Topic 5: Igneous Rocks

Igneous Rock Formation

The formation of this rock requires melting rocks to form magma, and then cooling and crystallization of the magma (either slowly underground or quickly above the surface)

Igneous Rocks

It is the product of a lengthy series of processes which influence the rock’s final composition, mineral makeup, and appearance as well as the shape and position of igneous bodies it forms

Magma Composition

Are determined in the source region by the type of rock that undergoes partial melting (basically the source of the magma is the type of rock) and can be modified during ascent and solidification

Felsic

High silica, eg. K-feldspar, quartz, biotite, hornblende (granite), tend to have light -coloured minerals

Intermediate

Medium silica, plagioclase feldspar, and amphibole (andesite), have somewhat light-coloured minerals

Mafic

Low silica, mostly calcium-rich plagioclase, pyroxene (basalt), tend to have dark-coloured minerals

Ultramafic

Ultramafic = very low silica, mostly pyroxene and olivine (peridotite), composed of fairly dark-coloured minerals

Xenolith

A piece of rock within an igneous rock that is not derived from the original magma but has been introduced from elsewhere, especially the surrounding country rock

Solidus

The temperature at which melting begins

Liquidus

The temperature at which the rock becomes completely molten

Water Saturated Solidus

A term describing the fact that excess water means that it takes a higher temperature to start melting the rock

Geotherm

The geothermal gradient, which is basically how the temperature changes with depth

Latent Heat of Fusion

When a solid is converted to a liquid at a constant temperature, a ton of heat is required for a phase change which is about 400 kj/kg for rocks, approximately 1200 kj to heat a kg of crystalline basalt from room temperature to the molten state; ⅓ of this heat is needed to convert solids to liquid

Partial Melting

In which rocks don't fully melt due to the latent heat of fusion; tends to be around 1-20%

Bowman's Reaction Series

A predictable order in which minerals form/melt, hence minerals at the top are last to melt and first to crystalize and vice versa with a continuous and discontinuous series with increasing amounts of silica and oxygen as we move toward the bottom, and we’re increasing in iron, magnesium, and calcium as we move from bottom to top

Decompression Melting

Occurs at mid-ocean ridge since they create a space that can be filled by hot rock that rises with nearly no heat loss (adiabatic), with pressure decreasing as it approaches the surface; as long as this hot rock rises faster than temperatures can cool off, it can melt because of decreasing pressure closer to surface with melting occuring when the rising asthenosphere eventually intersects the solidus

Flux Melting

Addition of volatiles (water, CO2) can lower the melting temperature of the rock, minerals in rocks along mid-ocean ridges become hydrated from seawater circulation, and when they reach subduction zones, this water is carried to the mantle, and as the cold slab of rock sinks and pressure rises, metamorphic reactions cause hydrous minerals to release the water, which rises into the overlying hot mantle, acting as a flux which allows the melt to form at the water-saturated solidus

Conduction

The transfer of heat by transmission of vibrational (thermal) energy from one atom to the next

Density

The degree of compactness of a substance, depends on magma, and provides the driving force for ascent; about 2600–2750 kg/m3 for basaltic magma and 2400 kg/m3 for granitic

Viscosity

The property of a liquid to resist flow when a shear stress is applied and is defined as the ratio of the applied shear stress to the rate at which the fluid deforms

Magma Viscosity

This affects flow behavior, movements of crystals and inclusions of foreign matter (xenoliths) within them, the diffusion of materials through them, the growth of crystals from them, and the explosivity of eruptions (when aided by growth of gas bubbles near the surface)

Volatiles

Volatile substances which can decrease the viscosity of magma through breaking bonds

Undercooling

To cool below the solidus temperature

Fire-Fountaining Volcanic Eruptions

Eruptions that have bubbles with low viscosity (eg. Holuhraun eruption in Iceland)

Explosive Reactions

These eruptions result in bubbles with high viscosity (eg. Mt. St. Helens)

Diffusion

The spreading of something more widely which is important for the growth of crystals and bubbles in magma, and determines the grain size of igneous rocks; low-viscosity=higher-rates and high viscosity=lower-rates

Magma Ascent

In which magma forms at the boundaries of the minerals in a rock as a network of interconnected thin films and tubes and flows as a result of changes in pressure gradient

Darcy's Law

Governs magma flow; rate = Jx = -KH dh/dx

Dike

A tabular or sheet-like body of magma that cuts through and across the layering of adjacent rocks

Diapir

A type of geologic intrusion in which a more mobile and ductility deformable material is forced into brittle overlying rocks

Batholith

A large mass of intrusive igneous rock >100km2 and tends to be felsic or phaneritic, and they can extend to considerable depth in the crust and can have surface exposures exceeding 100 km2

Magma Differentiation

The change the composition of the magma in a process as it cools and rises; processes may separate crystals from liquid

