Download Volcanoes - Simone Damiano Ph.D.

S c i e n c e s
Volcanoes
Key words: cinder cones, pyroclastics, eruption cloud, lava flows, tephra, fissure and hydrothermal vent,
viscousity, mafic and felsic rocks, volcanic winter, lahar, volcanic gas discharge, atoll
Volcanoes
A volcano is a rupture in a planet's crust due to the
movements of the plates; thus allows hot magma, volcanic
ash and gases to escape from the mantle
Plate boundary: relation to volcanoes
Volcanoes are generally found where tectonic plates are diverging or converging.
A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent
tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent
tectonic plates coming together.
“Hot spot” volcanoes may form where plumes of lava rise from deep within the mantle to the Earth’s
crust far from any plate margins
Volcanoes
acidity
Composition of magmatic rocks
Mafic
Felsic
SiO2< 45%
SiO2> 63%
Basalt
Granite
Mafic: lowest content of silicon (Si); these rocks (e.g. basalt) are
usually dark coloured, and have a lower viscosity than felsic rocks.
Basalt
Felsic: highest content of silicon (Si); these rocks (e.g. granite)
are usually light coloured and more viscous than mafic.
Granite
Volcanoes
Explosive or not?
The chemical composition of magma influences his physical
properties. The viscosity of lava depends on the Si content.
Felsic magmas are lighter in colors and are thicker, therefore
move slowly.
Mafic magmas are low in Si, darker in colors and thinner. These lava
flow more easily (faster).
acidity
Composition of magmatic rocks
Mafic
Felsic
SiO2< 45%
SiO2> 63%
Basalt
Granite
Felsic lava
Felsic (or silicic) lavas such as rhyolite typically form, lava domes
(stratovolcano) and are associated with pyroclastic deposits. The high
viscosity and strength are the result of their chemistry, which is high in
silica, aluminum, potassium, sodium, and calcium, which have a higher
viscosity than other elements.
Felsic magmas can erupt at temperatures as low as 650 to 750 °C.
Granite
Volcanoes
Explosive or not?
The chemical composition of magma influences his physical
properties. The viscosity of lava depends on the Si content.
Felsic magmas are lighter in colors and are thicker, therefore
move slowly.
Mafic magmas are low in Si, darker in colors and thinner. These lava
flow more easily (faster).
acidity
Composition of magmatic rocks
Mafic
Felsic
SiO2< 45%
SiO2> 63%
Basalt
Granite
Mafic lava
Mafic or basaltic lavas are characterized by their high ferromagnesian
content, and generally erupt at temperatures in excess of 950 °C.
Basaltic magma is high in iron and magnesium, and has relatively lower
aluminum and silica.Viscosities can be relatively low.
Basalt lavas tend to produce shield volcanoes, because the fluidal lava
flows for long distances from the vent.
Underwater they can form "pillow lavas"
Granite
There are different kinds of volcanoes classified by structure and
composition
Fuji Mountain - Japan
Stromboli - Italy
Mauna Loa - Hawaii
Hekla Volcano - Iceland
Cinder cones Volcanoes
Cinder cones are the result from volcanic activity that erupts mostly small pieces of scoria and
pyroclastics that build up around the vent.
These can be relatively short-lived eruptions that produce a cone-shaped hill.
Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones.
Stratovolcanoes
Stratovolcanoes or composite volcanoes are tall
conical mountains composed of viscous lava flows
and other matter (tephra) in alternate layers.
Cinders and ash pile on top of each other, lava flows
on top of the ash, where it cools and hardens, and then
the process begins again.
Classic examples include Mount S. Helens, Mount
Vesuvius and Stromboli.
Shield volcano
A shield volcano is a type of volcano built almost entirely of fluid lava flows. They are named so
because of their large size and low profile.
Shield volcanoes are built up by effusive eruptions, which flow out in all directions to create a shield-like
structure. The types of eruptions that occur at shield volcanoes have been named Hawaiian eruptions,
Because of their gradual build up and near-continuous eruptive characteristics, shield volcanoes vary
widely in size with their age.
Mature shield volcanoes are the largest volcanoes on Earth. The largest shield volcano (and the largest
active volcano) in the world is Mauna Loa in Hawaii.
Shield volcano
Shield volcanoes are not only limited to
Earth, but have been found on Mars.
Mars' shields are much more massive than
those of Earth, reaching more than 27 km in
height and 563 km in diameter.
The most famous example is Olympus Mons,
a shield volcano that is the highest known
mountain in the solar system.
Olympus Mons
Fissure vent
A fissure vent, is a linear volcanic vent through which lava erupts,
usually without any explosive activity.
The vent is usually a few meters wide and may be many kilometers
long.
Fissure vents can cause large flood basalts and lava channels.
Fissures are usually found in or along rifts and rift zones, such as
Iceland and the Great Rift Valley in Africa. Fissure vents are also
often found in shield volcanoes.
Supervolcano
A supervolcano is a large volcano that usually has a large caldera and can potentially produce
devastation on an enormous, scale.
Such eruptions would be able to cause severe cooling of global temperatures for many years because
of the huge volumes of sulfur and ash erupted. They are the most dangerous type of volcano
Submarine volcanoes
Submarine volcanoes are common on the ocean floor.
Many lie at such great depths that the tremendous weight of the water above them prevents the
explosive release of steam and gases, although they can be detected by hydrophones.
Because of the rapid cooling effect of water, submarine volcanoes often form Pillow lava.
They may become so large that they break the ocean surface as new islands.
