Igneous Rocks

• Reminder of silicic minerals:

  • quartz
  • orthoclase feldspar
  • plagioclase feldspar
  • muscovite mica
• Mafic minerals:

  • biotite mica
  • hornblende
  • augite
  • olivine
• Igneous rocks are formed by the crystallisation of molten magma. When magma cools, minerals crystallise from the melt and eventually fuse together to form solid igneous rocks. There are two main categories: plutonic or intrusive (both formed by the crystallisation of magma under-ground), and volcanic (formed by the crystallisation of surface lava).
• What are the boundaries for each grain size of igneous rocks?

  Coarse- 3.0 mm

  Medium- 0.1 - 3.0 mm

  Fine- <0.1 mm
• What are the three main types of igneous texture?

  holocrystalline, glassy, porphyritic
• Porphyritic textures are classified as rocks that contain large phenocrysts, set in a matrix of smaller groundmass crystals. These rocks were formed in two-stage crystallisation.
• Glassy textures are characterised by the absence of any visible crystal structure. The rock is composed entirely of very small crystals which cannot be seen with an ordinary hand lens. This type of texture can only occur when the cooling rate was extremely rapid.
• Crystals found in igneous rocks can be either euhedral or anhedral in shape; this refers to how developed the crystals are- whether they have clear, straight edges or not.
• Granular texture is made up of small silicate crystals that are not easily distinguished from one another; they all grew at the same rate.
• Vesicular rocks are formed by the accumulation of vesicles when gas escapes in lava rocks.
• Fragmental (tuff) texture is a type of texture that is formed by the breakage of the rock into fragments following a volcano eruption.
• Types of magma can be classified by the percentage of silica in the magma: Silicic/Acid magma- > 66% SiO2 , Intermediate magma- 65 - 55% SiO2 , Mafic/Basic magma- 54 - 45% SiO2 , Ultra mafic/Ultra basic- < 45%
• Rock formed by acid magma:

  Granite (+microgranite)
• Rock formed by intermediate magma: 


  Andesite
• Rocks formed by mafic/basic magma:


  gabbro, dolerite, basalt
• Rock formed by ultramafic/ultrabasic magma:

  peridotite
• Describing granite:

  • crystalline
  • texture: holocrystalline
  • grain size: coarse
  • minerals present: quartz, plagioclase, orthoclase, biotite mica, some hornblende
  • crystals: phenocrysts and groundmass
• The size of crystals in igneous rocks is dependent on the rate of cooling of magma as rocks crystallise. For example, gabbro has larger grains so the rate of cooling is very slow over many years (in a magma chamber), dolerite has medium grains so the rate is slow , over months (so formed in dykes/sills), basalt has very fine grains so the magma cooled very quickly over only days (in lava flows on the surface).


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• Igneous structures formed under the sea when lava erupts are:

  Pillow lavas
• Igneous structures formed when magma cools and contracts:

  Columnar jointing (six sided columns of rock)
• Amygdales are the secondary minerals that infill vesicles formed by escaping gas during formation.
• Pahoehoe is a ropey structure seen on the surface of a lava flow which is caused by the compression of surface lava into ridges because the lava under the surface is moving faster.
• Aa is highly vesiculated volcanic rock seen on the surface of lava flows, formed by rapid loss of gas as the lava erupts.
• Xenoliths are the name for rock fragments/ included fragments in igneous rocks- become involved when the rock is still magma.
• One form of igneous intrusion are dykes; these are formed when magma is injected into a fracture/fault, it has a discordant relationship with the surrounding rock. These structures can range in size from a few cm across to several kms. Heat from the intrusion create thin baked margins composed of metamorphic rock. The rocks in dykes are usually medium grained and include microgranite and dolerite. All dykes also display chilled margins made of fine grained crystals due to the more rapid cooling of magma in contact with the cool country rock.
• Another igneous intrusion similar to a dyke is a sill; these form when the magma injects along a bedding plane of sediment, so it has a concordant relationship with the rock. Heat from the intrusion creates thin baked margin composed of metamorphic rock. The rocks in sills are usually medium grained also, so also include microgranite and dolerite. Sills also have chilled margins composed of fine grained crystals due to the rapid cooling of the magma in contact with the country rock.
• Plutons (/batholiths) are another, larger form, of igneous intrusion that are formed by the slow rise of a huge mass of buoyant magma. There is a considerable amount of heat emitted from the intrusion and this creates a metamorphic aureole. The rocks in plutons are coarse grained due to the long period of crystallisation. Batholith rocks include granite and gabbro.
• At constructive plate margins, basic or basaltic magma is produced. This is by the partial melting of the mantle peridotite below the thinning crust (being pulled apart by plate movement). Melting is caused by the reduction of pressure as this happens- this is called decompression melting.
• At destructive plate margins, intermediate magma is formed. This is again by the process of partial melting of the mantle peridotite, this time above a subduction zone (heavier plate subducting at the boundary). The mantle begins to melt due to the addition of water that is released from the subducting plate, so this is called wet partial melting or flux melting.
• At convergent plate margins, acid/granitic magma is produced. Partial melting of the continental crust occurs above a subduction zone/ at the base of the continental crust is what forms this magma type.