Volcanoes - Simone Damiano · Volcanoes Key words: cinder cones, pyroclastics, eruption cloud, lava ﬂows, ... Stratovolcanoes or composite volcanoes are tall conical mountains composed

S c i e n c e sVolcanoes

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

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

Plate boundary: relation to volcanoes

Composition of magmatic rocksComposition of magmatic rocks

Mafic FelsicSiO2< 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.

Granite

Basalt

Felsic: highest content of silicon (Si); these rocks (e.g. granite) are usually light coloured and more viscous than mafic.

acidity

Volcanoes



Basalt Granite

Granite

acidityVolcanoesExplosive 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).

Felsic lavaFelsic (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.



Basalt Granite

Granite

acidityVolcanoesExplosive 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).

Mafic lavaMafic 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"

Fuji Mountain - Japan

Hekla Volcano - Iceland

Mauna Loa - Hawaii

Stromboli - Italy

There are different kinds of volcanoes classified by structure and composition

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.

Cinder cones Volcanoes

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.

Stratovolcanoes

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.

Shield volcano

Olympus Mons

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.

Fissure vent

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

Supervolcano

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)

Submarine 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.

Effects of volcanoes

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.


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

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

Effects of volcanoesA 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


A volcanic tsunami

Effects of volcanoesVolcanic 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 distributionThere a re about 500 active volcanoes on the Earth, with about half of these located around the Pacific “Ring of Fire”.

10 active volcanoes are located in Italy: Colli Albani Volcano complex; Vesuvio, Campi Flegrei, Ischia and Procida; Etna, Stromboli, Lipari, Vulcano and Pantelleria

Active volcano

Dormant volcano

Have erupted in recorded history

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

Classification of volcanoes

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”

Erta Ale “smoking mountain” volcano

Atoll formation and coral reef

Ngerebelas Atoll, Palau Islands - Philippines


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


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

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

1- All around the volcanic island a reef form

4- If the island continues sinking, an atoll can form, with a new lagoon in the middle

new corals

dead corals

forming reef

Castro!Huber: Marine Biology, Fourth Edition

I. Principle of Marine Science

2. The Sea Floor © The McGraw!Hill Companies, 2003

40 Part One Principles of Marine Science www.mhhe.com/marinebiology

Hot spots, the columns of hot magma that rise up from deep in themantle, remain more or less stationary. When they occur under anoceanic plate, the plate moves along above them, each new eruptionof magma breaking out at a slightly different place. The result is alinear chain of volcanoes. The Hawaiian Islands are part of one suchchain called the Hawaiian Ridge. The Ridge is connected to theEmperor Seamount chain, a string of seamounts stretching to thenorthwest. Each seamount and island in the chain was producedwhen magma erupted while the plate was over the hot spot. Thevolcanoes therefore get older moving away from the hot spot. Theisland of Hawai‘i, the youngest of the Hawaiian Islands, beganforming less than a million years ago. It is still erupting today, andthe island grows as the lava from the eruptions solidifies into newrock (see Fig. 11.1). Much of the island is smooth, bare volcanicrock because there has not been enough time for erosion or for veg-etation to grow. Kaua‘i, at more than 5 million years old, is the old-est of the main Hawaiian Islands. It is densely vegetated, anderosion has produced steep, jagged cliffs.

The remaining islands and seamounts of the Hawaiian Ridgeand the seamounts of the Emperor chain get progressively oldermoving to the northwest. For example, Midway, about two-thirdsof the way up the Hawaiian Ridge, is about 25 million years old.The bend that separates the Emperor chain from the HawaiianRidge was caused when the Pacific Plate changed its direction ofmovement a little less than 40 million years ago. Meiji Seamount, atthe northernmost end of the chain, is 70 million years old.

The ancient Hawaiianshad a remarkably similar expla-nation for the formation oftheir islands centuries beforethe days of detailed geologicalstudies. They recognized boththat the islands were formed byvolcanoes and that the eroded,densely forested Kaua‘i wasolder than the erupting, rela-tively barren island of Hawai‘i.They had no way of knowingthat their islands were movingalong on a lithospheric plate, of course, so they concludedthat it was the volcanoes thatmoved. Hawaiian tradition hasit that Pele, the Goddess ofFire, originally lived on Kaua‘ibut, under repeated attacksfrom her sister Namakao-kaha‘i, Goddess of the Sea,moved from island to island to her present home in the Kilauea Volcano on Hawai‘i.

The formation of theHawaiian Islands by a hot spotprovides the solution to a mystery

that once puzzled biologists. The Hawaiian Islands contain morethan 800 different species of fruit flies, most of which are foundnowhere else. And yet the islands are geologically very young. Howcould so many different kinds of fruit fly have evolved in such ashort time?

The answer is that they didn’t. The fruit flies have beenaround a lot longer than today’s Hawaiian Islands. They once livedon older islands in the chain, most of which have sunk below sealevel and are now seamounts. The flies island-hopped to the newislands that appearedover the hot spot.

A new Hawaiianisland may be on theway. A young subma-rine volcano calledLoihi has been foundsoutheast of the islandof Hawai‘i. Loihiprobably won’t breakthe surface for severalmillion years, so don’tget your surfboardready yet.

HOT SPOTS AND THE CREATION OF THE HAWAIIAN ISLANDS

Pele’s house: volcanic eruption at Kilauea,Hawai‘i.

Siberia Alaska

Canada

Aleutian Islands

Meiji Seamount(70 million yr)

55–60 million yr

45–50 million yr

Em

perorS

eamounts

Hawaiian RidgeMidway Island(25 million yr)

Stationaryhot spot

Moving lithosphericplate

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)

VolcanoesOceaniccrust

Astheno-sphere

HawaiianIslands

The Emperor Seamounts and Hawaiian Island chain.



Ngerebelas Atoll, Palau Islands - Philippines

The viscosityThe 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.

Low viscosity

High viscosity

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.

Remember: viscosity is different from density

The viscosityViscosity of fluids with variable compositions

Fluid Viscosity [Pa·s]

peanut butter ≈ 250

lard ≈ 100

ketchup 50–100

molten chocolate 45–130

honey 2–10

blood (37 °C)[8] (3–4)·10−3

Liquid Viscosity [Pa·s]

olive oil 81

castor oil 985

milk 3 ·10-3

mercury 1.526·10−3

benzene 0,604·10−3

water 0,894·10−3

Viscosity of common liquids

Incr

easi

ng V

isco

sity

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Volcanoes - Simone Damiano · Volcanoes Key words: cinder cones, pyroclastics, eruption cloud, lava ﬂows, ... Stratovolcanoes or composite volcanoes are tall conical mountains composed