Hazardous Earth

• Eruptions from least to most hazardous

  • Icelandic
  • Hawaiian
  • Strombolian
  • Vulcanian
  • Vesuvian
  • Plinian
• Eruptions at hot spots

  • A hot spot is a fixed area of intense volcanic activity where magma from a rising mantle plume reaches the Earth’s surface
  • As the plate moves over the plume, volcanoes that were once active become dormant and then extinct, with the area directly above the plume being the most active
  • Continental drift and the movement of a plate over a plume creates a chain of volcanic islands
  • For example the movement of the Pacific plate northwest at an average rate of 10cm per year has created the basaltic Hawaiian Islands
• Super-volcanoes

  • A volcano that erupts more than 1000 km3 of material in a single eruption event
  • E.g. Yellowstone in Wyoming (caldera of 75km diameter)
  • Most recent super-volcano eruption was 27,000 years ago at Taupo, New Zealand
• Hazards produced by volcanoes

  • Lava flows
  • Pyroclastic flows
  • Tephra
  • Toxic gases
  • Lahars
  • Floods
  • Tsunamis
• Lava flows

  • Basic (basaltic) lava is free-flowing so flows easily
  • Acidic lavas such as rhyolite are viscous so do not flow easily
  • Destroy infrastructure, property and crops through burning, bulldozing and buired
  • Lava flows rarely cause injuries or fatalities
• Pyroclastic flows

  • Combination of very hot gases, ash and rock fragments travelling at very high speed
  • Follow the contours of the ground and destroy everything in their path
  • Inhaling the gas and ash causes almost instant death e.g. Mount Vesuvius and Pompeii in AD 79
• Tephra

  • Any material ejected from a volcano into the air
  • Ranges from very fine ash to large volcanic bombs
  • Buries farmland in layers of ash and destroys crops
  • Eruption columns can carry material into the stratosphere because the temperature of the ejected material is hotter than the surrounding air, causing it to rise very fast
  • Transport can be disrupted on the ground and in the air e.g. Iceland in 2010 - 100,000 flights cancelled
  • Buildings can collapse due to the weight of accumulated ash and people with respiratory diseases may struggle to breathe
• Toxic gases

  • Includes carbon monoxide, carbon dioxide and sulphur dioxide
  • Deadly and silent threat to humans
  • When sulphur dioxide combines with atmospheric water, acid rain is produced
  • This can damage crops and pollute surface water and soils
• Lahars

  • A type of mud flow with the consistency of wet concrete
  • Snow and ice on a volcano summit melt and flow rapidly down the cone, mixing with rock fragments, ash and soils
  • Lahars can travel at speeds up to about 50 km/h
  • E.g. lahars from the Nevado del Ruiz eruption, 1984, caused 23,000 deaths in Armero, Colombia
• Floods

  • Volcanic eruptions beneath ice fields or glaciers cause melting
  • During an eruption, vast quantities of water accumulate until they find an exit from under the ice
• Tsunamis

  • The eruption of some island volcanoes can cause displacement of ocean water and tsunami waves which can travel at speeds of up to 600 km/h
  • In deep water, they have a height that is usually less than 1 metre and a very long wavelength of up to 200 km
  • As they get closer to the shore, tsunami waves increase in height and when they break, transfer energy and water along the shore and inland
  • E.g. tsunamis from the Krakatoa eruption in 1883 are believed to have drowned 36,000 people
• Long-term impacts of volcanic hazards

  • Ash blocks sunlight from reaching the Earth's surface and can lead to global temperatures reducing
  • E.g. Toba, Indonesia led to a fall in the global human population
  • Release of sulphur dioxide also added to cooling
  • Mixed with water in the atmosphere, sulphur dioxide forms sulphuric acid, which reflects insolation
• Explosive volcanoes

  • Found at destructive plate boundaries
  • Pyroclastic flows and other projectiles due to pressure build up due to high silica content of the magma
  • Lava flows slow moving
  • Tsunamis may be caused when the eruption is close to the coast / underwater and where there is a displacement of water
• Effusive volcanoes

  • Found at constructive plate boundaries
  • Lower viscosity water which moves much faster
  • Less pressure build-up before the eruption means little tephra and ash, but gas emissions still possible
  • If under an ice sheet, then flooding from the meltwater is possible
• Geological evidence to support Wegener's theory

  • South America and Africa fit together despite being on different sides of the Atlantic
  • Evidence from 290 million years ago suggests glaciation in southern Africa, Australia, Antarctica etc. suggesting that they were joined and near the South Pole
  • Mountain chains and some rock sequences are similar e.g. northeast Canada and northern Scotland
• Biological evidence to support Wegener's theory

  • Similar fossil brachiopods (marine shellfish) found in Australian and Indian limestones
  • Similar fossil animals found in South America and Australia, especially marsupials
  • Fossils from rocks younger the Carboniferous period, in places such as Australia and India, showing fewer similarities, suggesting that they followed different evolutionary paths
• Evidence for sea floor spreading

  • Palaeomagnetism - involves measuring small changes in the magnetism of rocks either side of a mid-ocean ridge
  • At divergent / constructive plate boundaries, lava flows cool and solidify, the iron particles are locked in alignment to the magnetic pole which changes orientation every 400 - 500,000 years, reversals not predictable and not periodic in nature
  • Sea-floor spreading as the newer, younger lava erupts it pushes the older solidified rock aside (ridge push process)
• Evidence for sea floor spreading

  • At ocean margins, subduction of oceanic plate can occur contributing to the slab pull process
  • Age of sea floor rocks - in the 1960s, a drilling programme recovered cores in water up to 7000m deep in the ocean floor
  • The cores revealed that the thickest and oldest sediments were nearest to continents
  • No oceanic crust was older than 200 million years
  • Evidence of continental drift - fossil evidence, glacial deposits showing evidence of the former supercontinent, shape of continents
• Movements of the Earth's crust form rift valleys

  • Rift valleys form on constructive plate margins e.g. central Iceland rift valley
  • The convection currents at this plate margin diverge, pulling the Eurasian and North American plates apart by on average 2.5 cm per year
  • The crust has stretched and become thinner - the stresses that this creates results in faulting parallel to the plate margin
  • Allowing sections of crust to sink to the mantle as they are no longer supported by the structure of the tectonic plate
  • In places lakes form in the sunken land
• Types of plate boundary

  • Divergent / constructive
  • Convergent / destructive
  • Conservative