Unit 3: Igneous Rock

What is a Batholith

A batholith is a large, discordant pluton with an outcrop area of >100 km2.

Define Cinder Cone Volcano

A cinder cone volcano is formed from loose rock fragments ejected from a central vent. Cinder cones tend to have steep sides (about a 30% gradient).

Define Composite Volcano

A composite volcano or stratovolcano is made up of alternating layers of volcanic rock fragments and solidified lava.

Favourable effects of volcanism:

1. production of fertile soil from weathered volcanic ash and lava (e.g., Hawaii) 2. creation of new land 3. generation of electricity and provision of space or water heating by geothermal energy

Difference between Granodiorite and Granite

A grano diorite has more plagioclase, less orthoclase (K-feldspar), and somewhat more ferromagnesian minerals than does granite.

What is a pluton

A pluton is a body of igneous rock that formed at great depth

What is a poryphritic texture

A porphyritic texture indicates two stages of solidification: phenocrysts form during the slow cooling, when the magma is underground; and the fine-grained matrix forms when the lava is erupted and cools rapidly.

Explain the process of magmatic differentiation.

A process whereby a single melt can produce a wide variety of different igneous rocks .The main processes involved in differentiation are assimiliation, exchange of volatiles, fractional crystallization, and magmatic mixing.

Rock Formed from rapidly cooling lava vs. Rock Formed from a slow cooling lava

A rock formed from a lava that cooled rapidly or was very viscous would probably have finer grains than a rock formed from a lava that cooled slowly or was relatively nonviscous.

What is a rock's most important textural characteristic?

A rock's most important textural characteristic is its grain size.

Define Shield Volcano

A shield volcano has a broad, gently sloping cone formed of solidified lava flows.

Define Spatter Cone

A spatter cone is a small, steep-sided cone built from lava sputtering out of a vent. Such a cone forms when a small pocket of gas is trapped in a cooling lava flow, and the gas leaks to escape.

What is a Stock

A stock is a small, discordant pluton with an outcrop area of <100 km2.

What is a vessicular texture, and how does it form? What is the difference between scoria and pumice?

A vesicular texture shows a series of cavities formed by bubbles of gas trapped in a lava when it solidified. Pumice is a frothy glass formed from viscous lava; scoria is a highly vesicular basalt.

How does a volcanic dome form? Describe the composition of its lava.

A volcanic dome forms by viscous, pasty lava (too viscous to flow) squeezing from a vent. The lava is felsic in composition.

According to Bowen's reaction series, which of each set of minerals below forms at a higher temperature? olivine or amphibole potassium feldspar or biotite sodium-rich plagioclase or calcium-rich plagioclase pyroxene or amphibole

According to Bowen's reaction series, a. olivine forms at a higher temperature than amphibole. b. biotite forms at a higher temperature than potassium feldspar. c. calcium-rich plagioclase forms at a higher temperature than sodium-rich plagioclase. d. pyroxene forms at a higher temperature than amphibole.

What is an intrusive rock

An intrusive rock is one which formed from magma that cooled below the Earth's surface.

Describe the composition of an ultramafic rock. Where does such rock form?

An ultramafic rock is composed almost entirely of ferromagnesian minerals. Ultramafic rocks form far below the Earth's surface, usually in the mantle. Very high temperatures are needed to melt ultramafic rocks.

Describe the texture of a tuff and that of a volcanic breccia.

Both tuffs and volcanic breccias have fragmental textures. In tuffs the fragments are <2mm, in volcanic breccias, the fragments are >2mm.

How does columnar jointing occur?

Columnar jointing results from contraction during the cooling of solidified lava.

What is differentiation?

Differentiation is the process by which different ingredients separate from an originally homogeneous mixture.

How are extrusive rocks named and identified?

Extrusive rocks are named and identified on the basis of their composition and texture.

Why are Felsic Rocks Light in colour?

Felsic rocks are light in colour because they contain abundant quartz and feldspar—light coloured minerals.

What are Felsic Rocks

Felsic rocks are silica-rich igneous rocks with a relatively high content of potassium and sodium; example: rhyolite.

What two factors determine the grain size of igneous rocks?

Grain size is determined by the rate of cooling and the viscosity of lava.

What causes the eruptions in the Hawaiian archipelago?

In the Hawaiian archipelago, an eruption is caused by a mantle plume or hot spot beneath the Pacific Plate, which creates magma in the asthenosphere. Eventually, this magma must erupt.

Describe the continuous branch of Bowen's reaction series.

In the continuous branch of Bowen's reaction series, plagioclase crystallizes, and as crystallization continues, the sodium content of the magma gradually increases and its calcium content decreases. The result is a series of zoned crystals of plagioclase that are more calcium rich in the centre and more sodium rich on the rim.

Describe the discontinuous branch of the Bowen's reaction series.

In the discontinuous branch, olivine begins to crystallize first and eventually recrystallizes as pyroxene. If the magma has not yet totally solidified, any remaining olivine and pyroxene react with the melt so that amphibole and biotite begin to form. In this branch, previously formed minerals are destroyed to create new minerals; this process does not occur in the continuous branch.

