VolcanoesVolcanoesA volcano is a place on the Earth’s surface A volcano is a place on the Earth’s surface where magma breaks through. where magma breaks through. A volcano is called A volcano is called activeactive if it erupts lava, if it erupts lava, rock, gas or ash, or if it shows seismic rock, gas or ash, or if it shows seismic activity. activity. A volcano is A volcano is dormantdormant when is not when is not presently erupting but is considered likely presently erupting but is considered likely to erupt in the future. to erupt in the future. (Foxworthy and Hill)(Foxworthy and Hill) An An extinctextinct volcano will never erupt again. volcano will never erupt again.

vent

cone

magma chamber

conduit

A volcano is a ventvent or 'chimney' that connects the magma chambermagma chamber (subterranean cavity containing the gas-rich liquid magma) to the Earth’s surface. The volcano includes the surrounding cone of erupted material.

In the Earth's In the Earth's mantlemantle the temperature the temperature is hot enough (800-1.600 °C) to melt is hot enough (800-1.600 °C) to melt rock and form a thick, flowing rock and form a thick, flowing substance called substance called magmamagma. Lighter than . Lighter than the solid surrounding rock, the solid surrounding rock, magmamagma is is buoyant much like a cork in water; buoyant much like a cork in water; being buoyant, it rises.being buoyant, it rises.

Most Most magmasmagmas contain abundant contain abundant gas , as the gas , as the magmamagma rises closer to the rises closer to the surface, the pressure decreases, surface, the pressure decreases, which cause the gases dissolved in which cause the gases dissolved in the the magmamagma to expand. This expansion to expand. This expansion propels the propels the magmamagma through openings through openings fractures in the Earth’s surface fractures in the Earth’s surface creating a volcanic eruptioncreating a volcanic eruption

Once Once magmamagma is erupted, it is is erupted, it is calledcalled lava lava..

Volcanic eruptions do not occur randomly, but are mainly Volcanic eruptions do not occur randomly, but are mainly localized at plate boundaries. localized at plate boundaries. Some volcanoes, can be found in the interior of plates at areas called hot spotshot spots. Although most of the active volcanoes we see on land occur where plates collide, the greatest number of the Earth's volcanoes are hidden from view, occurring along spreading oceanic ridges.

Two styles of volcanic eruption: Two styles of volcanic eruption: Explosive and Effusive Explosive and Effusive ExplosiveExplosive: where rapidly escaping gas : where rapidly escaping gas bubbles (= vesicles) rip apart the bubbles (= vesicles) rip apart the magma, fragmenting it.magma, fragmenting it.EffusiveEffusive: where the magma leaks out : where the magma leaks out onto the surface passively as lava onto the surface passively as lava flows.flows.

VOLCANIC ACTIVITYVOLCANIC ACTIVITY

Sometimes basaltic lava pours out quietly from Sometimes basaltic lava pours out quietly from long fissures and floods the surrounding long fissures and floods the surrounding countryside with lava flow upon lava flow, countryside with lava flow upon lava flow, forming broad plateaux. forming broad plateaux. Lava plateauxLava plateaux can be can be seen in Iceland, India and USA. seen in Iceland, India and USA. (Tilling, 1985)(Tilling, 1985)

Shield volcanoesShield volcanoes are built almost entirely of fluid are built almost entirely of fluid lava flows forming a broad, gently sloping cone, lava flows forming a broad, gently sloping cone, with a profile much like that a warrior's shield. They with a profile much like that a warrior's shield. They are built up slowly by highly fluid basaltic lava that are built up slowly by highly fluid basaltic lava that spread widely over great distances.spread widely over great distances. The Hawaiian The Hawaiian Islands are composed of linear chains of these Islands are composed of linear chains of these volcanoes, including Kilauea and Mauna Loa, two volcanoes, including Kilauea and Mauna Loa, two of the world's most active volcanoes.of the world's most active volcanoes.

