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Home›Earth science

Volcanoes and earthquakes

Lava can reach about 1,200 °C, and one step on the magnitude scale means about 32 times more energy. This course follows molten rock from the mantle to the surface, sorts volcanoes by shape and eruptions by size, visits the Hawaiian hotspot, explains how earthquakes are measured and ends with general safety information.

Progress

About this course

Lava can reach about 1,200 °C, and one step on the magnitude scale means about 32 times more energy. The course follows molten rock from the mantle to the surface. Magma forms many tens of kilometres down, is lighter than the solid rock around it, and is pushed upward by buoyancy and by the gas dissolved in it. It often pauses in a magma chamber a few kilometres below the volcano, and if it finds a way out, an eruption begins.

The six units are magma and lava, types of volcano, eruptions and the VEI, hotspots and Hawaii, earthquakes and magnitude, and staying safe. From VEI 2 up, each step of the explosivity index is ten times more erupted material, and between VEI 1 and 2 the step is a hundred times. Mount St. Helens in 1980 shows how an eruption builds: a magnitude 4.2 earthquake on 20 March gave the first clear sign that the volcano was waking, and on 18 May an earthquake under the bulging north flank set off a huge landslide. The hotspot unit turns numbers into a speed. Kauai and Kilauea are about 528 km apart and Kauai's oldest rocks are about 5.5 million years old, which gives roughly 9.6 cm per year for the Pacific Plate.

Magnitude scales are logarithmic. Each whole number step is a tenfold increase in amplitude and about 32 times more energy, exactly 10 to the power 1.5, or 31.6, so a magnitude 7 releases about 1,000 times the energy of a magnitude 5. The safety unit uses Pinatubo: because scientists forecast the eruption and people were moved out of the danger zones in time, at least 5,000 lives were saved.

Before you start

Volcanoes and earthquakes happen mostly where plates meet, so Plate tectonics is a useful first step, especially its units on convergent boundaries and the Ring of Fire. The Hawaiian hotspot unit makes more sense once you know why Hawaii sits so far from any plate boundary.

When you finish

Afterwards you can sort volcanoes into shield, composite and caldera types, place an eruption on the VEI scale from 0 to 8, and explain how the Hawaiian chain grew over a hotspot. You can also work out that one step in magnitude means about 32 times the energy, put the steps of earthquake safety in order, and say why a first tsunami wave is not the last.

  1. 1Magma and lavaOn Kilauea, a geologist pushed a thermocouple into lava breaking out 3 km from its vent and read 1,100 °C.
  2. 2Types of volcanoMeasured from the sea floor, Mauna Kea and Mauna Loa in Hawaii rise more than 9 km, yet their slopes are gentler than 10 degrees.
  3. 3Eruptions and the VEIThe 1912 eruption of Novarupta in Alaska put out about 30 times as much magma as Mount St. Helens did in 1980.
  4. 4Hotspots and HawaiiThe Hawaiian Islands are more than 3,200 km from the nearest plate boundary, yet Kilauea is one of the most active volcanoes on Earth.
  5. 5Earthquakes and magnitudeA magnitude 7 earthquake shakes the seismograph needle 10 times as much as a magnitude 6, but releases about 32 times as much energy.
  6. 6Staying safePyroclastic flows can move faster than 100 km per hour at more than 400 °C, so the only safe plan is to leave before they start.

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Sources for this course (23)
  • USGS Volcano Hazards Program, About Volcanoes (usgs.gov/programs/VHP/about-volcanoes): magma and lava, cinder cones, composite volcanoes, shield volcanoes, lava domes, about 1,350 potentially active volcanoes
  • USGS VHP, Temperatures at the surface reflect temperatures below the ground (lava up to about 1,200 °C; Kilauea flow 1,100 °C)
  • USGS FAQ How much of the Earth is volcanic? (more than 80% of the surface)
  • British Geological Survey, Discovering Geology: Types of volcano (bgs.ac.uk/discovering-geology/earth-hazards/volcanoes/how-volcanoes-form/): slopes 30 to 35° and under 10°, Tambora caldera 6 km wide and 1.1 km deep
  • BGS Volcanic hazards (pyroclastic flows 100 to 600 °C, 110 km/h or faster)
  • USGS Yellowstone Volcano Observatory, The Volcanic Explosivity Index (usgs.gov/observatories/yvo/news/volcanic-explosivity-index-a-tool-comparing-sizes-explosive-volcanic) and VHP image description of the VEI (VEI 0 under 10,000 m³, VEI 8 over 1,000 km³, tenfold steps above VEI 2)
  • US National Park Service, Volcanic Explosivity Index (nps.gov/subjects/volcanoes/volcanic-explosivity-index.htm): Novarupta 1912 about 30 times Mount St. Helens, Huckleberry Ridge 2,450 km³
  • Smithsonian Global Volcanism Program eruption records (volcano.si.edu): VEI of Mount St. Helens 1980 (5)
  • Pinatubo 1991 (6)
  • Tambora 1815 (7)
  • USGS Fact Sheet 113-97, The cataclysmic 1991 eruption of Mount Pinatubo (pubs.usgs.gov/fs/1997/fs113-97/)
  • USGS This Dynamic Earth, Hotspots and The long trail of the Hawaiian hotspot (pubs.usgs.gov/gip/dynamic/hotspots.html, Hawaiian.html)
  • USGS Hawaiian Volcano Observatory, Evolution of Hawaiian volcanoes (usgs.gov/observatories/hvo/evolution-hawaiian-volcanoes) and Volcano Watch: Kamaʻehuakanaloa (2022)
  • USGS Earthquake Hazards Program, Earthquake magnitude, energy release and shaking intensity (usgs.gov/programs/earthquake-hazards/earthquake-magnitude-energy-release-and-shaking-intensity): tenfold amplitude and about 32 times energy per whole magnitude
  • USGS HVO Volcano Watch, What was that on the Richter scale? (about 31.6 times energy per unit, 1,000 times per two units)
  • USGS FAQ How are earthquakes measured? (moment magnitude)
  • USGS, Are you ready to ShakeOut? (usgs.gov/news/featured-story/are-you-ready-shakeout-october-19): Drop, Cover, and Hold On
  • NOAA National Weather Service, Tsunami safety during a tsunami (weather.gov/safety/tsunami-during)
  • USGS Pyroclastic flow hazards at Mount St. Helens (over 100 km/h, over 400 °C; at least 17 pyroclastic flows on 18 May 1980)
  • USGS Volcanic Ash Impacts, Mt St Helens 1980 emergency management (volcanoes.usgs.gov/volcanic_ash/mount_st_helens_emergency_management.html)
  • USGS Earthquake magnitude page, Northridge (3 to 11 miles deep) and Nisqually (30 to 36 miles deep)
  • General safety information only
  • follow local officials.

Checked: 2026-10-17