Lecture 9: Magma and Volcanoes

Flashcards covering key concepts, terms, and processes about magma, volcanic activity, weathering, erosion, sedimentary processes, and soil formation.

What are the three main components of magma?

Liquid (molten rock), solid (crystals), gas (dissolved or immiscible gas bubbles).

What is the primary rock group for most magma on Earth?

Silicate group.

Why is silica content important in magma?

It controls most physicochemical properties like density and viscosity.

What did N.L. Bowen discover in the Skaergaard Intrusion?

Different layers in a plutonic igneous body had differences in mineralogy.

Which minerals were more abundant in the lower rock units found by N.L. Bowen?

Olivine was more abundant.

What are the upper rock units dominated by according to N.L. Bowen?

They are dominated by plagioclase and hornblende.

What is Bowen's Reaction Series?

It describes the order of crystallization of minerals from cooling magma.

What mineral forms first from silicate melts according to Bowen's Reaction Series?

Olivine.

What occurs after olivine in Bowen's Reaction Series?

Pyroxene starts crystallization as olivine growth slows.

What is the sequence of crystallization in Bowen's Reaction Series?

Olivine → Pyroxene → Calcium-rich plagioclase → More Na-rich plagioclase → Hornblende amphibole → K-feldspar, muscovite, and quartz.

What does Bowen’s Reaction Series reveal?

The relationship between temperature, composition, and mineralogy.

What are the limitations of Bowen’s Reaction Series?

It does not account for initial composition effects, pressure, water content, or interruptions such as magma mixing.

What is viscosity?

The resistance to flow when shear stress is applied.

What are Newtonian fluids? Give an example.

Fluids that flow immediately; for example, water.

What are Bingham fluids? Give an example.

Fluids that only flow after a certain amount of stress; for example, glass or toothpaste.

What does magma viscosity largely depend on?

Composition of the melt, polymerization, and temperature.

What are volatiles in magma?

Gases like H2O, CO2, SO2, F, and Cl that affect magmatic viscosity.

What happens to eruptive behavior if a volatile can become a separate phase?

It greatly changes the eruptive behavior.

How is pumice created?

From high concentration of volatile gas where bubbles try to escape, creating a vesicular texture.

What is solubility in the context of magma?

How much of a component can be dissolved in magma.

What does solubility control regarding volatiles?

It determines whether volatiles will be dissolved or present as a separate phase.

What is the universal solvent?

Water.

How do we classify igneous rocks and magma?

Chemical classification is typically based on SiO2 content.

What is the difference between felsic and ultramafic rocks based on SiO2 content?

Felsic has greater SiO2 content, while ultramafic has less.

What is a liquidus?

The maximum temperature at which a solid can coexist with a liquid.

What is solidus?

The temperature at which a liquid will turn into a solid.

What effect does increasing pressure have on melting temperature?

Increasing pressure usually increases melting temperature.

What happens to bonds in magma as pressure increases?

Bonds get shorter and denser.

What is the geothermal gradient?

The rate at which temperature increases as you go deeper into the Earth.

How can we melt rocks?

By reducing pressure, adding heat, or adding water.

Where is pressure reduced, leading to melting?

Mid-ocean ridges and continental rifts.

Where is heat added to promote melting?

Hotspots like Hawaii and crustal melting due to heat from intrusions.

Where is water added that leads to melting?

Subduction zones, such as the Andes.

What does adiabatically melt mean?

Melts due to decrease in pressure with minimal heat and matter transfer.

What happens to magma as it ascends in divergent plate boundaries?

It adiabatically melts.

Where does adiabatic melting occur?

At mid-ocean ridges (MOR) and continental rifts.

What happens when hot basaltic magmas enter continental crust?

Rocks can melt to generate felsic magmas.

What does increased water content do to melting temperature?

It decreases the temperature at which melting occurs.

What occurs when water is added in subduction zones?

Hydrous minerals dehydrate and release water into the mantle wedge above the subducted slab.

What is the result of water rising into the mantle wedge?

It creates a linear/curved belt of volcanoes above the point where the slab loses its water.

What rocks are formed due to magma mixing?

Andesites, diorites, etc.

What is fractional crystallization?

The process where minerals crystallize from cooling magma at different temperatures.

What type of magma is created at mid-ocean ridges?

Basaltic/magmatic.

What types of magma are created at continental rifts?

Basaltic and rhyolitic (bimodal).

What type of magma is created at subduction zones?

Mostly andesitic but can range from basaltic to rhyolitic.

What types of magma are found at continental collisions?

Little magma, mostly rhyolitic (forms plutons).

What types of magma are produced at hotspots?

