Hawaii's volcanoes pour out lava that runs like a river and builds broad, gentle mountains, while other volcanoes pile sticky lava near the vent into steep, dome-shaped peaks and can explode violently.
The difference comes from how sticky the magma is, its viscosity, which rises with the amount of silica (SiO₂) in the magma.
When an earthquake strikes underground, the fast P wave arrives first and the slower but stronger S wave later, and the gap between their arrival times (the S-P time) grows the farther you are from where the quake happened.
Change the viscosity to see how the shape of the volcano and the way it erupts change, and change the epicentral distance to compare how the S-P time stretches on the seismogram.
Magma Viscosity and Volcano Shape
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The shape of a volcano and how it erupts depend on the magma's viscosity
5 / 10
💡 Magma and the Shape of Volcanoes
①Magma is molten rock underground; once it flows out at the surface it is called lava.
②Low-viscosity magma flows easily and spreads far, building a broad, gentle shield volcano.
③High-viscosity magma barely flows and piles up near the vent, building a steep lava dome.
④In high-viscosity magma gas cannot escape easily, so pressure builds until it bursts out explosively.
⑤The viscosity values 1 to 10 in the picture are relative, for comparison only.
Volcanic Material and Kinds of Volcano
Comparing volcanoes
Kinds of volcano by magma viscosity
Feature
Shield volcano
Stratovolcano
Lava dome
Shape
Broad and gentle, like a shield
Cone
Rounded and steep, like a bell
Viscosity
Low
Medium
High
Eruption
Lava flows out quietly
Lava flows and explosions alternate
Explosive
SiO₂ content
Low (about 52 % or less)
Medium
High (about 63 % or more)
Color of the rock
Dark (basalt)
Medium (andesite)
Light (rhyolite)
Example
Mauna Loa, Hawaii
Mount Fuji, Japan
Lassen Peak, USA
🌋 What Volcanoes Throw Out
①Volcanic gas: mostly water vapor, mixed with carbon dioxide, sulfur dioxide and more.
②Pyroclastic material: by size, volcanic ash (2 mm or less), lapilli and volcanic bombs.
③Lava: hardens into volcanic rock, becoming basalt, andesite or rhyolite depending on its viscosity.
④Volcanic ash spreads high into the sky and can wreck aircraft engines, but where it settles it becomes fertile soil over time.
Earthquakes and Seismic Waves
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Seismogram: the gap between P- and S-wave arrivals (S-P time)
30 km
S-P time
S-P time = S-wave arrival time − P-wave arrival time
P and S waves travel at different speeds, so the gap between their arrivals widens with distance; the longer the S-P time, the farther away the earthquake.
🔍 Kinds of Seismic Wave
①P wave: the fastest, so it arrives first; its shaking is small, and it passes through solids and liquids.
②S wave: arrives after the P wave but shakes harder and causes more damage; it passes only through solids.
③In the picture, an epicentral distance of 40 km gives an S-P time of about 4.8 s, and 80 km about 9.5 s: it grows in proportion to distance.
④The focus is where the earthquake starts underground; the epicenter is the point on the surface directly above it.
Exam Key Points
Summary
viscosity ↑ → steeper volcano, explosive eruption · S-P time ↑ → distance to focus ↑
Magma viscosity sets the shape of a volcano, and the S-P time gives the distance to an earthquake.
🎯 Exam Points
①The more SiO₂ in magma, the higher its viscosity, the lighter the rock it forms and the more explosive the eruption.
②Order of viscosity: shield volcano (gentle) < stratovolcano (cone) < lava dome (steep).
③P waves are fast with small shaking; S waves are slower with strong shaking.
④The longer the S-P time, the farther the station is from the focus.
⑤Magnitude is the energy released by the earthquake itself; intensity is how strongly the ground shook and how much damage was done at a given place.
Examples and Unit Test
Example 1
What shapes of volcano form when magma viscosity is low and when it is high?
1
With low-viscosity magma, lava flows easily and spreads far, building a broad, gentle shield volcano.
2
With high-viscosity magma, lava barely flows and piles up near the vent, building a steep lava dome, and the eruption is explosive.
▸ Low viscosity = shield volcano (gentle) / high viscosity = lava dome (steep)
Think of pouring water and honey: runny water spreads wide, sticky honey piles up high.
Example 2
At a station, the S wave arrived 8 seconds after the P wave. What is the S-P time, and how does the distance to the epicenter change as the S-P time gets longer?
1
The S-P time is the S-wave arrival time minus the P-wave arrival time.
2
The S wave came 8 s after the P wave, so the S-P time is 8 s. Because the two waves travel at different speeds, the farther away the station, the longer the S-P time.
▸ S-P time = 8 s / the longer it is, the farther the epicenter
Draw circles with the distances measured at three or more stations; the point where they meet is the epicenter.
Unit-test style
Which statement about volcanoes and earthquakes is correct?
①The higher the magma viscosity, the gentler the volcano
②The longer the PS time, the farther the distance to the epicenter
③The S wave arrives before the P wave
④The S wave passes through both solids and liquids
⑤Low-viscosity magma erupts explosively
▸ ② The longer the PS time, the farther the distance to the epicenter
1
The S-P time grows with distance from the focus, so a longer S-P time means a farther epicenter; ② is correct.
2
Higher viscosity makes steeper volcanoes (①), P waves arrive before S waves (③), S waves pass only through solids (④), and low-viscosity magma flows out quietly (⑤).