Igneous Processes

• • Igneous rocks are the products of cooling magma and lava
  • Volcanism is the surface expression of igneous activity
  • Classified based on texture and composition - mainly crystal size and composition
• • Glassy - rapid cooling, no time for crystals to form. Exclusively volcanic e.g. obsidian
  • Crystalline - made of interlocking crystals. Most igneous rocks are crystalline.
  • Equicrystalline - crystals are all of equal size. Can be fine, medium or coarse
  • Porphyritic - Two stages of cooling, different crystal sizes. Large crystals are called phenocrysts and form from slow cooling. Coarser crystals are surrounded by a finer groundmass. Groundmass grain size defines the rock grain size.
• • Vesicular - holes present in rock. Formed when gas bubbles are trapped as lava cools quickly. Tend to be located towards the top of a lava flow.
  • Amygdaloidal - forms when vesicles are infilled by minerals from groundwater
  • Flow banding - layers of light and dark minerals form due to mineral separation normally within silicic lava flows. Minerals aligned parallel to flow direction.
• • Green → ultramafic
  • Coarse → peridotite
• • Grey/Black → Mafic
  • Coarse → Gabbro
  • Medium → Dolerite
  • Fine → Basalt
• • White/Pink → Silicic/Felsic
  • Coarse → Granite
  • Fine →Rhyolite. Lots of vesicles = pumice
• Melts to solids

  • If a melt stayed at its point of origin and nothing in its surrounding changed, it would stay molten.
  • Melts eventually freeze due to cooling beyond freezing temperature
  • Cooling happens when magma rises because the crustal temperature decreases upwards
  • If magma becomes trapped underground as an intrusion it slowly loses heat to the surrounding rock and freezes
  • If a magma reaches the surface and extrudes as lava it cools in contact with air or water
• Factors affecting cooling rate

  • Depth of intrusion
  • Magma intruded deep in the crust, surrounded by hot wall rock, cools slower than magma intruded near the ground surface where wall rock is relatively cool
  • Shape and size of magma body
  • For a given shape, smaller volume cools faster. For a given volume, a flatter shape cools faster
  • Presence of circulating groundwater
  • Water passing through wall rock carries away heat
  • Magma interacting with circulating groundwater cools faster than magma that intruded dry rock
• Types of igneous intrusion

  • Pluton - a body of intrusive igneous rock that is crystalized from magma slowly cooling below Earth’s surface. In practice, it usually refers to a distinctive mass of igneous rock, typically several km in dimension, without a tabular/flat shape like sills and dikes. They
  • Batholith - type of pluton, more than 100km wide.
  • Sill - small igneous intrusion that cuts between beds, making them parallel to them - concordant. Sills dip at the same angle as the beds.
  • Dyke - small igneous intrusion that cuts through the beds so that it runs at an angle - discordant
• Types of igneous intrusion

  • Lava flow
  • Pahoehoe - Smooth, ropy, surface due to very fluid lava under a solidifying surface crust
  • A’a - free chunks of very angular pieces of lava. Reflects flow dynamics - forms when lava flows rapidly. Rapid heat loss and a resulting increase in viscosity
• Types of igneous intrusion

