EAS 2600 Exam 2

Intrusive igneous rocks

Plutonic igneous rocks form when magma solidifies underground (e.g., granite)

Extrusive igneous rocks

Volcanic igneous rocks form when lava solidifies on the surface (e.g., basalt)

How does intrusive vs. extrusive formation affect crystal size?

Intrusive rocks cool slowly and have large crystals (granite). Extrusive rocks cool quickly and have small crystals or glass (rhyolite).

Name the four composition categories and the coarse-grained intrusive rock for each.

Felsic: granite. Intermediate: diorite. Mafic: gabbro. Ultramafic: peridotite.

What are the extrusive (fine-grained) equivalents of granite, diorite, and gabbro?

Rhyolite, andesite, and basalt

Dike vs. sill?

Both are tabular intrusions. A dike cuts across the layering in the country rock. A sill runs parallel to the layering.

What is a pluton?

A large, blob-shaped intrusive body of coarse-grained rock

What are stocks?

Plutons with less than 100 km of exposed igneous rock

What are batholiths?

Plutons with 100 km or more of exposed igneous rock

What is a volcanic neck?

An intrusion formed when magma solidifies in the throat of a volcano

Describe an aphanitic texture.

Fine-grained crystals; formed extrusively through rapid cooling

Describe a phaneritic texture.

Coarse-grained crystals; formed intrusively through slow cooling

Describe a pegmatitic texture.

Very large crystals; very slow cooling; formed intrusively

Describe a porphyritic texture.

Large crystals within an aphanitic (fine-grained) matrix

Describe a vesicular texture.

Holes (gas bubbles) in the rock

Describe a glassy texture.

No crystals; smooth and shiny

Why isn't the geothermal gradient alone enough to melt rock in the lower crust and upper mantle? What else is needed?

The geotherm stays below the melting curve. Melting needs additional heat, a pressure change, volatiles, and partial melting.

What is decompression melting? Where does it usually occur?

A drop in pressure lowers the melting temperature, so rock melts as it ascends. Occurs at divergent boundaries.

What is volatile melting and where does it happen?

Volatiles lower the melting temperature of rock. Occurs in subduction zones, where water released from the subducting plate melts the asthenosphere.

What are two sources of additional heat?

Friction in subduction zones and rising, hot mantle rocks

Mid-ocean ridge volcanism

Ridges produce mafic melts from partial melting of the ultramafic mantle, with gentle eruptions.

Arc volcanism

Arcs produce felsic melts from re-melting and contamination by the crust, with explosive eruptions.

Where is igneous activity most common, and what magma is typical?

Most common at divergent boundaries, hot spots, and subduction zones. Divergent boundaries and hot spots produce mostly basaltic (mafic) magma. Subduction zones produce mafic to felsic magma.

Mafic magma

Hotter, low viscosity (maple syrup), less explosive because of lower water and gas content

Felsic magma

Cooler, highly viscous (peanut butter), more explosive because of higher water and gas content

What three factors determine the violence of an eruption?

Composition, temperature, and amount of gas in the magma

Order the four volcanic rock types by silica content.

Basalt, andesite, dacite, rhyolite

What is the importance of silica content in volcanic eruptions?

More silica means a lower eruption temperature and less mobile lava.

Describe a shield volcano.

Rounded, low profile; built from successive fluid basalt flows. Usually not explosive (Mauna Loa, Kilauea).

Describe a stratovolcano (composite volcano).

Built from successive layers of extrusive rock and forms high peaks. The lava is less fluid than basalt, so these volcanoes can be more explosive (Mt. Rainier, Mt. St. Helens).

What are cinder cones?

Cone-shaped piles of lapilli and bombs with straight sides and steep slopes. Small (under 300 m) and often on the flanks of large volcanoes (SP Crater, Cerro Negro).

What is the Volcanic Explosivity Index (VEI)?

A relative scale (0-8) for comparing eruptions by plume height and erupted volume

Effusive eruption

Long-distance lava flows and gas emissions

Explosive eruption

Felsic. Ash, pyroclastic flows, lahars, tsunamis, and climate change.

Hawaiian eruption

Effusive and mafic. Comes from a central vent and builds shield volcanoes.

Icelandic eruption

Effusive and mafic. Comes from long fissures.

Aa lava

Rough, blocky, and jagged

Pahoehoe lava

Smooth and ropy

Strombolian eruption

short-lived explosions of intermediate lava thrown high.

Vulcanian eruption

Viscous magma traps gas, causing explosions that eject blocks and bombs, with a higher eruptivity but less material thrown high.

What is a phreatic eruption?

magma heat interacting with water creating steam (gas). includes no new magma.

Plinian eruption

Huge explosive eruptions sending particulate matter into the stratosphere, affecting climate

What are the three main products of volcanic eruptions?

Lava flows, volcaniclastic deposits (tephra), and volcanic gas

Basaltic lava behavior

Thin and fluid, moves fast (up to ~30 km/h), and travels long distances

Rhyolitic lava behavior

Rarely flows. It plugs the vent as a lava dome, and pressure builds toward an explosive eruption (Novarupta, Alaska).

