Transcript Lecture 6: Four Classic Eruptions

Four Classic Eruptions
• Thera/Santorini - 1,620 BC
– Impact on Minoan Civilization; legend of Atlantis
• Vesuvius - 79 AD
– Detailed eyewitness accounts; well excavated ruins
• Krakatau - 1883
– First instrumental data during major eruption
• Mt. Pelée - 1902
– >30,000 people killed in a single dome collapse event
that generated dense block and ash flows
Volcanic Explosivity Index
Eruption magnitude can either be measured based on the volume of lava
ejected as tephra (ash and pumice) during an explosive eruption or the
volume of lava extruded during an effusive eruption.
VEI is a log scale and is empirically derived from historical data.
VEI
0
1
2
3
4
5
6
7
8
Description
non-explosive
gentle
explosive
severe
cataclysmic
paroxysmal
colossal
super-colossal
mega-colossal
Plume Height Volume
<100 m
1000s m3
100-1000 m 10,000s m3
1-5 km
1,000,000s m3
3-15 km
10,000,000s m3
10-25 km
100,000,000s m3
>25 km
1 km3
>25 km
10s km3
>25 km
100s km3
>25 km
1,000s km3
Volcano
Vesuvius
Vesuvius
Vesuvius
Etna
Santorini
Country
Italy
Italy
Italy
Italy
Greece
Classification
Hawaiian
Haw/Strombolian
Strom/Vulcanian
Vulcanian
Vulcanian/Plinian
Plinian
Plin/Ultra-Plinian
Ultra-Plinian
Ultra-Plinian
Date
5960 BC
3580 BC
79 AD
1500 BC
1650 BC
VEI
5
5
6
5?
6
How often
daily
daily
weekly
yearly
10's of years
100's of years
100's of years
1000's of years
10,000's of years
Example
Kilauea
Stromboli
Galeras, 1992
Ruiz, 1985
Galunggung, 1982
St. Helens, 1981
Krakatau, 1883
Tambora, 1815
Yellowstone, 2 Ma
Volcano Type
Complex
Complex
Complex
Shield
Shield
VEI Redux
NB: Volumes are
DRE (dense rock equivalent)
Source: USGS
Deadliest Eruptions Since 1500 AD
Eruption
Year
Casualties
Major Cause
Nevado del Ruiz, Colombia
Mont Pelée, Martinique
Krakatau, Indonesia
Tambora, Indonesia
Unzen, Japan
Lakagigar (Laki), Iceland
Kelut, Indonesia
1985
1902
1883
1815
1792
1783
1586
25,000
30,000 (29,025)
36,000 (36,417)
92,000
15,000 (14,030)
9,000 (9,350)
10,000
Mudflows
Pyroclastic flows
Tsunami
Starvation
Volcano collapse, Tsunami
Starvation
Hellenic Arc - Regional Tectonic Setting
NASA Space Shuttle Image
November 13, 1995.
Eruption Frequency vs. VEI
Similar frequency-magnitude
behavior as observed for
earthquakes.
1600-1982
Volcanic Deaths by Country
Total = 238,000
Source: O. Melnik, Bristol
Location Map: Thera/Santorini, Greece
NASA 1983 Shuttle Image
Steep cliff faces
define ancient
caldera wall
Minoan Tephra - 1,650 BC
Basalts and Andesites
Caldera Wall - formed
during 21 ka event
Santorini
Stratigraphy
50 m deposit of
Minoan Tephra
Photos: R. Decker
Kameni Islands
Islands for after the caldera forming event that deposited the
Minoan Tephra in 1,620 BC. Eleven lava flow eruptions since
197 BC, with most recent being in 1950 on Nea Kameni.
Photo: R. Decker
Thera Summary
• Thera was an important center of Minoan civilization.
Eruption has been linked to Atlantis legend.
• Plinian eruption and caldera collapse event at 1,620 BC.
Contributed to the decline and ultimate demise of Minoan
culture ~1,450 BC.
• ~20 km3 of tephra was deposited in the sea and on adjacent
islands
• Large tsunamis were generated during the eruption, which
affected coastal Mediterranean cities.
• City of Akroteri was buried in 1-2 m of ash, but the
population of ~30,000 was apparently evacuated prior to the
eruption as no bodies were found during excavations.
