Igneous

• Causes of Earth's Internal Heat:

  Primordial Heat (1/3 of heat)

  • proto-earth formed by collisions with planetoids (asteroids) releasing kinetic energy that transferred to earth (increased thermal energy); in early earth, surface was magma ocean
  • segregation of crust/mantle/core resulted in gravitational potential energy of siderophiles transferred to kinetic energy as they sunk to core (some transferred to thermal energy); energy enough to melt high density siderophiles forming molten core (elements accumulate and release energy)
• Causes of Earth's Internal Heat:

  Primordial Heat (1/3 of heat)

  • a change in earth's cores state; the change from molten to solid at the boundary between the inner/outer core transfers heat into surrounding material (inner core grows 0.5mm/yr)

  Higher temperatures in core, but mantle stores greatest amount of heat energy as silicates are poor conductors.
• Causes of Earth's Internal Heat:

  Radiogenic Heat from decay of radioactive elements (2/3 of heat)

  • unstable parent atoms change to stable daughter atoms resulting in thermal energy being transferred to surrounding earth; radioactive isotopes (potassium/uranium/thorium) are lithophiles so occur in mantle/crust minerals
  • isotopes with short half-lives contributed to geothermal energy of young earth; resulted in higher temperature gradients + higher rates of mantle convection (high temperatures = ultramafic lava)
• • Extrusive igneous rocks - molten rock/lava that upon reaching the earth's surface via volcanoes solidifies, eg. basalt.
  • Intrusive igneous rocks - molten rock/magma that solidifies at depth and may eventually be exposed at earth's surface following period of uplift/erosion.
• Crystal Size and Cooling Rates:

  • instantaneous cooling of lava erupted under water as pillow lavas results in a glassy texture (devoid of crystalline form), eg. obsidian
  • rapid cooling in lava flows at earth's surface over a few months results in crystals of <0.5mm in diameter forming volcanic rocks, eg. basalt and rhyolite
  • slower cooling in dykes/sills over hundreds to thousands of years results in hypabyssal crystals 0.5-2mm in diameter, eg. dolerite
  • slow cooling in magma chambers deep undergrounds over millions of years result in plutonic crystals >2mm in diameter, eg. granite and gabbro
• Crystal Shape

  • Euhedral - well formed crystals with a regular and recognisable shape; they form when a crystal can grow freely in a melt and are not impeded by the presence of surrounding pre-existing crystals.
  • Subhedral - partially formed crystals with some recognisable shape; they have been partially impeded as they grew by the surrounding pre-existing crystals.
  • Anhedral - no regular crystalline shape visible; the shape of the growing crystal is controlled by the arrangement/orientation of surrounding pre-existing crystals.
• • Phenocrysts - large well formed (euhedral) crystals in an igneous rock, eg. shap granite with 3cm wide orthoclase phenocrysts.
  • Groundmass - the remainder of the igneous rock made up of smaller crystals, eg. biotite mica/quartz in shap granite.
• Textures:

  • Equigranular - all the crystals in the rock are roughly the same size, produced by a steady/constant cooling rate.
  • Porphyritic - large crystals (phenocrysts) set in a finer grained groundmass, produced by two-stage cooling.
  • Vesicular - small spherical/ellipsoidal cavities found in lavas, formed by gas bubbles being trapped during solidification of rock.
  • Glassy - no crystals visible; rocks are often dark green/black and show conchoidal fracture.
  • Amygdaloidal - vesicles in lava are later infilled by secondary minerals precipitated from solution (quartz/calcite).
• • Flow Banding (Texture) - occurs where layers of dark/light minerals form due to separation of minerals within a lava flow; they align parallel to flow direction (often contorted as slow moving lava), eg. rhyolite.
  • Ophitic Texture - forms when an elongate crystal is enclosed by another mineral (common in dolerite/gabbro).
  • Cumulate Texture - forms when crystals settle out of the magma, typically on floor/wall/roof of magma chamber, and accumulate in mutual contact (crystals continue growing after settling).
• • Felsic/Silicic igneous rocks - composition rich in silica (>66%) + light coloured (leucocratic) + quartz, orthoclase/plagioclase feldspars, biotite/muscovite mica
  • Intermediate igneous rocks - silica cement 52-66% + grey in colour (mesocratic) + some quartz, orthoclase/plagioclase feldspar, biotite mica and hornblende
  • Mafic igneous rocks - silica cement 45-52% + dark colour (melanocratic) + plagioclase feldspar (Ca), augite, and olivine
  • Ultramafic igneous rocks - silica cement <45% + augite and olivine.
• • Felsic Minerals - quartz/feldspar (rich in silica); quartz only found in silicic/intermediate rocks + magma must be oversaturated with silica so excess is left over after other rock forming minerals crystallise - feldspars most common rock forming mineral in igneous rocks (60%); K Feldspar, Plagioclase Feldspar, and Orthoclase Feldspar.
  • Mafic Minerals - contain magnesium/iron (ferromagnesian); biotite (silicic/intermediate) and muscovite (silicic) mica - amphiboles common in intermediate rocks + pyroxenes main minerals in mafic and ultramafic rocks (+olivine when silica is undersaturated).
• Bowen's Reaction Series

