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Environmental Science > 4.3 - Volcanic Deposits > Flashcards

4.3 - Volcanic Deposits Flashcards

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1
Q

AD 79 Vesuvius eruption

A

Pompei was smothered in ash from a pyroclastic flow. There were thousands of fatalities.

The ash preserved bodies and even loafs of bread which provided exceptional insights into the life of ancient pompei.

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2
Q

What are Plinian columns?

A

They are columns of ash and pumice that rise several kilometres in the air. They are named after Pliny the Younger who provided an eyewitness account to the historian Tacitus about the event of pompei in AD 79.

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3
Q

What can the distribution of ancient volcanic deposits help provide answers to?

A

The distribution of ancient volcanic deposits can help answer:

  • If volcanic eruptions trigger mass extinctions
  • If we get more volcanism following deglaciation
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4
Q

What are columnar joints?

A

Columnar jointing forms in lava flows. Dikes, ignimbrites and shallow intrusions of all compositions

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5
Q

How do cracks form in columnar joints?

A
  1. Conductive heat loss from magma filled sill or dyke to country rock
  2. Cracks form due to shrinkage on further cooling of the hot rock-shrinkage creates tension within the rock that is relieved by cracking. The cracks propagate inwards from the cooling surfaces.
  3. Rock thickens inwards, cools, shrinks, and cracks grow
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6
Q

What is entablature columnar jointing?

A

It is the ingress of water in upper parts of lava, flow modifies isotherm and produces fan like columns

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7
Q

What is pillow lava?

A

Pile of pillows ‘budding’ one from another = dyke feeding magma to the surface

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8
Q

What is tephra?

A

It is a collective term for pyroclasts

Pyroclasts/tephra can include:

  1. Fragments of new lava (juvenile)
  2. Individual crystals
  3. Accidental/lithium fragments (non juvenile)
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9
Q

What is lapilli?

A

It is ‘little stones’ that can include silicic pumice (juvenile), mafic scoria (juvenile), and lithic (non-juvenile) clasts

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10
Q

What do different eruptive styles do?

A

They produce different pyroclastic deposits which can be closely related and often occur during the same eruption

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11
Q

What are the 3 pyroclastic eruption styles?

A
  1. Pyroclastic fall (fall of ash or tephra)
  2. Pyroclastic flow
    - Pumice flows (ignimbrites)
    - Block and ash flows (Nuee ardentes)
  3. Pyroclastic surges
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12
Q

What are pyroclastic fall deposits?

A

It is fallout from an eruptive column (pilinian)

  1. Velocity maxima is reached in the core of the plume
  2. At edges, particles encounter velocities that are insufficient to keep particles aloft
  3. The tephra falls back to the surface
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13
Q

What are the characteristics of fall deposits?

A

Mantle topography

  • Parallel bedding
  • Well sorted
  • Often shows size grading
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14
Q

What influences the deposit of pyroclastic fall?

A

In theory deposits will be equally thick at any given distance and direction from the vent (wind complicates this)

Particle size and exit velocity influence its lifetime in atmosphere and dispersion from the vent

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15
Q

What are pyroclastic flow deposits?

A

They are produced by pyroclastic flows (pyroclastic density currents)

  • They are high speed avalanches of hot ash, rock fragments and gas, ground hugging
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16
Q

What are the characteristics of pyroclastic flows?

A

They fill the topography

  • Bedding is relatively rare
  • Poorly sorted
  • Often graded
17
Q

What are block and ash flows?

A

These contain dense lava fragments derived from the collapse of a growing lava dome or dome flow eg. Mt Unzen and Merapi (2006)

18
Q

What are pumice flows (ignimbrites)?

A

These contain vesiculated, low-density pumice derived from the collapse of an eruption column (found at a volcanic field in mexico)

19
Q

What is a pyroclastic surge?

A

It is a more energetic and dilute mixture of searing gas and rock fragments

Surges move easily up and over ridges and flows tend to follow valleys

20
Q

What are the characteristics of pyroclastic surges?

A
  • Partially topographic ally constrained
  • Fine grained
  • Cross bedding characteristics
  • Intermediate sorting
  • Often graded
  • Energetic eruptions so may find evidence of volcanic bombs
21
Q

What is the Minoan eruption (1625 BC)?

A
  • 70km3 of material and ash was dispersed throughout the eastern med
  • The eruption column reached around 40km in height
  • Deposits were roughly 60km on santorini
  • It buried the city of Akrotiri, although the city’s appears to have been evacuated before the eruption as there were no bodies
  • It likely impacted Minoan civilisation although the extent of its impact is controversial
  • The eruption gave birth to the legend of Atlantis
22
Q

What was the Caldera eruption?

A

It was a plinian eruption that generated a sustained plume estimated at a height of 36km

It produced a pumice fall deposit of up to 6m on santorini

23
Q

What was phase 2 of the caldera eruption?

A

Access of seawater to the vent initiated violent phreatomagmatic explosions and triggered the generation of base surges that spread radically away from the vent, and formed stratified deposits up to roughly 12m thick

24
Q

What was phase 3 of the caldera eruption?

A

Pumiceous pyroclastic flows transitional to mud flows, increasing water to magma ratios produced denser, partly wet, low temperature density currents

25
Q

What was phase 4 of the caldera eruption?

A

Venting of high temperature (300-500 degrees) pyroclastic flows, which produced fine-grained, non welded ignimbrites around the caldera rim and the coastal plains

26
Q

How can we estimate eruptive volume?

A

Mapping out tephra dispersal can provide key information about past eruption magnitude

  • there is a volcanic explosivity index
27
Q

Volcanic explosivity index

A

VEI - measures magnitude of eruptions

  • Based on volume of explosive products plus height of the eruption column
  • Each successive category equals a 10 fold increase in explosive power
28
Q

Eruption size distribution

A

VEI 0 : quiet, effusive eruptions of lava, typically a threat to local property only

VEI 1-3 : progressively more violent explosive eruptions capable of local damage

VEI 4-5 : moderate explosive eruptions capable of regional damage and disruption

VEI 6-7 : large to gigantic explosive eruptions capable of global impact through climate modification

VEI 8 : super-eruptions capable of severe climate modification

29
Q

Magnitude and frequency of eruptions

A

The relationship between magnitude of eruption, expressed as volume of ejecta erupted, and the frequency in number of eruptions per thousand years

  • Several small explosive eruptions (VEI 4) every year
  • Moderate explosive events (VEI 5) every decade or so and have regional impact
  • Large explosive eruptions (VEI 6-7) have return periods of a century or more
  • 2 VEI 8 events every 100 millennia
30
Q

What do you need to understand the size and frequency of past volcanism?

A

You need to accurately describe and quantify their eruptive deposits

31
Q

What are pyroclastic eruption deposits related to?

A

The wide range of deposits are related to magma water interactions, whether there was a sustained eruptive column etc.

32
Q

What is useful about deposits eg. Minoan and Santorini?

A

Can use to understand how ancient eruptions unfolded and the likely environmental impacts using VEI scale

Environmental Science (15 decks)

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