Vesuvius, Italy
Photo: C. Wood and NASA (above)
Primary Plinian & Sub-Plinian Eruptions
of Somma-Vesuvius
Name of the Eruption
Age (years before present or AD)
Codola
25000
Basal Pumices ( Sarno)
17000
Greenish Pumices
15500
Mercato (Ottaviano)
7900
Novelle
no date available
Avellino
3750
Pompeii
1900 (79 AD)
Pollena
472 (AD)
1631
1631 (AD)
Source: http://vulcan.fis.uniroma3.it/vesuvio/vesuviustext.html
79 AD Eruption Chronology - I
• Greek and Roman scholars knew that Vesuvius was a volcano
before 79 AD.
• Feb. 5 AD 62, large earthquake jolted region that is now
Naples. Common for region, so they were largely ignored.
Events caused significant damage to Pompeii and Oplonti.
Seneca stated that quakes lasted several days and weakened
with time; he advocated for future hazard planning.
• Pliny the Elder was the admiral of the roman fleet based in
Misenum; he went to rescue people close the volcano and died
(probably of a heart attack) once on shore near Stabiae
(downwind of the eruption).
• Pliny the Younger observed the eruption from Cape
Misenum (~20 km away) and his letters form the basis of our
detailed knowledge of a large explosive event and subsequent
column collapse. “Plinian” eruptions are named in his honor.
The Route of Pliny the Elder
Source: http://vulcan.fis.uniroma3.it/vesuvio/79_eruption.html
79 AD Eruption Chronology - Pliny’s Account
• 1 PM on 24 August large “cloud” appeared in sky above
Vesuvius. Reported to resemble a Mediterranean pine tree.
• By nightfall, pumice fragments were falling on Stabiae.
Earthquakes shook the area.
• At dawn, sunlight was blocked from heavy ash fall.
Sulphurous fumes were widespread. Lightning discharged
within the ash cloud.
• Tsunamis were generated by the earthquakes. Similar
phenomena were described for the 1631 eruption.
• Sky was completely blackened at Misenum.
• Smooth cone of Vesuvius was revealed to be a stump, once
ash had finally cleared.
79 AD Eruption Destroyed Pompeii
Victim of a pyroclastic surge
View of Vesuvius from Pompeii
Photo: R. Decker (right) and
Pompeii Museum (above)
Pumice Deposits from Early Airfall
Victims in Later Pyroclastic Surge Unit
Indonesian Island Arc Volcanoes
Location Map, Krakatau
LANDSAT
Anak Krakatau Emergent since 1927
Photo: R. Decker
Tsunami Inundation and Ship Tracks - 1883 Eruption
All three ships survived the eruption despite their proximity.
Atmospheric Pressure Record at Batavia Gasworks
Sound waves
took ~5 m
to reach
Jakarata.
First volcanic
infrasound
measurements!
Note multiple
pressure
pulses with
largest at
10:15 AM
local time,
which caused
the barometer
to go offscale.
Discrete
explosions or
PFs?
Tide Gauge Record Tandjong Priok
Arrival of tsunami
Normal Tides
~2 hr wave period
superimposed on
tides.
Pre-1883 Reconstruction
Post-1883 Reconstruction
1883 Krakatau Eruption Summary
• Rakata volcano was the largest center on Krakatau Island prior to eruption.
800 m AMSL.
• Krakatau and nearby islands were uninhabited.
• Vague reports of eruptions on the islands in 1680 and 1681.
• Frequent minor earthquakes in 1870’s near Sunda Straits.
• Eruption sequence began suddenly on 20 May 1883. Audible explosions
>150 km away. Ash deposited from air fall >500 km away.
• Volcano quieted down by May 27. Close approach by ship yields reports of
deafening noise.
• Climatic eruption began on August 26 and continued until August 27, 1883.
Several ships were in close proximity during eruption.
• Most powerful explosion occurred 10:02 AM and was heard 4811 km away
at Rodriguez Island in the Indian Ocean. Pressure transient of 1.45 mbar
recorded in Tokyo.
• Marine ignimbrite deposit is 40 m thick in places. ~20 km3 DRE erupted.
Lesser Antilles Island Arc and Caribbean Plate
PRVI
seismicity: epicenters < 30 km depth from USGS
Lesser Antilles
Caribbean
Lesser Antilles Active Volcanoes
Mt. Pelée and St. Pierre in 1987
Map of Devastation of May and August
PFs at Mt. Pelée
Volcano was active for weeks
prior to major dome collapse.
Citizens were forced to stay
in town for local election that
was to take place on May 11.
Pelée Stratigraphy - 14,000 yBP
Distribution of Pelean Deposits
Distribution of Plinian Deposits
Mt. Pelée: Future Volcanic Hazards