  A) Ultramafic

  B) Basaltic

  C) Andesitic

  D) Granitic

  E) Calcium

  F) Sodium

  G) Olivine

  H) Pyroxene

  I) Amphibole

  J) Biotite

  K) Muscovite

  L) Quartz

  M) Potassium

  N) 1400

  O) 650

  
• Extrusive Lava

  Lava (extrusive form of igneous activity) represents molten rock that has been erupted/extruded onto the surface via volcanoes. Lava cools quickly over months/years/decades depending on thickness of flow; as it cools quickly the volcanic crystals are <0.5mm in diameter.

  • Basaltic, eg. Hawaii/Iceland at constructive margins or hotspots
  • Rhyolitic, eg. Yellowstone (explosive volcanic events)
  • Andesitic, eg. Mt St. Helens (creates intermediate rocks)
  • Pumice, eg. Jemez Mountains, Mexico
• • Pillow Lavas - formed by submarine eruptions of basaltic magma whereby the exterior chills and crystallises rapidly against cold seawater (5$^o$C); outside layer will have glassy texture with vesicles from gas bubbles trapped below the surface (vesicular texture).
  • Glassy Texture - formed by instantaneous cooling of lava so that there is no time for crystals to form; structure shows conchoidal fracture when broken.
• • Pahoehoe - as the external layer of lava cools and solidifies it forms a rope-like pattern of rock as the liquid lava continues to move underneath (faster flowing than Aa).
  • Aa - Aa lava has a surface made of sharp, angular and jagged blocks of basalt; the cooled surface layers break up into these fragments as molten lava continues to flow beneath.
• Lava Flow Structure - lava flows typically have rubbly bases/tops (sometimes shown pahoehoe/aa texture) + interior shows columnar jointing and vesicles often trapped in upper parts of flow - ground underneath lava will be a baked margin (contact metamorphism).

  
• • Lava Tubes - sub-surface tunnels within lava flows that have been formed as the fluid lava has continued to flow down a slope; the solid top/base is formed as it cools and solidifies against ground or atmosphere - top portion solidifies insulating underlying fluid lava flowing beneath hardened crust until supply of new lava slows.
  • Flood Basalts (Mega-eruptions of lava) - basaltic eruptions that last between 0.5-2 million years and have a global impact on climate/life, eg. Deccan Trapp Mountains, India (formed 65Ma).
• Columnar Jointing - forms when thick lava flows (10s of metres) cool slowly; as the lava cools discrete cooling centres develop and the rock contracts towards these centres producing a polygonal pattern of vertical joints. Some columns of basalt have 4-7 sides and the base/top of flows are often rubbly as they have chilled margins against the ground and air.