Mauna Kea (left) and Mauna Loa (right),

Some of the Earth's highest mountains are Some of the Earth's highest mountains are composite volcanoescomposite volcanoes; sometimes called ; sometimes called stratovolcanoesstratovolcanoes. They are typically steep-sided, . They are typically steep-sided, symmetrical cones of large dimension built of symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, alternating layers of lava flows, volcanic ash, cinders, blocks, and bombscinders, blocks, and bombs

Fuego (left) and Acatenango - Guatemala

In a "In a "StrombolianStrombolian"-type eruption huge clots "-type eruption huge clots of molten lava burst form the summit crater of molten lava burst form the summit crater to form luminous arcs through the sky. to form luminous arcs through the sky.

Stromboli

Stromboli is a volcan characterized by frequent activity Stromboli is a volcan characterized by frequent activity and high viscosity lavas.and high viscosity lavas.

A "A "VulcanianVulcanian"-type eruption is characterized by "-type eruption is characterized by a dense cloud of ash-laden gas that explodes a dense cloud of ash-laden gas that explodes from the crater and rises high above the peak. from the crater and rises high above the peak. Steaming ash forms a whitish cloud near the Steaming ash forms a whitish cloud near the upper level of the cone. upper level of the cone.

In a "In a "PeleanPelean" eruption a large quantity of gas, dust, " eruption a large quantity of gas, dust, ash, and incandescent lava fragments are blown out ash, and incandescent lava fragments are blown out of a central crater, fall back, and form a dense cloud of a central crater, fall back, and form a dense cloud that move down-slope at velocities as great as 100 that move down-slope at velocities as great as 100 miles per hour. Such eruptions can cause great miles per hour. Such eruptions can cause great destruction and loss of life if it occurs in populated destruction and loss of life if it occurs in populated areas, as demonstrated by the devastation of St. areas, as demonstrated by the devastation of St. Pierre during the 1902 eruption of Mount Pelee on Pierre during the 1902 eruption of Mount Pelee on Martinique, West Indies.Martinique, West Indies.

before after

““PlinianPlinian” eruption: on August 24, 79AD Vesuvius ” eruption: on August 24, 79AD Vesuvius blew its top, erupting tonnes of molten ash, pumice blew its top, erupting tonnes of molten ash, pumice and sulfuric gas miles into the atmosphere. and sulfuric gas miles into the atmosphere. Pyroclastic flows flowed over the city of Pompeii Pyroclastic flows flowed over the city of Pompeii and surrounding areas.and surrounding areas.

WHY DO COMPOSITE VOLCANOES TEND TO WHY DO COMPOSITE VOLCANOES TEND TO BE EXPLOSIVE AND SHIELD VOLCANOES BE EXPLOSIVE AND SHIELD VOLCANOES

NON-EXPLOSIVE??NON-EXPLOSIVE??

1) CHEMISTRY (COMPOSITION) OF LAVA

2) LAVA TEMPERATURE

COMPOSITE

- Higher silica content of lavas make them more viscous (thick).

- Lava temperatures are generally a few hundred degrees cooler than those of shield volcanoes, making the lavas “thicker” in consistency.

SHIELD

-Lower silica content of lavas make them more fluid (runny).

- Lava temperatures are quite hot, ~ 2200oF, making it easier for these lavas to “flow like water”.

PRODUCTS OF EXPLOSIVE ERUPTIONSPRODUCTS OF EXPLOSIVE ERUPTIONS

Pyroclastic materialPyroclastic material• RockRock fragments created by explosive eruptions fragments created by explosive eruptions

•magma explodes from volcano and solidifies in the airmagma explodes from volcano and solidifies in the air•existing rock is shattered by powerful eruptionsexisting rock is shattered by powerful eruptions

Volcanic blocks

Volcanic bombs

Lapilli

Volcanic ash

• Three products from an explosive eruptionThree products from an explosive eruption– Ash fall: volcanic ash that has fallen through Ash fall: volcanic ash that has fallen through

the air from an eruption cloud. A deposit so the air from an eruption cloud. A deposit so formed is usually well sorted and layered. formed is usually well sorted and layered.

