above the focus of an earthquake. The strength of the shock waves generally decrease away from the epicentre.
• Focus - The position within the Earth where an earthquake occurs. Earthquakes may be divided into shallow-focus and deep-focus earthquakes depending on how far below the Earth's surface they occur.
• Intensity - The power of an earthquake is generally measured using the Richter scale or sometimes the Mercalli scale.
• Richter scale - An open-ended scale to record magnitude of earthquakes – the higher the number on the scale, the greater the strength of the earthquake. There are more small earthquakes than large earthquakes.
• Mercalli scale - Relates ground movement to commonplace observations of, for example, light bulbs, book cases and building damage.

Different types of volcanoes and their characteristics

Volcanoes vary in shape and structure depending on the nature of the lava they produce. The type of lava influences whether the volcano has gentle slopes or steep sides, and their locations often correlate with specific geological settings.

Categories of volcanoes based on lava type

• Shield volcano - Gently sloping volcano produced by very hot, runny lava.
• Cone volcano - Steeply sloping volcano produced by thick lava.

Composite volcanoes may have several secondary cones rather than just a single cone.

Features and measurement scales of earthquakes

Earthquakes are sudden and intense movements within the Earth's crust, releasing energy in the form of seismic waves. Their impact and strength are measured using specific scales that provide insight into their power and effects.

Core features of earthquakes

• Sudden movement - An earthquake is a sudden, violent movement of the Earth.
• Measurement scales - Two primary scales are used to assess earthquakes:
  • Richter scale - The Richter scale is logarithmic, so an earthquake measuring 7.0 on the Richter scale is 10 times more powerful than one measuring 6.0.
  • Mercalli scale - Focuses on the observable impacts, such as structural damage or the movement of everyday objects, to gauge intensity.

Understanding earthquake strength

In terms of frequency and magnitude, there are more small earthquakes than large earthquakes.

Locations and examples of volcanic formations

The distribution of volcanoes is closely linked to tectonic activity, with specific types of volcanoes found in distinct geological zones. Real-world examples help illustrate these patterns and characteristics.

Typical locations of volcanic types

• Composite volcanoes - Found at destructive plate margins.
• Shield volcanoes - Found at constructive plate margins and hotspots.

Notable examples of volcanoes

• Mount Vesuvius (Italy) - A typical composite volcano with multiple historic eruptions.
• Kilauea (Pacific) - Has produced extensive lava flows due to its fluid basaltic magma.