What are Intermediate Rocks

Intermediate rocks are rocks with a chemical content between that of felsic and mafic rocks; example: andesite.

What minerals are present in a granite, but absent in a basalt or a gabbro?

K-feldspar and quartz are present in granite, but absent in basalt and gabbro

Why do felsic lavas erupt more violently than mafic lavas?

Mafic lavas are low in silica, while felsic lavas are high in silica and have a greater likelihood of containing silica tetrahedral that have begun to form small frameworks.

Why are Mafic Rocks dark in colour?

Mafic rocks are dark in colour because they contain an abundance of ferromagnesian minerals, which are dark.

What are Mafic Rocks

Mafic rocks are silica-poor igneous rocks with a relatively high content of magnesium, iron, and calcium; example: basalt.

What is Magma

Magma is molten rock

Is quartz present in andesite? —in granite? —in basalt?

Neither andesite nor basalt contain quartz; granite does contain quartz.

Why doesn't a cone build up around the vents through which magma that forms plateau basalt flows?

No cone is built up around the vent of plateau basalts because the associated lava is fluid enough (non-viscous) to flow smoothly.

What is obsidian?

Obsidian is volcanic glass and is not composed of minerals.

What is partial melting?

Partial melting is a process in which only the chemical components of the asthenosphere with the lowest melting temperatures liquefy, while minerals with the higher temperatures remain crystalline

What is pegmatite? How does pegmatite form? What rare elements are found in pegmatite?

Pegmatite is extremely coarse-grained igneous rock. Pegmatites form from very slow cooling of low viscosity fluids made up mostly of water under high pressure. A rare element commonly found in pegmatites is lithium.

What do pillow structures in basalt indicate?

Pillow structures indicate submarine eruption of lava.

What is the mineral composition of a rhyolite? —of a basalt?

Rhyolite contains quartz, k-feldspar, plagioclase, and ferromagnesians. Basalt contains plagioclase and ferromagnesians.

List six possible mechanisms by which granitic and andesitic magma could form at convergent plate boundaries.

Six possible mechanisms by which granitic and andesitic magma could form at convergent plate boundaries: • partial melting of basalt • melting of sedimentary rock • partial melting of the mantle • assimilation of crustal rocks • partial melting of the lower crust • heating by transient magma or under plating

What is the geothermal gradient?

The geothermal gradient is the rate at which temperature increases with increasing depth beneath Earth's surface.

What kind of igneous rock would you expect to find under the oceans? —on continents? —on or near continental margins?

The igneous rock you would expect to find under the oceans is basalt; on continents, granite; and on or near continental margins, andesite.

Which intrusive igneous rock corresponds in composition to basalt? —to rhyolite? —to andesite?

The intrusive igneous rock corresponding to basalt is gabbro; to rhyolite, granite; to andesite, diorite.

What does the violence of an eruption depend on?

The violence of an eruption depends on the amount of gas in a lava or magma and the ease or difficulty with which such gas escapes. Lava from which gas has difficulty escaping is likely to erupt more violently than lava from which gas can escape easily.

What does the viscosity of lava determine?

The viscosity of lava determines the ease or difficulty with which gas escapes, and the viscosity of lava is determined by the following factors: • the temperature of the lava compared to the temperature at which it solidifies. • the silica content of the lava. • the degree to which small frameworks of silica tetrahedra have formed in the lava.

What two factors could cause melting in the asthenosphere?

Two factors that could cause melting in the asthenosphere are the addition of extra heat, which raises the temperature above the melting points for some components of the asthenosphere, and the reduction of pressure.

Define aa Lava

aa lava: a lava flow that solidifies as a spiny, rubbly surface; lava that is cool enough to have partly solidified; therefore it moves as a slow pasty mass.

Define Caldera

caldera: a volcanic depression over a vent at the summit of the cone.

Define Chill Zone

chill zone: finer-grained rock on the outer rim of an intrusive body. Chill zones result from quick solidification because of rapid loss of heat to the cooler country rock.

What is Country Rock

country rock: older rock into which igneous rock has intruded.

Define Crater

crater: a basin-like depression over a vent at the summit of the cone.

What is a Dike

magma that forces itself across rock layers and hardens

What is a Sill

magma that is parallel to layers of rock

Define Pahoehoe Lava

pahoehoe lava: a lava flow with a ropy or billowy surface

What is a Laccolith

similar to a sill but mushroom shape

Define Vent

vent: the opening through which an eruption takes place.

What is Xenolith

xenolith: a fragment of rock that is distinct from the body of igneous rock in which it is enclosed.

Destructive effects of volcanism:

• loss of life and burial of cities, etc., by hot ash • destruction of landforms and possibly whole islands through explosive eruptions and the creation of giant sea waves • alteration of world weather patterns through the solar filtration effect of fine particles and gases propelled into the atmosphere