PART 4: (SEC7)

What is an igneous rock?

Forms when molten rock (magma below ground, lava at surface) cools and crystallises.

What is the field identifier of an igneous rock?

A random network of interlocking crystals.

Most igneous rocks are built around which structure?

The silicon–oxygen tetrahedron SiO₄⁴⁻, which bonds with cations such as Ca, Na, K.

Magma deep underground is kept under pressure by the surrounding rock, holding dissolved gas inside it — like what?

Like an unopened, shaken bottle of soft drink.

Reaching the surface is like opening the bottle: what happens to the gas?

The gas can suddenly escape.

How explosive the release of gas is depends on two things. What are they?

How much gas the magma originally contained, and how easily that gas can escape — which comes down to silica content and viscosity.

Low-silica (mafic) magma → low viscosity → what happens to the gas?

Even a high gas content can bubble/escape relatively freely → gas comes out as the magma erupts, producing vesicle-rich rock and small, "sputtering" volcanoes rather than a single huge blast.

High-silica (felsic) magma → high viscosity → what happens?

Gas cannot easily bubble out → pressure builds up hugely as sticky magma blocks the vent from below → the result is a violent, highly dangerous explosive eruption (e.g. Mount Fuji, Japan; Krakatoa, Indonesia).

What was the global consequence of Krakatoa’s 1883 eruption?

It blasted so much debris into the atmosphere that it caused measurable global cooling for the following seasons and disrupted agriculture — an example of how a single very explosive, high-silica eruption can have global consequences.

What are caldera eruptions?

Very large, shallow magma chambers can collapse catastrophically (e.g. Yellowstone); none in recorded human history.

What is the difference between pāhoehoe and ʻaʻā lava?

Pāhoehoe (fluid, ropey) vs ʻaʻā (rough, blocky) lava.

What is the difference between dikes and sills?

Dikes (cut across layers) vs sills (spread between layers).

What are laccoliths?

Dome the rock above.

What are batholiths?

Large, deep intrusions, often granite.

What are xenoliths?

Foreign rock fragments preserved in solidified magma.

List the volcanic products and features mentioned.

Lava tubes; pillow lavas (underwater); columnar joints.