Fractional Crystallization

The removal and segregation from a melt of mineral precipitates, changing the composition of the magma

Cumulates

An igneous rock formed by gravitational settling of particles in a magma

Crystal Mush Compaction

Occurs when crystal settling may be difficult in basaltic magmas where plagioclase has a neutral buoyancy and dense minerals may not separate, and a crystal mush forms when crystals are abundant enough to make contact with each other (~1/3 to 2/3 crystallization)

Assimilation/Contamination

Occurs near the margins of many igneous bodies where most magmas do not have enough heat to totally melt xenoliths, but heat liberated by crystallization can partially fuse them, creating a low-melting fraction that is added to the magma, changing its composition

Desilication

This reaction requires heat, which is provided by the latent heat of crystallization of magma, causing the magma to solidify

Magma Mixing

The process by which two magmas meet, commingle, and form a magma of a composition somewhere between the two end-member magmas, and is common in volcanic magma chambers

Igneous Rock Bodies

A large mass of igneous rock formed when magma cools and solidifies either beneath the Earth's surface (intrusive) or on the surface (extrusive)

Hyperabyssal

Occurs if igneous rocks crystallize near the surface

Plutonic

Occurs if igneous rocks crystallize at a great depth

Concordant

An igenous rock body that is parallel to the layering in the surrounding rock

Discordant

An igenous rock body that cuts across the layering in the surrounding rock

Sill

A tabular sheet intrusion of igneous rock that forms between existing rock layers, often sedimentary or metamorphic rocks; it is concordant

Phaneritic

Coarse grained, individual crystals can be seen by the naked eye

Aphanitic

Fine grained, need to use a microscope or magnifier to see individual grains

Pegmatitic

Very coarse grains larger than 3 cm

Laccolith

A mass of igneous rock, typically lens-shaped, that has been intruded between rock strata causing uplift in the shape of a dome

Cone Sheet

Thin, inclined igneous intrusions arranged in a curving array, dipping inward towards a common source and forming a circular or elliptical shape in outcrop

Ring Dike

An intrusive igneous body that is circular, oval or arcuate in plan and has steep contacts

Caldera

A large, cauldron-like hollow that goes shorty after the emptying o a magma chamber in a volcanic eruption

Nested Caldera

A geological structure where one or more smaller calderas (large, bowl-shaped depressions formed by volcanic collapse) are located within a larger caldera

Pipe

These intrusions are formed by the violent, supersonic eruption of gas-rich magma

Diatreme

These consist of a narrow carrot-shaped cone of solidified magma and is usually composed of kimberlite or lamproite

Explosive Diatremes

These can form when magma contacts a shallow body of groundwater, resulting in a rapid expansion of steam, a very explosive reaction

Maar

A broad, shallow crater, typically filled by a lake, formed by a volcanic eruption with little lava

Tuff Ring

Low-lying cones with a wide, bowl-shaped crater and are made of compacted pyroclastic deposits like volcanic ash and tephra

Lopolith

A large saucer-shaped intrusion, which can have a diameter of hundred of kilometers which is often formed by large rocks consisting of basaltic magma, and is bassically the opposite of a laccolith; eg Bushveld Complex

Stoping

When magma rises through the crust and shatters, but doesn’t melt, the surrounding rocks

Rift Valley

A long, linear valley formed when tectonic plates pull apart, creating a lowland region between uplands or mountain ranges

Hot Spot

A volcanic region where magma rises from the Earth's mantle and erupts through the crust, forming volcanoes, and can create chains of volcanoes as tectonic plates move over them

Tephra

Any fragmented rock material ejected from a volcano during an eruption, regardless of size, composition, or how it was deposited

Tuff

A type of rock primarily composed of consolidated volcanic ash and other pyroclastic material ejected from explosive volcanic eruptions

Icelandic Eruption

Characterized by effusions of molten basaltic lava that flow from long, parallel fissures; outpourings often build lava plateaus and result from fluid (low-viscosity) lava flows with little explosivity

Hawaiian Eruption

Involves fluid lava with little associated explosive activity; similar to Icelandic eruptions, but erupt from a volcano’s summit and radial fissures

Strombolian Eruption

Liberate more gas in the form of moderate bursting bubbles that eject blobs of magma tens of meters in the air; particles can build up around the vent to build steep-sided splatter cones

Bombs

Large ejected particles >64 mm

Lapilli

Particles between 2–6 mm

Ash

Particles <2 mm

Vulcanian Eruption

Occur when the lava in the vent solidifies and prevents gas from escaping until the pressure builds sufficiently to cause a more violent eruption; characterized by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak; ejected fragments are dominantly solid and angular

Pelean Eruption

Are associated with explosive outbursts that generate pyroclastic flows; the fluidized slurries produced by these eruptions are heavier than air but are of low viscosity and pour valleys and slopes at great velocities

