Volcanoes

A volcano is a place on the Earth’s surface where magma breaks through. A volcano is called

active

if it erupts lava, rock, gas or ash, or if it shows seismic activity. A volcano is

dormant

when is not presently erupting but is considered likely to erupt in the future. (Foxworthy and Hill) An

extinct

volcano will never erupt again.

cone conduit vent A volcano

vent

is a or 'chimney' that connects the

magma

magma)

chamber

(subterranean cavity containing the gas-rich liquid to the Earth’s surface.

The volcano includes surrounding cone of material.

the erupted magma chamber

In the Earth's mantle the temperature is hot enough (800-1.600

°C) to melt rock and form a thick, flowing substance called magma. Lighter than the solid surrounding rock, magma is buoyant much like a cork in water; being buoyant, it rises.

Most magmas contain abundant gas , as the magma rises closer to the surface, the pressure decreases, which cause the gases dissolved in the magma to expand. This expansion propels the magma through openings fractures in the Earth’s surface creating a volcanic eruption Once magma is erupted, it is called lava.

Volcanic eruptions do not occur randomly, but are mainly localized at plate boundaries. Some volcanoes, can be found in the interior of plates at areas called

hot 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.

VOLCANIC ACTIVITY

Two styles Explosive and Effusive

Explosive

: where rapidly escaping gas bubbles (= of volcanic vesicles) magma, fragmenting it.

rip eruption: apart the

Effusive

: where the magma leaks out onto the surface passively as lava flows.

Sometimes basaltic lava pours out quietly from long fissures and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaux.

Lava plateaux can be seen in Iceland, India and USA. (Tilling, 1985)

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

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

Some of the Earth's highest mountains are composite volcanoes ; sometimes called stratovolcanoes . They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs Fuego (left) and Acatenango - Guatemala

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

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

A " Vulcanian "-type eruption is characterized by a dense cloud of ash-laden gas that explodes from the crater and rises high above the peak.

Steaming ash forms a whitish cloud near the upper level of the cone.

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

Pierre during the 1902 eruption of Mount Pelee on Martinique, West Indies.

before after

“ Plinian ” eruption: on August 24, 79AD Vesuvius blew its top, erupting tonnes of molten ash, pumice and sulfuric gas miles into the atmosphere.

Pyroclastic flows flowed over the city of Pompeii and surrounding areas.

WHY DO COMPOSITE VOLCANOES TEND TO BE EXPLOSIVE AND SHIELD VOLCANOES NON-EXPLOSIVE??

1) CHEMISTRY (COMPOSITION) OF LAVA 2) LAVA TEMPERATURE COMPOSITE (runny).

SHIELD

-

Lower silica content of lavas make them more fluid

-

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

-

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.

PRODUCTS OF EXPLOSIVE ERUPTIONS Pyroclastic material

• Rock fragments created by explosive eruptions •magma explodes from volcano and solidifies in the air •existing rock is shattered by powerful eruptions Lapilli Volcanic bombs Volcanic blocks Volcanic ash

•

Three products from an explosive eruption

–

Ash fall: volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered.

–

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

–

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

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

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

Direct measurements of pyroclastic flows are extremely dangerous!!!

Explosive volcanic activity can also produces mud flows called lahar Hot lava can melt snow and ice on the top of the cone, so that melt water picks up ashes and rocks forming fast flowing, high energy torrents.

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

Effusive (lava) activity

If magma is thin and runny, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano.

Lava flows rarely kill people because they move slowly enough for people to get out of their way.

Lava flows, however, can cause considerable destruction to buildings in their path.

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

When lavas contain large amounts of gases, bubbles tend to escape while lava is flowing giving rise to highly irregular surfaces.

Aa (a Hawaiian term), is lava that has a rough, spiny and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior. (Tilling, Heliker, and Wright, 1987)

If magma has a low content of gas, lava flow quikly solidifies at his top, while inside the hot fluid keeps on running downhill.

Pahoehoe is a Hawaiian term for basaltic lava that has a smooth, or ropy surface. A pahoehoe flow typically advances as a series of small lobes that continually break out from a cooled crust.

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

FEATURES ASSOCIATED WITH COMPOSITE AND SHIELD-TYPE VOLCANOES Sometimes basaltic lava flows give rise to some spectacular geological features such as the “Giant Causeway” in Northern Ireland

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

(Foxworthy and Hill, 1982)

Cinder cones, also known as scoria cones, represent the smallest type of volcano, rarely exceeding 300 m in height.

They are entirely composed of pyroclastic materials (volcanic ash). They are quite common and 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 volume of material onto the surface the cone may collapse into the voided reservoir, forming a steep, bowl-shaped depression called a Caldera . These can be quite large, 1-100 km in diameter!

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

Diatreme is a general term for a volcanic vent or pipe drilled through enclosing rocks (usually flat-lying sedimentary rocks) by The the energy charged bearing pipes Africa of diatremes.

explosive of gas magmas.

diamond kimberlite South are

References

•

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

• http://www.nature.nps.gov/views/kcs/volca nism • http://geology.com/volcanoes/volcanic hazards