– Pyroclastic flow: a hot, fast-moving and high-Pyroclastic flow: a hot, fast-moving and high-density mixture of fine and coarse particles density mixture of fine and coarse particles and gas formed during explosive eruptions or and gas formed during explosive eruptions or from the collapse of a lava domefrom the collapse of a lava dome

– Base surge: turbulent, low-density cloud of Base surge: turbulent, low-density cloud of rock debris and water and (or) steam that rock debris and water and (or) steam that moves over the ground surface at high speed. moves over the ground surface at high speed.

Deposits associated to ash fall are generally called Deposits associated to ash fall are generally called tuffstuffs, volcanic rock made up of rock and mineral , volcanic rock made up of rock and mineral fragments in a volcanic ash matrix. Tuffs commonly fragments in a volcanic ash matrix. Tuffs commonly are composed of much shattered volcanic rock are composed of much shattered volcanic rock glass chilled magma blown into the air and then glass chilled magma blown into the air and then deposited regularly on the ground like a snow fall. If deposited regularly on the ground like a snow fall. If the deposits are made by coarse – grained materials the deposits are made by coarse – grained materials they are called they are called volcanic breccia.volcanic breccia.

Pyroclastic flow consists in the lateral flowage consists in the lateral flowage of a turbulent mixture of hot gases and of a turbulent mixture of hot gases and unsorted pyroclastic material. The deposits are unsorted pyroclastic material. The deposits are not layered and tend to fill ground hollows, they not layered and tend to fill ground hollows, they are generally called are generally called ignimbrites ignimbrites

Direct Direct measurements measurements of pyroclastic of pyroclastic flows are flows are extremely extremely dangerous!!!dangerous!!!

Explosive volcanic activity can also produces Explosive volcanic activity can also produces mud flows called mud flows called laharlahar

Hot lava can melt snow and ice on the Hot lava can melt snow and ice on the top of the cone, so that melt water top of the cone, so that melt water picks up ashes and rocks forming fast picks up ashes and rocks forming fast flowing, high energy torrents.flowing, high energy torrents.

Base surges are generated by Base surges are generated by phreatomagmatic eruption:phreatomagmatic eruption: a type of a type of volcanic explosion that occurs when water volcanic explosion that occurs when water comes in contact with hot rocks or ash near comes in contact with hot rocks or ash near a volcanic vent, causing steam explosionsa volcanic vent, causing steam explosions . .

If magma is thin and If magma is thin and runny, gases can escape runny, gases can escape easily from it. When this easily from it. When this type of magma erupts, it type of magma erupts, it flows out of the volcano. flows out of the volcano. Lava flows rarely kill Lava flows rarely kill people because they people because they move slowly enough for move slowly enough for people to get out of their people to get out of their way. Lava flows, way. Lava flows, however, can cause however, can cause considerable destruction considerable destruction to buildings in their pathto buildings in their path..

Effusive (lava) activityEffusive (lava) activity

Because of basalt's low silica content, it has Because of basalt's low silica content, it has a low viscosity .Therefore, basaltic lava can a low viscosity .Therefore, basaltic lava can flow quickly and easily move >20 kilometers flow quickly and easily move >20 kilometers from a vent. from a vent.

