FoG: T6 - Igneous Rocks Magmatism

What is the definition of magma?

An extremely hot, molten or semi-molten natural material from which all igneous rocks are formed, located beneath the Earth's surface.

What is the primary distinction between magma and lava?

Magma is molten rock located beneath the Earth's surface, while lava is magma that has flowed onto the Earth's surface.

Besides a mixture of minerals, what other components are found in magma?

Small amounts of dissolved gases, such as water vapor, carbon dioxide, and sulfur.

What is a magma chamber?

A large pool or mass of molten rock (magma) that pools beneath a volcano as it rises through the crust.

Within a magma chamber, where do lighter, silica-rich minerals tend to accumulate?

They float to the top of the magma chamber.

Within a magma chamber, where do heavier, iron-rich minerals tend to accumulate?

They sink to the bottom of the magma chamber.

In lava sampling, what chemical element is analyzed to determine the temperature of the lava when it was erupted?

Magnesium (Mg); the more magnesium, the hotter the lava.

What is the term for the melting process that occurs as hot rock rises through the mantle, causing a decrease in surrounding pressure?

Pressure-relief melting.

At subduction zones, what substance escapes from the subducting plate to lower the melting temperature of the overlying mantle rocks, triggering melting?

Volatile compounds, especially water.

What geological process is thought to produce rocks of different compositions from the same initial magma as it rises and cools?

Fractional crystallization.

The process where only a portion of a rock melts, producing magma with a different chemical composition than the original rock, is known as _____.

partial melting

What is magma contamination?

The process where rising magma causes overlying crustal rocks to partially melt, mixing molten rock of a different composition into the magma.

Felsic rocks, such as rhyolite, are typically formed from the partial melting of what type of crust?

Continental crust.

Mafic rocks, such as basalt, are typically formed from the melting of what part of the Earth?

Mantle rocks.

What is the most abundant volatile gas found in most magmas?

Water vapor (H2​O).

What is the second most abundant volatile gas found in most magmas?

Carbon dioxide (CO2​).

What is the definition of viscosity in the context of magma?

It is the resistance to flow (the opposite of fluidity).

How does the concentration of silica (SiO2​) affect magma viscosity?

Viscosity increases with increasing silica (SiO2​) concentration.

How does temperature affect magma viscosity?

Viscosity decreases with increasing temperature; hotter magmas are less viscous.

The viscosity of magma affects the shape of a volcano. Volcanoes with steep slopes are formed from magma with _____ viscosity.

very high

Flatter volcanoes are formed from magma that has _____ viscosity and flows easily.

low

What are the three main types of magma?

Basaltic (Mafic), Andesitic (Intermediate), and Rhyolitic (Felsic).

What is the typical silica (SiO2​) content range for Basaltic (Mafic) magma?

45-55%.

What is the typical temperature range for Basaltic (Mafic) magma?

1000°C - 1200°C.

Which type of magma has low viscosity and is described as being 'fairly fluid'?

Basaltic (Mafic) magma.

Basaltic (Mafic) magma is high in which three elements?

Iron (Fe), magnesium (Mg), and calcium (Ca).

What is the typical silica (SiO2​) content range for Andesitic (Intermediate) magma?

55-65%.

What is the typical temperature range for Andesitic (Intermediate) magma?

800°C - 1000°C.

How is the viscosity of Andesitic magma described?

High viscosity.

What is the typical silica (SiO2​) content range for Rhyolitic (Felsic) magma?

65-75%.

What is the typical temperature range for Rhyolitic (Felsic) magma?

650°C - 800°C.

Which magma type has the highest viscosity?

Rhyolitic (Felsic) magma.

Rhyolitic (Felsic) magma is high in which two elements?

Potassium (K) and sodium (Na).

A _____ is an intrusion of magma that solidifies after welling up from below the surface.

pluton

What is a magmatic dike?

A large slab of magmatic material that has intruded into, and cuts across, another rock body.

What is a xenolith?

A piece of rock trapped in another type of rock, often torn from inside the Earth and embedded in cooling magma.

When magma solidifies in the air during a violent volcanic explosion, it forms volcanic rock called _____.

tephra

In the atmosphere, tephra is more commonly called _____.

volcanic ash

The gases in magma give it an explosive character because their volume _____ as pressure is reduced during ascent.

expands

Why do high-silica magmas tend to produce violent, dangerous explosions?

