Igneous Rocks, Volcanoes, and Eruption Dynamics: Key Concepts and Classifications

What two criteria determine how igneous rocks are classified?

Igneous rocks are classified by composition and texture.

Explain how and where volcanic glass forms and how its cooling rate compares with plutonic and volcanic rocks (glassy, phaneritic, aphanitic).

Volcanic glass forms from instant cooling, producing a glassy texture compared to aphanitic (fast cooling) and phaneritic (slow cooling) textures.

Explain how a porphyritic texture forms and what type of igneous rock a porphyry is.

Porphyritic texture forms from slow then fast cooling, making it an igneous rock with large crystals in a fine matrix.

How are the two most common crust elements related to mafic, intermediate, and felsic compositions (include silica percentages)?

Oxygen and silicon form silicate minerals whose silica content defines mafic (45-52%), intermediate (53-65%), and felsic (66-76%) compositions.

How can color be used to determine silica content of igneous rocks?

Light-colored rocks have high silica, medium colors have intermediate silica, and dark colors indicate low silica.

What two main factors determine volcanic explosivity?

Explosivity mainly depends on silica content and gas content.

Give examples of viscous and non-viscous fluids.

Viscous fluids include honey and rhyolite lava; non-viscous fluids include water and basaltic lava.

How do composition and temperature affect a volcano's explosivity?

High silica and low temperature increase viscosity and gas trapping, making eruptions more explosive.

Describe the steps of a volcanic eruption as magma rises through the conduit.

As magma rises, pressure drops, gases expand, and if trapped, cause an eruption.

What constitutes pyroclastic debris?

Pyroclastic debris includes ash, lapilli, blocks, bombs, and pumice.

What are the three most abundant volcanic gases?

The three most abundant volcanic gases are H2O, CO2, and SO2.

Describe the differences between the three volcanic cones and how each forms.

Shield volcanoes are broad from lava flows, stratovolcanoes are steep from layered lava + pyroclasts, and cinder cones are small and made of loose pyroclasts.

What triggered the 1980 Mt. St. Helens eruption?

Mt. St. Helens erupted after a bulge collapsed in a landslide that released pressure and triggered an explosive blast.

What is a lava dome and which cone type is it associated with?

Lava domes are steep piles of viscous lava that usually form on stratovolcanoes.

How does a caldera differ from a crater? Calderas are large collapse depressions, while craters are small explosion pits.

How do temperature, silica, viscosity, and gas content affect volcanic products and landforms?

Silica, temperature, viscosity, and gas content determine whether eruptions produce lava flows or pyroclasts and what landforms result.

Why are lavas the greatest hazard in Hawaii?

Lavas are Hawaii's greatest hazard because fluid basalt flows destroy large areas of infrastructure.

How does volcanic ash form and what hazards does it pose?

Volcanic ash forms from shattered magma and causes hazards like engine failure, breathing issues, and roof collapse.

Compare and contrast bombs and blocks and where they are most dangerous.

Bombs are molten when ejected, blocks are solid, and both are most dangerous near the vent.

What is a pyroclastic flow and why is it dangerous?

Pyroclastic flows are fast, hot avalanches that threaten populations on the flanks of stratovolcanoes.

What is a lahar and why are they hard to predict?

Lahars are volcanic mudflows difficult to predict and most dangerous in river valleys.

Why is Mt. Rainier considered dangerous?

Mt. Rainier is dangerous because glacier-covered slopes can produce massive lahars flowing into populated areas.

How do volcanic island arcs form and in what tectonic environment?

Volcanic island arcs form at ocean-ocean subduction zones where melting creates chains of volcanoes.

How is a continental volcanic arc different from a volcanic island arc?

Continental arcs form above ocean-continent subduction zones, while island arcs form above ocean-ocean subduction zones.

How do volcanoes form?

Volcanoes form when magma rises from the mantle through the crust and erupts at the surface due to pressure from gases and molten rock.

How does silica content affect magma?

Higher silica content increases viscosity, making magma thicker and more explosive.

How does silica content relate to igneous rock composition?

Silica-rich magma forms felsic rocks, medium silica forms intermediate rocks, and low-silica magma forms mafic rocks.

How does silica content influence color?

Low-silica mafic rocks are dark, intermediate silica rocks are medium-colored, and high-silica felsic rocks are light-colored.

What determines magma viscosity?

Viscosity is controlled mainly by silica content and temperature.

Give examples of viscous and non-viscous magmas.

Rhyolitic magma is viscous, while basaltic magma is fluid and low-viscosity.

How does volcanic glass form?

Volcanic glass forms when lava cools so rapidly that crystals cannot grow, producing a glassy texture.

How does cooling rate affect igneous textures?

Slow cooling creates phaneritic textures, fast cooling creates aphanitic textures, and extremely fast cooling creates glassy textures.

What is the difference between phaneritic and aphanitic textures?

Phaneritic textures have large visible crystals from slow cooling, while aphanitic textures have tiny crystals from fast cooling.

How does color help identify igneous rocks?

Light-colored rocks are felsic with high silica, medium-colored rocks are intermediate, and dark rocks are mafic with low silica.

What makes magma more explosive?

High viscosity and high dissolved gas content create more explosive eruptions.

What does viscosity mean in volcanic processes?

Viscosity describes how easily magma flows, with high viscosity resisting flow and trapping gas.

How do gas content and viscosity interact during eruptions?

High viscosity traps gas, building pressure that leads to explosive eruptions, while low viscosity lets gas escape gently.

What is volcanic glass also known as?

Volcanic glass is commonly known as obsidian.

How does temperature affect magma behavior?

Hotter magma flows more easily because lower viscosity allows gas to escape.