Mostly basaltic, with some rhyolitic. Oceanic hotspots produce mafic magmas and continental hotspots produce bimodal compositions.

What are dikes?

Thin, tabular, discordant intrusions that cut across foliation or bedding.

What are dike swarms?

Genetically related dikes with the same orientations, typically in response to extensional stress.

What is a geological example of a vertical eruption flow deposit?

Mt. Pinatubo.

What is a geological example of a lateral blast flow deposit?

Mt. St. Helens.

What is a geological example of a low-pressure boiling over flow deposit?

Mt. Lamington, Papua.

What is a sill?

Thin, tabular, concordant bodies that lie parallel to surrounding rock layers.

What is a stock?

Plutons with an exposed area of less than 100 km².

What is a batholith?

Plutons with exposed areas greater than 100 km².

What drives the formation of stocks and batholiths?

Erosion.

What is diapirism?

A process suggested to form large plutons driven by buoyancy differences.

What is crack seal growth?

A method of pluton emplacement involving multiple injections of magma in the same area.

What is a pluton?

A large body of intrusive igneous rock formed when magma cools and solidifies underground.

What do volcanic processes and shapes depend on?

Magmatic properties and chemistry.

What are vents?

Single points from which magma erupts.

What are fissures?

Long, linear cracks where magma erupts.

What are shield volcanoes characterized by?

Large size, mostly basaltic composition, low slope, and typically one primary central vent.

What causes shield volcanoes to form?

Low viscosity magma.

What is a good example of a shield volcano?

Hotspot volcanoes.

What are the characteristics of stratovolcanoes?

Size up to a few kilometers across, mostly andesitic, steep slope, and multiple vents.

What contributes to the formation of stratovolcanoes?

High viscosity magma and medium-high volatile content.

What are cinder cones?

Collections of ash, lapilli, and blocks from weakly explosive eruptions.

What are characteristics of cinder cones?

Size up to a few hundred meters tall, mostly basaltic, slope around 33°, with a central bowl crater.

What is a caldera?

A bowl-like structure formed from major collapse features due to magma drainage.

Where does caldera formation typically occur?

Above stratovolcanoes.

What are the geological implications of caldera eruptions?

Involve pyroclastic flows and can lead to catastrophic events.

What types of lava flows are generated?

Pahoehoe, A’a, blocky lava, lava tubes, basaltic lava, pillow lava.

What characterizes pahoehoe flows?

Smooth flow with very low viscosity.

What are volatiles?

Gases dissolved in magma that can drive explosive eruptions.

What is the nature of A’a flows?

Forms a crust and may solidify and tumble at the surface.

What characterizes blocky lava flows?

Forms from higher viscosity flows with irregular surfaces and blocks.

What defines lava tubes?

Natural tunnels where molten lava flows beneath the surface.

What is the significance of basaltic lava flows?

They can flow extensive distances and impact the environment.

What typically causes columnar jointing?

Thick flows or shallow basalt emplacements.

What happens to columnar joints during cooling?

Volume decreases, causing fractures in characteristic patterns.

What is pillow lava?

Lava that forms when low viscosity lava erupts underwater or flows into standing water.

What primarily controls eruption explosivity?

Composition, particularly the amount of volatiles.

What are pyroclastic flows?

Masses of hot ash, lava fragments, and gases ejected explosively from a volcano.

What allows dissolved gas to exsolve in pyroclastic flows?

Decreased pressure as magmas rise to shallow depths.

What is exsolution?

The process where dissolved gases in magma come out of solution and form bubbles.

What drives explosive eruptions?

Expanding exsolved gases.

What are pyroclastic rocks?

Volcanic rocks generated by explosive eruptions.

How are pyroclastic deposits divided?

Into falls and flows/surges.

What characterizes pyroclastic falls?

Well sorted with grain size distributed around the vent.

What are isopachs?

Lines of constant thickness indicating particle distribution around eruptions.

What are mechanisms that generate pyroclastic flows?

Vertical eruption, lateral blast, low pressure boiling over, dome collapse.

What historical event wiped out Pompeii?

A pyroclastic flow.

What types of deposits do pyroclastic flows commonly produce?

Poorly sorted deposits such as ignimbrites.

What are Plinian eruptions?

Explosive eruptions characterized by high ash output and air dispersion.

What do eruptions get classified by?

Their explosivity.

What are Icelandic/Fissure eruptions?

Found at mid-ocean ridges, characterized by line-like forms and non-explosive activity.

What are Hawaiian eruptions?

Dominantly basaltic magma with low viscosity.

What are Strombolian eruptions?

Weakly explosive, mostly basaltic with higher volatile content.

What can Strombolian eruptions be compared to?

Volcanic burps.