When lavas contain large amounts of gases, bubbles When lavas contain large amounts of gases, bubbles tend to escape while lava is flowing giving rise to tend to escape while lava is flowing giving rise to highly irregular surfaces.highly irregular surfaces. Aa Aa (a Hawaiian term), is (a Hawaiian term), is lava that has a rough, spiny and generally clinkery lava that has a rough, spiny and generally clinkery surface. In thick aa flows, the rubbly surface of loose surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense clinkers and blocks hides a massive, relatively dense interior. interior. (Tilling, Heliker, and Wright, 1987)(Tilling, Heliker, and Wright, 1987)

If magma has a low content of gas, lava flow quikly If magma has a low content of gas, lava flow quikly solidifies at his top, while inside the hot fluid keeps on solidifies at his top, while inside the hot fluid keeps on running downhill.running downhill. Pahoehoe is a Hawaiian term for Pahoehoe is a Hawaiian term for basaltic lava that has a smooth, or ropy surface. A basaltic lava that has a smooth, or ropy surface. A pahoehoe flow typically advances as a series of small pahoehoe flow typically advances as a series of small lobes that continually break out from a cooled crust. lobes that continually break out from a cooled crust.

Fluid lava erupted under water may form a special Fluid lava erupted under water may form a special structure called structure called pillow lavapillow lava. Such structures form when . Such structures form when molten lava breaks through the thin walls of underwater molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly tubes, squeezes out like toothpaste, and quickly solidifies. Typically ranging from less than a foot to solidifies. Typically ranging from less than a foot to several feet in diameter, each pillow has a glassy outer several feet in diameter, each pillow has a glassy outer skin formed by the rapid cooling of the lava by water.skin formed by the rapid cooling of the lava by water.

FEATURES ASSOCIATED WITH COMPOSITE FEATURES ASSOCIATED WITH COMPOSITE AND SHIELD-TYPE VOLCANOESAND SHIELD-TYPE VOLCANOES

Sometimes basaltic lava flows give rise to some Sometimes basaltic lava flows give rise to some spectacular geological features such as the “Giant spectacular geological features such as the “Giant Causeway” in Northern IrelandCauseway” in Northern Ireland

A A lava domelava dome is a steep-sided mass of viscous lava is a steep-sided mass of viscous lava extruded from a volcanic vent, often circular in plan view extruded from a volcanic vent, often circular in plan view and spiny, rounded, or flat on top. Its surface is often and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome. cooler, outer crust during growth of the dome. (Foxworthy and Hill, 1982)(Foxworthy and Hill, 1982)

Cinder cones, also known as scoria cones, represent Cinder cones, also known as scoria cones, represent the smallest type of volcano, rarely exceeding 300 m in the smallest type of volcano, rarely exceeding 300 m in height. They are entirely composed of pyroclastic height. They are entirely composed of pyroclastic materials (volcanic ash). They are quite common and materials (volcanic ash). They are quite common and associated with both composite or shield volcanoes.associated with both composite or shield volcanoes.

Volcano Sudoeste and the smaller cinder cones at its base are part of the San Quintin volcanic field located in Mexico’s Baja Peninsula.

When volcanoes explode or eject a significant When volcanoes explode or eject a significant volume of material onto the surface the cone may volume of material onto the surface the cone may collapse into the voided reservoir, forming a steep, collapse into the voided reservoir, forming a steep, bowl-shaped depression called a bowl-shaped depression called a CalderaCaldera. These can . These can be quite large, 1-100 km in diameter!be quite large, 1-100 km in diameter!

Crater Lake, Oregon (10km) Aniachak Caldera, Alaska

DiatremeDiatreme is a is a general term for a general term for a volcanic vent or volcanic vent or pipe drilled through pipe drilled through enclosing rocks enclosing rocks (usually flat-lying (usually flat-lying sedimentary rocks) sedimentary rocks) by the explosive by the explosive energy of gas-energy of gas-charged magmas. charged magmas. The diamond-The diamond-bearing kimberlite bearing kimberlite pipes of South pipes of South Africa are Africa are diatremesdiatremes..

References

• http:// egsc.usgs.gov /isb//pubs/teachers-packets/volcanoes

• http://www.nature.nps.gov/views/kcs/volcanism

• http://geology.com/volcanoes/volcanic-hazards