Hydrothermal vents (black smokers and cold seeps) are common near these volcanoes, and some
support peculiar ecosystems based on dissolved minerals (chemosynthesis)
Effects of volcanoes
The concentrations of volcanic gases can vary considerably from one volcano to the next. Water
vapor (H2O) is typically the most abundant volcanic gas, followed by carbon dioxide (CO2) and
sulfur dioxide (SO2) .
Other volcanic gases are hydrogen sulfide (HS), hydrogen chloride (HCl), and hydrogen fluoride (HF).
Dust increases the Earth's reflection of radiation (albedo) from the Sun back into space, thus cools
the Earth's lower atmosphere.
Many eruptions during the past century have caused a decline in the average
temperature at the Earth's surface for periods of one to three years.
Effects of volcanoes
A volcanic winter is a reduction of temperatures caused by volcanic ash and droplets of
sulfuric acid obscuring the sun and raising Earth's albedo (Earth's reflectivity of solar radiation),
during a large particularly explosive type of volcanic eruption
The Great Famine of 1315–1317 in Europe may have been precipitated by a volcanic
event, perhaps that of Kaharoa, New Zealand, which lasted about five years
Effects of volcanoes
A lahar is a type of mudflow composed of pyroclastic material, rocky debris, and water. The material
flows down from a volcano, typically along a river valley.
The term is a shortened version of "berlahar" which originated in the Javanese language of Indonesia
Effects of volcanoes
A volcanic
tsunami
Effects of volcanoes
Volcanic gas discharge
Lake Nyos is a crater lake in the Northwest Region of
Cameroon.
It’s a deep lake on the top of an inactive volcano.
The lake suddenly exploded, emitting a massive cloud of
about 1.6 million tons of CO2 on August 21, 1986, which
suffocated 1,700 people and 3,500 livestock in nearby
villages.
Volcanoes distribution
There are about 500 active
volcanoes on the Earth, with about
half of these located around the Pacific
“Ring of Fire”.
Classification of volcanoes
Active volcano
Have erupted in recorded
history
Dormant volcano
Have not erupted in
recorded history
A popular way of classifying magmatic volcanoes is by their frequency of eruption.
Those volcanoes that erupt regularly are called active while those that have erupted in
historical times but are now quiet are named dormant (or inactive).
Extinct volcanoes are those that scientists consider unlikely to erupt again, because the
volcano no longer has a magma supply. Examples of extinct volcano is mt. Amiata
10 active volcanoes are located in Italy: Colli Albani Volcano complex; Vesuvio,
Campi Flegrei, Ischia and Procida; Etna, Stromboli, Lipari,Vulcano and Pantelleria
Erta Ale “smoking mountain” volcano
Erta Ale is a basaltic shield volcano in the Afar Region and is the most active volcano in Ethiopia. It
is 613 mt high, with one or two active lava lakes at the summit which occasionally overflow. Volcanoes
with lava lakes are very rare: there are only five in the world. Erta Ale means “smoking mountain” in
the Afar language and its southernmost pit is known locally as “the gateway to hell”
Atoll formation and coral reef
Ngerebelas Atoll, Palau Islands - Philippines
Atoll formation and coral reef
The formation of an atoll starts from an active volcano in
correspondence of a hot spot region where plumes of lava
rise from the deep mantle
Atoll formation and coral reef
forming reef
1- All around the volcanic island a reef form
2- With the movements of the plate caused by the
oceanic floor expansion, the volcano cools and
the island sinks in the ocean. A coral reef grows
upward
new corals
dead corals
3- If the climate, light and temperature conditions
are satisfying, corals begins to form. The island
continues sinking and new corals form on the top
of dead corals’ skeletons
4- If the island continues sinking, an atoll can
form, with a new lagoon in the middle
, is 70 million years old.
Atoll formation and coral reef
Siberia
Alaska
Canada
Meiji Seamount
(70 million yr)
Ale u t
sl a
I
n
ia
n
ds
Kaua‘i (3.8–5.6 million yr)
O‘ahu (2.2–3.3 million yr)
Moloka‘i (1.3–1.8 million yr)
Maui (less than 1 million yr)
Hawai‘i (0.8 million yr to present)
Loihi (submarine; present)
E m pe r or
55–60 million yr
S eam
45–50 million yr
oun
Moving lithospheric
plate
ts
Ha
wa
ii a n
Midway Island
(25 million yr)
Rid
ge
Oceanic
crust
Hawaiian
Islands
Volcanoes
AsthenoStationary sphere
hot spot
Atoll formation and coral reef
Ngerebelas Atoll, Palau Islands - Philippines
The viscosity
The viscosity of a fluid is a measure of its resistance to deformation by shear stress.
For example, honey has a higher viscosity than water.Viscosity is due to the friction
between neighboring particles in a fluid that are moving at different velocities.
Liquids with higher viscosities make smaller
splashes when poured at the same velocity.
A fluid with low viscosity flows easily
because its molecular makeup results in very
little friction when it is in motion.
Low viscosity
Remember: viscosity is different from density
High viscosity
The viscosity
Viscosity of fluids with variable compositions
Viscosity of common liquids
Viscosity [Pa·s]
Liquid
Viscosity [Pa·s]
peanut butter
≈ 250
olive oil
81
lard
≈ 100
castor oil
985
ketchup
50–100
milk
3 ·10-3
molten chocolate
45–130
mercury
1.526·10−3
honey
2–10
benzene
0,604·10−3
blood (37 °C)[8]
(3–4)·10−3
water
0,894·10−3
Increasing Viscosity
Fluid