ESCI 111 Week 5-6

What controls volcanic activity?

  • Tectonic settings
  • Eruption styles
  • Types of magma
  • “Type” of volcano

Where are Volcanoes on earth?

  • Divergent margins = mid ocean ridges
  • convergen t margins = subduction zones (oceanic arc)
  • Intraplate = hot spots

Types of magma

  • Not all magma is made equal

Silica content is the mane chemical index for what magma we have

As we move up in silica we deal with a declining temperature - increase in viscosity (Resists flow)

Rhyolite brakes brottaly (Gas content)

Eruption styles

Form lava flows or domes

Magma viscosity (magma type) controls how fast/far the lava flows

  • Mauna Loa (Hawaii) - Basalt
  • Kelud (Indonesia) - Andesite
  • Chaiten (Chile) - Rjyolite (generally more explosive)

Low gas content and low viscosity make for a mild explositivy of fluids

  • Builds steep sided cones of loose pyroclastic fragments

Make for mild explositivy or fluid materials

Volcano can be a single cone or the cones can be small satellites on a larger volcanoes

Intesse explositivy: Rhyolite plinian deposit

Plumes can go buoyant into the stratosphere, to traverse the globe

Widespread deposits that evenly mantle the pre-existing topography

The tast eruption of Taupo was small compared to some in the geologic record, but still reached to well beyond Gisborne

Explosive eruptions

Proclastic flows:

  • Explosive plumes sometimescannot ascend into the atmosphere
  • They collapse to form a Proclastic flows
  • They are ground hugging flows of hot particles and gasses

If water is added repid heat transfer from magma to water can chill a lava flow to rwstrict its movement

Steam eruptions are very hard to predict \

Water can increase the vigour of an otherwise mellow eruption

Plumes

  • Magma is broken into particles by fragmentation
  • Pyroclast = Broken Fire
  • Degree of explosivity is controlled by volatile content, magma viscosity

Lower gas content - gas can escape

What controlls volcanic activity?

Fissure volcano:

A linear volcanic vent or series of vents

Most often associated with effusive basaltic eruptions but can be explosive

Mid oceanic ridge volcanism

Shield Volcano:

Broad low profile volcano built by a succession of basaltic lava flows

Eg. mauna Loa (Hawaii) the biggest volcano on earth

Mosly basaltic lava flowsbut can have cinder cones and occanisal explosive activity

(Rangitoto volcano)

Olympus Mons on Mars - biggest volcano in the solar system

Composite cones / straddle volcanoes:

Steep sided uilt up in successive layers of lava and pyroclastic material

Usually intermediate magma types - andesite and dacite

Common in continental volcanic areas

Classical volcano shape

(Ruapehu, Taranaki)

Combination of eruption styles

Caldera Volcanoes:

Highly explosive, large volumes

Subsidence structures formed from underlying chambers being empty

Crate big pyroclastic flows

(Lake Taupo)

Large igneous provinces:

Similar to hot pots but on an extreme scale

None in historical time

Deccan traps (india)

  • 500,000 km2
  • 2000m thick
  • 66.25 million years ago, and lasted -30,000 years

Size of eruptions

  • Hard to but a measure on it / classify them

Hunga Tonga - Hunga Ha’apai

  • Shock waves strong enough to shatter windows
  • Shockwaves traveled round the earth at least 4 times
  • Eruption was heard in NZ (2000 km away) some bangs heard in Alaska (9700 km)
  • Mesosphere altitude of plume suggests that the main blast was more powerful that the most recent eruption of Taupo

Volcanology

Combination of fieldwork, chemistry, physics, material science, mathematics

Monitoring active volcanoes

geography/seismology

Seismology used to detect and read volcanic earthquakes

Activity detected by field instruments

Volcanic earthquakes have a different character to tectonic earthquakes, they show movement of magma in the subsurface

Mauna Loa permanently installed volcano monitoring networks

Seismology is used to detect and read volcanic earthquakes

Pinatubo eruption was predicted to the day by the use of an emergency network of seismometers

Deformation

Geodetic techniques are used to monitor volcanic deformation

  • GPS
  • Tilt meters
  • Satellites
  • Level lines

Think of volcanoes being static but they very much aren’t - magma chambers etc

Satellite techniques (INSAR) can detect deformation remotely

Deformation of Mt Etna, Sicily 2002

It still worked, just cant use it as reliable data

Volcanic gasses/fluids

The amount of chemistry of gas discharge from a volcano can tell a lot about its magma

Can be measured from afar with spectroscopic methods, or up close with direct sampling.\

UV spectroscopy can measure these gasses

Remote sensing

Many parameters can be measures from satellites, be remote sensing

Good for monitoring volcanoes when you can’t have a team on the ground

Can measure SO2 emissions

Direct observation

Field observations of active eruptions and their deposits

Some processes can only be determined by direct observation

Rhyolite eruption in progress - explosivity city and effucitvy happened at the same time

Drones

Look at things you otherwise could not see - get gear into places you can't reach

Geological mapping

When volcanoes are close to people or infrastructure, hazards become risks

Geological mapping allows eruption scenarios to be estimated and planned for

How people perceive risk

Stratigraphy / field geography

Distribution of deposits tells about the size and dynamics of eruptions

Small detail about the conditions during the eruption

Petrology and geochemistry

The minerals in lava and the microchemistry of the minerals tells a lot about the history of the magma

Textural analysis

The physical relationship between minerals, melt and bubbles tells us about the magma ascent

The bubbles represent evolved gasses (volatiles)

Numerical modeling

Theoretical and mathematical studies can be used to predict how volcanic eruption will proceed

Analogue modeling

Can make magma in the lab and sede what it does

Allows processes we can’t witness to be proven

Bad stuff can happen

Some volcanic hazards are directly associated with eruptions, others can be ongoing

Lava flows you can usually outrun a small lava flow, they can be very destructive (houses and property damage)

Large igneous province development can be linked to mass extinctions and transitions in the geological record

Pyroclastic flows

Ground hugging flows - destroy everything in their path

Tephra fall

Locally can pile up to cave in houses, burry victims

Mummified bodies

Can be widespread and disrupt air traffic

makes fine ash

Volcanic gas

During eruption can load atmosphere with aerosoles that affect global climate

Can be deadly even in absence of eruption

Lake Nyos, Cameroon 1986 C02 released from lake killed 1700 people overnight

Mitigated by pipes that go into the lake and release CO2

Lahars (big mud flow)

Volcanic mudflows can occur with eruption, or long after

Can range from tiny to enormous

Tsunami

Eruptions can generate a tsunami

Secondary hazard