  • Pillow structure - basaltic lava flows forming underwater. Cools very quickly. The tough outer skin of the pillow initially stops the advance of the flow but the pressure of the lava squeezing into the pillow eventually breaks through and a new blob of lava squeezes out forming another pillow.
  • Columnar joints - Joints are fractures in rocks. The magma/lava cools rapidly from the outside towards the centre causing shrinkage cracks to form, commonly in a hexagonal pattern.
• • Graph that shows how temperature increases naturally with depth (geothermal gradient)
  • Shows the temperature at which rock melts at different depths and consequently pressures, inside the Earth
  • The natural increase of temperature with depth (the geothermal gradient) is such that down to a depth of 75km, the Earth is pretty much in the solid state. There must be other forces operating in the volcanically active regions allowing melting to take place and magma to form.
• • Melting by adding volatiles
  • Dehydration melting is when water is transferred from the ocean crust to mantle. This lowers the melting point of mantle peridotite - produces a new solidus, the wet solidus.
  • Mantle plume - hotspot
  • An upwelling of abnormally hot rock within the Earth’s mantle. The heads of mantle plumes can partially melt when reaching shallower depths. This results in hotspots
  • Decompression melting
  • Reduction in pressure decreases melting point causing partial melting.
  • The melting of rock can be caused by heating, pressure decrease and addition of water.
• • Fissure vents - a linear vent through which lava erupts. They can be km long and are gently effusive and usually found along rift zones such as Iceland and East Africa
  • Shield volcano - runny lava will low silica content. Layers of lava representing succeeding eruptions with gentle slopes normally less than 30 degrees.
• • Stratovolcano - steep slopes with alternating layers of ash and lava. The magma is viscous (silicic or intermediate). These are common at destructive plate boundaries as water from the subducting plate adds volatiles and makes the volcano explosive.
  • Caldera - collapse of the volcano into itself resulting in a large volcanic crater. Formed by emptying of the magma chamber beneath the volcano. If enough magma is erupted, the empty chamber cannot support the weight of the mountain above so collapses into a massive crater called a caldera.
• • Icelandic - persistent fissure eruption
  • Hawaiian - runny basaltic lava travels down sides of volcano in lava flows. Occasional pyroclastic activity
  • Strombolian - medium-low viscosity magma. Regular explosions following explosion of large gas bubbles. Less mobile lava and more pyroclastic debris
• • Vulcanian - andesite magma type with violent explosions. Lava domes form steep sides subject to collapsing in avalanches. More viscous lava forming a crust between eruptions. Involves release of large amounts of pyroclastic debris
  • Vesuvian - very powerful blasts of gas pushing gas clouds into sky. Ash falls cover surrounding area.
  • Plinian - violent upward eruption of gas and debris.
• • Silica content/viscosity - high silica content results in explosive but less frequent eruptions. Low silica content results in frequent and gentle eruptions.
  • Partial melting results in a more silicic magma than a parent rock.
  • Volatiles are added at destructive plate boundaries resulting in explosivity. Local groundwater conditions can also add water into the magma chamber.
  • Assimilation - continental rocks are silica rich. Melted and incorporated into rising magma resulting in an intermediate/silicic magma. Can occur wherever there is continental crust to rise through.
• Hazards

  • A hazard is a natural event that has the possibility to cause loss of life or economic damage. A natural disaster is when the natural event DOES cause loss of life and damage to property.
  • Exogenic hazards - occur as a result of natural Earth surface processes
  • Endogenic hazards - derive their energy from within the Earth e.g seismic, volcanic and radon emissions
• • Lava flows - can form fast moving streams or spread out in broad thin sheets as much as several miles wide
  • Pyroclasts - when magma is explosively ejected into atmosphere as particles called pyroclasts
  • Falls
  • Ash - Significant impact on urban settlements, agricultural land and aviation. Amount of ash produced and its effect depends on the nature of the eruption prevailing winds at the time.
  • Lapilli
  • Bombs and blocks
  • Flows
  • Hot pyroclastic flows occur when the explosion from the volcanic vent directs a lethal mixture of hot gas, rock fragments, lava and ash in a downslope direction
• • Landslides - caused by ground deformation of volcanic slopes by rising magma, explosive eruptions, large earthquake, heavy or long lived rainfall that saturates the ground
  • Lahars - mixture of water and rock fragments flowing rapidly down the slopes of a volcano - stratovolcanoes.
  • Tsunamis - generated by explosive eruptions on volcanic islands or by submarine caldera collapse
• • Gases - eruptions are associated with the emission of a range of toxic gases including carbon monoxide and carbon dioxide. Carbon dioxide collects in hollows as it is denser than air. Collects in low lying volcanic areas.
  • Sulphur dioxide - emissions can cause acid rain and inhaling the gas can cause serious health problems. Reflects light can cause cooling.
• • Seismic activity - seismometers used to detect vibrations beneath volcanoes. Buried to maximise their sensitivity to seismic vibrations and to shield them from wind vibration.
  • Gas emissions - as magma rises into magma chambers gases escape due to the depressurising of magma. Just before an eruption, sulphur dioxide levels can drop rapidly due to sealing of gas passages by hardened magma leading to an increase in pressure.
• • Ground deformation - movement of magma within the lithosphere can deform the ground above. Swelling of volcano shows that magma has collected near surface. Rate of swelling is monitored.
  • Gravity measurements - used to measure gravity changes in the magma chamber below the volcano. A decrease in gravity could be due to magma draining out of the chamber