What are the tephra size classes in order?

Fine ash (under 2 mm), lapilli (2-64 mm), blocks and bombs (over 64 mm). Blocks are angular and bombs are smooth.

What are Pele's tears and Pele's hair?

Basaltic tephra. Pele's tears are frozen droplets. Pele's hair is thin glass strands.

Name the three most abundant volcanic gases and what SO₂ does.

H₂O is most abundant, CO₂ is second, and SO₂ is third. SO₂ reacts with water to form sulfuric acid aerosol. Gas makes up 1-9% of magma, with more in felsic magmas.

What is a pyroclastic flow?

A fast-moving mass of hot gas and volcanic matter that moves away from a volcano. Causes the largest share of volcanic fatalities.

What caused the 1816 "Year Without a Summer"?

Volcanic aerosols from a large eruption in 1815, which caused widespread crop failures

What happened at Lake Nyos, Cameroon?

CO₂ built up in the water. A landslide disturbed the lake and released a CO₂ cloud that suffocated cattle.

Which eruption is the largest of the 21st century so far?

Hunga Tonga-Hunga Ha'apai (2022)

What happened at Nevado del Ruiz (1985)?

A pyroclastic flow mixed with snowmelt and river water. This sent debris flow downstream and killed ~23,000 people.

What is a lahar?

volcanic mudflow

Mt. St. Helens

VEI 5. Felsic. pyroclastic flow impacted timber industry. deadliest us eruption

Mt. Pelée (1902) key facts?

VEI 4. A pyroclastic flow (~100 mph) destroyed St. Pierre and killed ~30,000 people. Some think it helped push the US to choose Panama over Nicaragua for the canal.

What happened at Toba?

(VEI 8) from 70,000 years ago in Indonesia; largest eruption in 2 million years

Why do so many strong volcanoes erupt in Indonesia?

Subduction boundaries

What happened at Tambora?

Large blast in Indonesia where pyroclastic flow and resulting famine and climatic effects (Year Without a Summer) led to mass death.

Where was the most devastating eruption in human history?

Tambora

How can volcanoes cause climatic change?

aerosols and other particulate matter block sunlight and pollute air in stratosphere where there is no convection (Year without Summer)

pillow basalts

mafic lava on sea floor forming boulder shapes. Proof of ancient part of the sea

caldera

when volcano vent collapses

what is a metamorphic rock?

oldest preexisting rocks that went under a certain amount of heat and pressure, changing the nature of the rock

what is metamorphic shield?

exposed basement rock

what is metamorphic platform?

rock buried under sediment

temperature metamorphism

“baked” rock. Higher temperature associated with further burial. Contact metamorphism

Pressure metamorphism

Grains (sediment become more interlocked (“squeezed”). Rapid pressure change. shock metamorphism(meteorites).

Combo of Temp and Pressure metamorphism

Most common method of metamorphism. Done through burial or subduction

How are minerals/atoms modified through metamorphism?

recrystallization, phase change, neocrystallization, pressure solution, plastic deformation

recrystallization

change in size and shape of grain without changing the mineral

phase change

change in packing of atoms

neocrystallization

new minerals grow (not in og rock) as rock reacts to new pressure, temp, and fluid conditions

pressure solution

differential stress when liquid is present

plastic deformation

squeezing/sheering at high temp and pressure

Foliation

texture. parallel surfaces/layers from the alignment of grain and layering of different minerals

rank grade of rocks

increasing grain size: slate,phyllite,schist,gneiss,migmatite

what does the composition of metamorphic rock depend on?

pressure and temp, composition of any fluid phase, bulk chem composition of the rock

Sandstone metarock

quartzite

granite metarock

gneiss

shale metarock

slate

conglomerate metarock

metaconglomerate

limestone metarock

marble

rank limestone, marble, and sediment with increasing pressure and temperature

sediment → limestone → marble

Where does blueschist rock occur

accretionary prism

Where does hornfeld rock occur

the heart

How does foliation form from randomly oriented mineral grains?

Directed pressure realigns the grains perpendicular to the squeezing direction

How do compositional bands (felsic vs. mafic) form in foliated rocks?

Felsic and mafic minerals dissolve and their ions migrate, segregating into separate bands

Which metamorphic rocks are NON-foliated?

Marble, quartzite, hornfels, anthracite coal, metaconglomerate

mylonite

forms in a shear zone

migmatite

high-grade banded rock

What defines an element?

Its number of protons (atomic number)

What are isotopes?

Atoms of the same element with different numbers of neutrons

What is an isotope's mass number?

# of protons + # of neutrons

What does it mean for an isotope to be radioactive?

unstable

How are ages measured in the lab?

Parent and daughter amounts are measured in a mass spectrometer, and age is calculated from their ratio

What condition must be met for radiometric dating to be reliable, and how is accuracy checked?

The sample must be a closed system; cross-checks are used

What is a half-life?

The time it takes for half of the original radioactive atoms to decay

What fraction of parent atoms remains after 3 half-lives?

1/8 (12.5%)

Relative age vs. absolute (radiometric) age?

Relative = order of features using logic; absolute = when an event occurred in exact units of time