They resist flowing, which traps expanding gases and allows pressure to build up until it blasts out.

Why do poor-silica magmas tend to result in fairly gentle eruptions?

They flow easily, allowing gas bubbles to move up and escape without building up significant pressure.

What type of energy can be generated by using magma, as demonstrated by the Icelandic Deep Drilling Project?

Geothermal energy.

What is the common name for igneous rocks, derived from the Latin word for 'fire'?

Fire rock.

Igneous rocks are formed through the cooling and _____ of magma or lava.

solidification

What are the two main types of igneous rocks, classified by where they form?

Intrusive (Plutonic) and Extrusive (Volcanic) rocks.

Which type of igneous rock forms when magma cools and solidifies within the planet's crust?

Intrusive (or Plutonic) igneous rocks.

Why do intrusive igneous rocks typically have a coarse-grained texture?

Because the magma cools slowly while surrounded by pre-existing rock, allowing large mineral grains to form.

Name a common type of intrusive (plutonic) igneous rock.

Granite, gabbro, or diorite.

Which type of igneous rock results from magma pouring onto the surface and cooling?

Extrusive (or Volcanic) igneous rocks.

Why do extrusive igneous rocks typically have a fine-grained texture?

Because the lava cools and crystallizes at a much faster rate due to exposure to air or water.

What common extrusive igneous rock forms lava flows, sheets, and plateaus?

Basalt.

What type of extrusive rock is formed from the products of an explosive volcanic eruption, such as ash and other blasted material?

Tuff.

What are the two most important variables used for the classification of igneous rocks?

Particle size (texture) and mineral composition.

A _____ is an intrusive igneous body that is concordant (parallel) with the existing layering of rock.

sill

A _____ is a discordant (not parallel) intrusive igneous body that cuts across existing layering.

dike

What is a batholith?

A large mass of intrusive igneous rock with an exposed surface area greater than 100 square kilometers.

What is a stock in geology?

An irregularly-shaped pluton similar to a batholith but smaller, with an exposed surface area of less than 100 square kilometers.

What term describes the texture of intrusive igneous rocks where crystals are large enough to be seen with the unaided eye?

Phaneritic.

What term describes the texture of extrusive igneous rocks where crystals are too fine to be seen with the unaided eye?

Aphanitic.

The color of igneous rocks is influenced by their _____ composition.

chemical

Mafic rocks are dark-colored due to a high content of which two elements?

Iron (Fe) and Magnesium (Mg).

Felsic rocks are light-colored due to a high _____ content and low Fe & Mg content.

silica (SiO2​)

What is the color of andesite, an intermediate igneous rock?

Intermediate shades or colors (green, gray, brown).

What is an example of a glassy igneous rock that is usually black despite having a felsic composition?

Obsidian.

What is the name of the series, formulated by Norman L. Bowen, that describes the temperature-dependent crystallization of minerals from a cooling magma?

Bowen's Reaction Series.

The _____ Series in Bowen's model describes the evolution of minerals like olivine, pyroxene, and amphibole, where early minerals react with the melt to form new minerals with different structures.

Discontinuous

The _____ Series in Bowen's model describes the evolution of plagioclase feldspars as they evolve from being calcium-rich to more sodium-rich.

Continuous

According to Bowen's Reaction Series, which mineral crystallizes at the highest temperature?

Olivine.

According to Bowen's Reaction Series, which mineral is the final product to crystallize at the lowest temperature?

Quartz.

If early formed crystals like olivine are removed from a magma through crystal fractionation, the remaining liquid becomes richer in what compound?

Silica (SiO2​).

Magma is a mixture of four parts: a hot liquid base (the melt), crystallized minerals, incorporated solid rocks, and _____.

dissolved gases

What is magmatism?

The emplacement of magma within and at the surface of a planet's outer layers, which solidifies as igneous rocks.

What is volcanism in relation to magmatism?

Volcanism is the surface expression of magmatism.

What tectonic setting is likely the largest producer of magma on Earth, responsible for creating oceanic crust?

Oceanic ridges (divergent plate boundaries).

What general compositional type of igneous rocks (e.g., mafic, felsic) are produced at oceanic ridges?

Mafic rocks (basalts and gabbros).

The cores of continental land masses are primarily composed of what felsic igneous rock?

Granite.