Petrology - texture, structure and mineral composition of rocks.

Magma stops before reaching surface but may be exposed by later erosion. Intrudes into country rocks and so is younger.

• Form depends on size, position, stress and properties of the magma.
• discordant - cut across bedding planes - e.g. dykes.
• concordant - parallel to bedding planes - e.g. sills.

Xenoliths - fragments of country rock broken off as magma flows through. Stoping is the process by which the roof of a batholith is eroded away and fragments enter. They tend to sink under gravity, melt and may become incorporated into the batholith ("digested"). They may circulate and so become rounded.

Chilled edge - part in contact with colder country rock - rapid cooling gives finer grains. Found at margins of batholiths, dykes and sills.

Baked margin - part of country rock in contact with hotter intrusion, batholith, dyke or sill. May metamorphose the surrounding rock forming an auriole (many km in the case of a batholith).

Large, volumes of many km³

• May have outcrops of many km, may have been exposed by
• Compact
• Coarse grained - cool and crystalise slowly because of large size

Forceful intrusions

Magma intruded into country rock by pressure underneath

Discordant - across the bedding

• large mass of (usually) granite or granodiorite which stops before it reaches the surface
• formed by melting induced by friction of a subducting plate + release of volatiles which lower melting point. Found in mountains near continental margin.

• stock - exposed irregular offshoot of a batholith
• boss - exposed regular (round) offshoot of a batholith
• diapir - a "blob" detached from the roof of a batholith and piercing the overlying rock
• (bysmalith - forget it!!!)

Concordant - parallel to bedding

• (lopoliths - basin shaped, fed from below from batholoth)
• laccoliths - domed top, fed from the side from batholith, causes uparching of rock
• phacoliths - like laccolith but does not cause uparching

• domes - intrusion spreads along beds, lifts up, rounded top (not the same as domed anticline)

Magma intruded into spaces formed by ring fractures or subsidence - form minor intrusions

Small bodies - discordant or concordant - formed as magma moves into spaces or fractures

• dyke - discordant intrusion. May be plate like.
• dyke swarm - series of dykes formed as magma moves into tension cracks, parallel.
• radial swarms - radiate from a centre , may be volcanic or stock
• parallel swarms - parallel plates, larger scale tension cracks

• ring dykes - concentric dykes, ± cylindrical (diameter a few km), may taper a bit towards the top.
• cone sheets - concentric dukes, taper downwards, form in cracks issuing from top of a batholith.

• sill - plate-like, concordant (along bedding plane) intrusion. May be stepped. E.g. Whin Sill
• vein - tabular body of rock or mineral intruded into another rock. Often applied to material deposited by solution.

• plug - solidified feeder pipe of a former volcano, magma or pyroclastic material.

Late stage igneous activity with water rich fluids

• Pegmatite - igneous rock, typically granite, with a very coarse grain, average size 3 cm but may be many m. A second phase of crystallisation which occurs when an intrusion has cooled and cracks formed. Mineral rich water and magma forced in, slow cooling to produce large crystals.
• Aplite - smaller versions of pegmatites, finer grained.

A late stage activity when the temperature lowers. Steam/hot water dissolves minerals, forced out through fissures, minerals deposited as lodes or veins

A volcano is an opening in the crust through which magma erupts.

• Fissure
• Central vent

• magma erupts from a crack in the crust
• tends to be basic (high iron, low silica, free flowing)
• non-explosive
• emerges as thin sheets
• repeated eruptions may build up layers

e.g. Deccan Trappe, Columbia Plateau (USA)

• may get weathering or beds of sedimentary rock between layers
• may be traversed by intrusions – feeder dykes) to different layers

Between eruptions, congealed lava blocks cracks. Therefore, fewer points next eruption. Fissure may be replaced by string of cones.

• cone shaped
• central crater
• vent – feeder pipe to magma chamber
• may be parasitic cones from subsidiary vents
• cones are built up from layers of lava and pyroclastic rocks

Three major forms of volcano

• basic lavas (basalt)
• low viscosity
• erupt frequently (keeps vent open) and quietly
• flows to form thin sheets up to 80 km
• cones broad and low, angle of 2° – 10° (shield shaped)

e.g. Mauna Loa – base 113 km, height 8 km (from sea floor)

• acid magma (high silica, less iron, viscous)
• forms a steep-sided dome
• vent blocked by viscous magma and pyroclastic rocks
• subsequent eruptions add to inside
• original dome upheaved
• outer layers eroded leaving steep-sided plug with no crater

• magma originates in the upper mantle (at about 1000°C) - remains solid
• as it gets nearer the surface it gets more fluid
• volatiles (gas and liquid) are come out of solution and migrate to the head of the plug
• pressure builds up
• volcano erupts
• plug, sides of vent shattered and carried upwards as pyroclastic material
• pyroclastic material falls down to form volcanic cone – sides 30° - 40°
• with loss of volatiles, magma become more viscous, solidifies and blocks vent

In terms of violence, which depends on viscosity of magma

Hawaiian type

• non-explosive

• basic lavas (basalt)
• low viscosity (fluid)
• erupt frequently (keeps vent open) and quietly

• cones broad and low, angle of 2° – 10° (shield shaped)

Strombolian type

• slightly more viscous (less fluid) magma
• small, frerquent explosions - dust and clots of magma
• vent kept open

