Igneous Rocks & Processes

Unit 7.1

Why are composite volcanoes more dangerous than other volcanoes?

Their high-viscosity magma resists movement until extreme pressures build, causing massive eruptions.

How does a shield volcano form?

Many low-viscosity lava flows stacked on top of each other.

What does low viscosity mean for basaltic (mafic) lava?

It flows easily.

What can be inferred from a rock with a phaneritic texture?

It formed slowly from a slow-cooling intrusive magma.

What is the main difference between intrusive and extrusive igneous rocks?

Intrusive rocks cool slowly underground; extrusive rocks cool quickly at the surface.

Which texture is typically associated with extrusive igneous rocks?

Fine-grained minerals.

What factor most strongly influences magma composition?

The layer of the Earth where the magma originates.

At which plate boundary is magma commonly produced in the mantle?

Divergent boundaries.

What is the composition of mafic magma?

High in iron and magnesium.

In which tectonic setting is felsic magma most commonly formed?

Convergent boundaries with subduction zones.

How does silica (SiO₂) content affect the viscosity of lava?

Higher silica increases viscosity

Which type of lava flows more quickly due to low viscosity?

Mafic lava.

Which type of volcano forms from eruptions of high-viscosity felsic lava?

Stratovolcano

How does mafic lava influence the size and shape of shield volcanoes?

Its low viscosity allows it to spread broadly, creating gentle slopes.

Why are stratovolcanoes highly explosive?

High-viscosity felsic/intermediate magma traps gases, causing explosive eruptions.

What material is commonly produced by mafic volcanism during underwater eruptions?

Pillow lavas.

Felsic volcanism produces which hazardous material?

Pyroclastic flows.

At which plate boundary is arc volcanism most common?

Convergent boundaries with at least one oceanic plate.

What magma composition is typical of arc volcanism, and how does it affect explosivity?

Intermediate to felsic magma, producing highly explosive volcanoes.