  
• • Hypabysall - when igenous rocks form at relatively shallow depths below the surface.
  • Plutonic - when igneous rocks form deep below the surface.
  • Intrusions - composed of igneous rock formed below the earth's surface, where magma is forced into pre-existing rocks.
• Batholiths

  • large-scale igneous intrusion with an exposed area over 100km$^2$; formed at destructive plate margins where partial melting occurs at base of continental crust to form felsic magma
  • felsic magma forms at depths up to 50km; once formed it rises to surface and begins to cool (granitic magma below surface and rhyolitic lava above surface)
  • batholiths cool/solidify at 5-12km depth in crust and exposed at surface after long periods of erosion, eg. Cornubian Batholith, South West England (Devon/Cornwall)
  • felsic composition; coarse grained plutonic rocks (granite/diorite)
• Plutons

  Major igeous intrusions are plutonic and solidify deep below surface; a pluton represents one magma body under 100km$^2$ - batholiths are much larger and represent a long period of repeated igneous intrusions; they are discordant, circular and have steep sides (solidify at 5-30km depth as plutonic rocks), eg. Cornubian Batholith, Dartmoor to Lands End and Isles of Scilly (235km long).
• Metamorphic Aureoles

  • heat from a pluton metamorphoses the country rock causing re-crystallisation and the growth of new minerals; metamorphic aureoles have to be at least 50m wide (otherwise it is a baked margin)
  • the rocks in the aureole are altered by contact metamorphism to produce new metamorphic rocks; width of an aureole depends on size/temperature of the intrusion, the dip of the intrusion and the composition of the country rock
• • Stoping - the process that accomodates the magma (as it moves upwards into the country rock) by mechanical fracturing of the surrounding country rock.
  • Assimilation - the melting process that incorporates blocks of country rock (freed by stoping) into the magma.
  • Xenoliths - clasts/blocks of pre-existing rock contained within an igneous rock.
  • Partial Melting - occurs when only a portion of rock is melted when a rock is heated; minerals with lower melting points will melt and those with high melting points will remain solid.
• • Dykes - discordant sheet-like intrusion; often vertical or steeply inclined and follow the path of least resistance along joints, faults, cleavages or bedding planes (result in crustal extension).
  • Sills - concordant sheet-like intrusions; often gently dipping, don't cut across older geological structures and intrude between/follow bedding planes for some distances.
  • Sills Vs Lava Flows: sills have chilled/baked margins above and below but lava flows only have them at the base + xenoliths found at bottom of lava flows and tops of sills + lava flows have rough, weathered surfaces but sills don't
• Constructive/Divergent Plate Margin

  • at mid-ocean ridges/continental rift valleys, the lithosphere is being pulled apart; the lithosphere stretches/thins and solid (but plastic) material rises by convection through the asthenosphere
  • asthenosphere upwells closer to the surface and pressure is reduced (5km depth at mid-ocean ridges); as pressure reduces, decompression of ultramafic peridotite causes partial melting and produces mafic magma - low temp minerals melt forming mafic gabbro/basalt

  
• Constructive/Divergent Plate Margin

  • basalts are produced (mostly erupt as pillow lavas on sea floor) + dolerite dykes/sills are intruded below the surface
  • in continental settings, magma rises through thicker continental crust and is modified
  • basaltic magma accumulates in magma chambers and erupt when the sideways movement allows magma to rise to surface
• Decompression Melting - involves the upward movement of Earth's mostly solid mantle; hot material rises to an area of lower pressure through convection (areas of lower pressure have lower melting points).
• Mantle Plumes/Hotspots

  • at hot spots, mantle plumes rise from deep in the mantle resulting in partial melting of ultramafic material
  • convection in the mantle slowly transports heat from core to surface; mantle plumes carry heat upwards in narrow rising columns of hot material (spreads out as plume meets base of rigid lithosphere)
  • lower pressures allow decompression/partial melting of mantle peridotite (concentrated zone in asthenosphere) forming basaltic magma

  
• Mantle Plumes/Hotspots

  • a hotspot is formed if the plume provides a continous supply of magma in a fixed location
  • as the lithosphere moves over this hot spot due to plate tectonics, the eruptions of magma forms a chain of volcanoes parallel to movement of plate
  • basalt may erupt onto the surface over very short time scales (less than 1 million years) to form flood basalts
• Orogenic Belts (Continental-Continental Plate Margin)

  • where two continental plates converge, neither continent will be subducted; high pressures and mass of sediments (deformed to form fold mountains) combine to force base of crust down
  • partial melting at base of continental crust will produce silicic (granitic) magma - silicic continental crust start to melt at 800$^o$C (temperature below mountains due to geothermal gradient and presence of water)
  • as magma rises it intrudes to form granite batholiths; no volcanoes present as silicic magma too viscous to rise to surface