Vulcanian type (*not on syllabus)

• more viscous magma
• less frequent and more violent explosions
• vent blocked between explosions
• dust clouds cauliflower shape (mushroom cloud)

Vesuvian type (*not on syllabus)

• viscous magma
• long periods of quiescence
• eruption preceded by series of Vulcanean explosions which relieve pressure
• rapid release of volatiles produces frothy lava (pumice)
• explosive ejection of mobile lava froth
• (AD 79 Pompeii and Herculaneum covered, people killed by gases)

Pelean type

• very viscous magma
• very violent
• vent gets blocked after eruption
• with increased pressure a lava dome rises
• skin cracks with pressure
• side blows out
• nuée ardents escapes and flows down sides very quickly
• lava spine usually forms - erodes fairly quickly
• speed up to 400 km h^-1
• ash to height of 20 km and distance of1600 km

Krakatoan type (*not on syllabus)

• very viscous magma

• most violent
• volcanic dome blown away
• collapses to form a caldera
• violent tsunami

Caldera formation

• roof of the underlying magma chamber collapses along curved (arcuate) fractures
• circular depression in instead of original cone original cone
• magma may be exposed as a magma lake
• may be associated with ring dyke complexes
• may get residual activity as vents, fumaroles, geysers, etc.
• as activity decreases a lake may form
• examples, Crater Lake, Oregon, Krakatoa, Yellowstone

Dormant volcanoes

• solfateric activity
• fumaroles - steam and gas vents
• encrustation of solid matter around vents
• ground warm

Lava is the molten (fluid) rock material that flows along the ground from volcanic vents and then consolidates as rock.

Pyroclastic rocks are formed of fragmented material ejected into the atmosphere by disruptive volcanic eruption. Individual fragments are pyroclasts - crystal, glass or rock not subjected to secondary redisposition.

Pyroclastic flow is a rapidly moving mass of lava buoyed up by gases released near the vent.

Pumice is a vesicular and light rock, which forms when gaseous lava cools. It forms from acid (high silica) lava, which is viscous and traps gases, typically from a Vesuvian type eruption. Floats on water.

Scoria is a rock made up of cinder-like vesicular lava, basic in composition.

Volcanic bombs are molten or semi-molten pieces of magma, ejected from a volcano and which solidify in flight, and are greater than 32 (or 64) mm in diameter.

Volcanic blocks are angular fragments of rock, which were solid before ejection, i.e. bits of the parent rock.

Tephra is the unconsolidated deposit, which forms from magma ejected from the vent (pyroclasts).

Lapilli are rounded pieces of tephra 4-32 mm in diameter (or 2-64 mm because Geologists can't make up their mind on definitions).

Volcanic ash is made up of pieces of tephra less than 4 mm (2 mm) in diameter

… is the typical product of a volcanic explosion consisting of fragments of varying sizes from fine dust to bombs.

Tuff is rock formed of consolidated lapilli in a matrix of ashes.

Pelé's Hair consists of long threads of volcanic glass, which form when jets of lava are blown by the wind. Typically found in Kilauea, Hawaii. Named after Pelé, Goddess of Fire rather than a footballer.

Ignimbrite is a pyroclastic rock formed when hot lava and glass fragments (vitric) are fused together as it cools. It is also known as welded tuff.

Agglomerate is a rock formed by the consolidation of angular fragments (blocks) in an ash matrix, and formed from explosive volcanic activity. It is also known as pyroclastic breccia. Some authorities include bombs in agglomerate.

Nuée ardente is a rapidly moving, hot, glowing cloud of volcanic ash buoyed up by gas. It is very destructive with speeds up to 150 km h^-1. Forms ignimbrite on cooling.

Volcanoes release lava, ash (pyroclasts) and gases - all of which are potential hazards

Lava and ash - may engulf people and property

• danger related to rate of flow from vent
• basic (mafic) low viscosity lava loses volatiles easily and hence moves slowly - Hawaii
• more siliceous higher viscosity lava erupt with more explosive violence, with less warning
• nuee ardente, high volatile content, may travel at 150 km h^-1 too fast to escape by running - Mt Pelee

Explosions and lateral blasts

• land may be destroyed leaving a caldera - Krakatoa
• ash may choke people and engulf town - Pompei

Lahars

• mudflows - saturated loose lava - ice melting
• may move at 40 - 50 km h^-1

Landslides

• build up of ash or lava to critical level

Floods

• dam may be breached
• displacement of water by landslide

Volcanic gases

• noxious gases - H₂S
• poison or choke
• CO₂ may be trapped in hollows
• may effect plants famine
• may form acid rain

Particles in atmosphere

• may be very high (stratosphere)
• darken the sun for months or years
• affect the climate
• affect plant growth

Tsunami

• cause damage thousands of km away

Earthquakes

• may be triggered - further damage

Economic disruption

• area may be devastated - businesses and farms destroyed
• years to recover - people may be reluctant to go back

Soil

• fertile with good texture and high nutrient level

New land

• most Pacific islands are volcanic
• Azores

Volcanic gases

• help maintain the balance of elements in the atmosphere

Commercial use

• ores - valuable minerals may be brought to the surface
• material for construction and abrasives
• geothermal energy - Iceland, California, New Zealand

Tourism

• people fascinated by active volcanoes - Vesuvius
• post volcanic terrain may be interesting - Yellowstone