grade 7 Volcanoes & Earthquakes

Volcanoes & Earthquakes

Magma viscosity shapes a volcano. Low viscosity builds a gentle shield; high viscosity makes a steep dome and more explosive eruptions. In quakes the P wave arrives first and the S wave later; a longer PS time means a farther epicenter. Change magma viscosity and epicenter distance and volcano form and seismograph traces line up.

Types of Volcanoes

Magma viscosity5 / 10

💡 Magma and Volcanoes

①Magma: rock melted underground

②Lava: magma reaching the surface

③Low viscosity → flows easily → shield volcano (gentle)

④High viscosity → barely flows → lava dome (steep)

⑤The viscosity slider from 1 to 10 is a relative scale.

Volcanic Materials and Volcano Types

Volcano Comparison

Types and Features of Volcanoes

Aspect	Shield	Stratovolcano	Lava dome
Shape	Wide & gentle	Conical	Steep & pointed
Viscosity	Low	Medium	High
Eruption	Quiet flow	Moderate	Explosive
SiO₂	Low (≤52%)	Medium	High (≥63%)
Color	Dark (basalt)	Medium	Light (rhyolite)
Example	Mauna Loa	Mt. Fuji	Lassen Peak

🌋 Volcanic Materials

①Volcanic gases: water vapor (most), CO₂, SO₂

②Pyroclastics: ash, bombs, lapilli

③Lava forms different rocks based on viscosity

④Volcanic ash is dangerous to airplane engines but enriches soil

Earthquakes and Seismic Waves

Epicenter distance30 km

PS Time

PS-time = arrival of S-wave − arrival of P-wave

Longer PS-time → farther epicenter

🔍 Types of Seismic Waves

①P-wave: fast, small motion, passes through solid + liquid

②S-wave: slow, large motion (more damage), only solids

③PS-time grows with distance — used to find epicenter

④Epicenter: directly above focus / Focus: actual quake source

Exam Key Points

Core Summary

Viscosity↑ → steeper, explosive | PS-time↑ → farther epicenter

Volcanoes: viscosity is key. Earthquakes: PS-time gives distance

🎯 Exam Key Points

①Higher SiO₂ → higher viscosity, lighter color, explosive

②Shield(gentle)-Strato(cone)-Lava dome(steep)

③P-wave fast/small ; S-wave slow/large

④Longer PS-time → farther epicenter

⑤Magnitude = quake energy / Intensity = local shaking

Examples and Unit Test

Example 1

When magma has low viscosity versus high viscosity, what shape of volcano forms in each case?

When magma has low viscosity, the lava flows easily and forms a broad, gentle shield volcano.

When viscosity is high, the lava barely flows and forms a steep lava-dome volcano, and the eruption is also explosive.

▸ Low viscosity = shield volcano (gentle) / high viscosity = lava-dome volcano (steep)

Low-viscosity magma makes a gentle volcano; high-viscosity magma makes a steep, explosive one.

Example 2

At a station, the S wave arrived 8 seconds after the P wave. What is the PS time at this station, and how does the distance to the epicenter change as the PS time gets longer?

The PS time is the S-wave arrival time minus the P-wave arrival time.

Since the S wave arrived 8 seconds after the P wave, the PS time is 8 seconds. The longer the PS time, the farther the epicenter.

▸ PS time = 8 seconds / a longer PS time means a farther epicenter

PS time = S arrival minus P arrival; the larger it is, the farther 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

The PS time gets longer the farther the epicenter, so a longer PS time means a farther epicenter.

Higher viscosity makes a steeper volcano (①), the P wave arrives before the S wave (③), the S wave passes only through solids (④), and low-viscosity magma flows out quietly (⑤).

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