  
• Destructive/Convergent Plate Margin

  Subduction Zone

  • where plates converge and oceanic plate is subducted, water in rocks of oceanic crust is carried down into hot mantle; descending plate heats up and water released into mantle rock above plate
  • water lowers melting points of minerals in mantle rock + partial melting of mantle creates granitic/andesitic magma (flux melting)
• Destructive/Convergent Plate Margin

  Oceanic-Oceanic

  • where the over-riding plate is oceanic, the erupting magma will be mafic-intermediate in composition as initially magma had only thin oceanic crust to rise through
  • if magma moves quickly at shallow depths it will be basaltic + in larger/older island systems (thicker crust) andesitic (intermediate) magma forms
• Destructive/Convergent Plate Margin

  Continental-Oceanic

  • where over-riding plate is continental, magma must rise through thicker silicic crust (partially melts due to high temperature); rising mafic magma may be at temperature excess of 1000$^o$C
  • results in mixing of mafic/silicic melts to give intermediate-silicic magma (mixing difficult as different viscosities); some magma will reach surface to form intermediate volcanoes (most intruded to form granite batholiths)
• Destructive/Convergent Plate Margin

  • different density plates; heavier/oceanic plate sinks beneath lighter/continental plate
  • two oceanic plates; larger one (moving with greater force) often one that sinks
  • two continental plates; sinking is difficult as both are low denisty, so the smaller one usually sinks

  
• Destructive Plate Margin

  Oceanic-Continental

  • oceanic plate subducts and components taken into areas of high temperatures; basalt lava (+mafic rocks) from oceanic crust is source of new lava + water from ocean/wet sediment lowers melting point of magma + salt water from sea provides sodium for Na plagioclase + sea floor sediment increases % silica available - andesitic (intermediate) volcanoes found here
• Oceanic-Continental Plate Boundary

  • composition of oceanic crust changes; more sodium plagioclase, hornblende forms over augite, and % silica increases
  • temperature increases as plate descends, hornblende/sodium plagioclase melt first (presence of water); high temperature minerals (augite/olivine/calcium plagioclase) don't melt
  • melted material accumulates as intermediate/andesitic magma; less dense than surrounding crust so rises through plate - andesitic volcanoes have high % silica/water so eruptions explosive
• • Adiabatic Heating - occurs when crust/mantle material descends and temperature rises as it contracts (no loss/gain of thermal energy).
  • Adiabatic Cooling - occurs when crust/mantle material rises, undergoes expansion and temperature falls (no loss/gain of thermal energy).
  • Flux Melting - water released from mantle rock as temperature rises lowers the melting point of minerals; partial melting of mantle produces magma.
  • Increasing temperature melting - rising high temperatures of mafic magma heats silicic rocks in crust producing melting.
• • Divergent Margin - partial melting of peridotite allows augite and plagioclase feldspar to separate and create basaltic magma + partial melting involves 5% (95% solid material left) + minerals with lower melting points melt first.
  • Convergent Margin - presence of water alters composition and allows formation of amphibole minerals (andesitic magma); andesite more silicic so more viscous than basalt (eruptions more violent).
• Lava Flows

  • More threat to property than human life – basalt fissures eruptions most dangerous reaching >50km per hour on steep slopes/can spread 10s of km from there source + andesitic/rhyolitic lavas move slowly and rarely spread more than 8km from their source.
  • Lava flows have buried roads, housing developments, and cars + Flood basalt eruptions: mega eruptions of basalt have correlated with mass extinction events – last flood basalt was 15Ma in the Columbia River Plateau, USA; 1,500km³ of basalt erupted (travelled 300km from source).
• • Paricutin, Mexico 1943-1952; andesitic lava flows covered 25km² + city of San Juan de Parangaricutiro located 5km away from active vent.  
  • Kilauea, Kawaii has been erupting since 1983 covering 78km² and destroying 180 houses + eruptions have added 120km² of new land.  
  • Nyiragongo, Congo January 10th, 1977; lava lake of basalt emptied followed a large fissure eruption + initial speeds of lava flow 100km per hour + lavas covered 20km² + several hundred killed.