Pyroclastic Flow

Dense mixtures of hot gas and ash

Plinian Eruption

Intensely violent kind of eruption where gases boiling out of gas-rich magma generate enormous and nearly-continuous jetting blasts that core out the magma conduit and rip it apart that resembles a gigantic rocket blast directed vertically upward

Volcanic Explosovity Index (VEI)

Is a logarithmic scale that quantifies the explosive power of eruptions, and it is gauged by the volume of tephra produced and the height of the explosive column

Shield Volcanoes

Eruptions of mafic magmas tend to be non-explosive, runny, and contain the least amount of tephra (aka pyroclasts: explosive fragmentary material, including ash, lapilli, cinders, and bombs), are tall and broad, gently sloping (5-10°)

Cinder Cones

Mafic to intermediate magmas can also produce cinder cones: small cones formed by the accumulation of cinders (pieces of magma) that spew out of a hole; slopes reflect the angle of repose, which depends on the size of the material: generally 25-35°

Conposite Volcanoes

Intermediate lavas erupt explosively and produce composite volcanoes (also known as stratovolcanoes); these have the classic ‘volcano’ shape and are stratified with alternating layers of lava flows and pyroclastics, and slopes are generally 10-30°

Lava Domes

Felsic lavas are the most viscous and can build up lava domes; tend to form at the end of an eruptive cycle at composite volcanoes (stratovolcanoes)

Mid-Ocean Ridges

These cover a length of ~65k km and are the most productive rock factories on Earth, melting of ultramafic mantle due to decompression; produces tholeiitic MORBs

Tholeiitic

This type of magma results from large-scale partial melting at shallow depths

Calc-Alkaline

This type of magma results from smaller-scale melting at greater depth

Continental Flood Basakts

They are the largest volumes of volcanic rock in the geologic record (100’s to 1000’s km3) erupted from long fissures that have little topographic relief and occurs in regions of crustal extension and are common precursors to the breakup of tectonic plates

Large Igneous Provinces (LIPs)

Are massive accumulations of igneous rocks, both intrusive and extrusive, that form during exceptional magmatic events; tend to be over 100,000 cubic kilometers of mainly mafic magma

Continental Rift Valleys

Where plates are moving apart (forming triple junctions) and igneous rocks tend to be alkaline due to smaller degrees of partial melting in the source region

Convergent Plate Boundaries

Areas where tectonic plates collide, resulting in one plate either sinking beneath the other (subduction) or colliding to form mountains; features include volcanic arcs and back-arc basins

Ocean-Ocean Convergence

Main rocks along the volcanic arc are high-Al basalt, basaltic-andesite, and andesite (and their intrusive equivalents), with only minor felsic rocks

Ocean-Crust Convergence

Main rocks along the volcanic arc are high-Al basalt, andesite, dacite, and rhyolite (and their intrusive equivalents); more continental rocks = more felsic rocks (derived from partial melting of crust)

Back-Arc Basins

Also have a huge range of compositions, from alkalic during initial rifting, evolving towards tholeiitic as extension continues to the point where there is passive mantle upwelling

Ore Deposits

A naturally occurring accumulation of minerals within a host rock that contains sufficient quantities of valuable elements for economic extraction (include: porphyry-type deposits, orogenic Au deposits, epithermal Au-Ag deposits, IOCG deposits, skarn-type deposits, etc.)

Pahoehoe Flows

A type of lava flow characterized by a smooth, ropy, or billowy surface

Aa Flows

Lava flows that have irregular rough surfaces made of jagged, spiny and rough clasts of lava called clinkers

Asthenosphere

A layer in the upper mantle extending from depths between 20 km and 50 km beneath oceans and 70 km and 220 km beneath continents that is marked by low seismic velocities

Peridotite

A plutonic igneous rock composed essentially of olivine and pyroxene

Adiabatic

A process in which no heat is lost or gained

Andesite

A volcanic rock that characterizes composite volcanoes formed along island arcs above subduction zones, composed of approximately equal proportions of pyroxene and plagioclase

Hydrostatic Pressure

The pressure at depth in a fluid (such as water) that has no shear strength, due entirely to the weight of the overlying fluid; at depth in the Earth, rocks have little shear strength, so pressures can be calculated assuming rocks behave like a fluid

Lithostatic Pressure

The calculated pressure at depth in the Earth based on the assumption that rocks have almost no shear strength (i.e., they behave the same as water)

Anhydrous Melting

The process where rocks melt without the presence of water, meaning that no water is involved in the chemical reactions that cause the rock to transform from a solid to a liquid (magma); mechanisms include temperature release and pressure reduction

Temperature Release

Raising the temperature of a rock above its solidus point (the temperature at which it starts to melt) will cause anhydrous melting

Pressure Reduction

Reducing the pressure on a rock, particularly at depth, can also lead to anhydrous melting