Volcano

INTRUSIVE AND EXTRUSIVE FEATURES

1Prepared by: A Smith

What is a Volcano?A volcano is a vent, or opening in the crust from

which pour molten rock, rock debris, gases and steamThe name comes from the Roman god of fire Vulcan.When magma penetrates the surface it is known as

lava.

Prepared by: A Smith 2

What is a Volcano?



Global distribution of volcanoes


Global distribution of volcanoesMost volcanoes are found along convergent and

divergent plate boundaries.At these points there are molten rocks and magma to

supply the volcanoes.There are about 1300 potentially active volcanoes in

the world.A small number of volcanoes are along way from

plate boundaries known as hot spots.


Global distribution of volcanoesAt these points the temperature at the boundary of

the mantle ad crust is unusually high and there are lines of weakness in the crust which magma can follow to reach the surface.

An example of an hot spot is the Hawaiian IslandsSome hot spots can also be found beneath continents

such as Yellowstone Basin in the USA


Hot spots


Volcanic Eruption?The sudden occurrence of a violent discharge of

steam and volcanic material.An eruption begins when pressure on a magma

chamber forces magma up through the conduit and out the volcano's vents.

When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma.

The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam.


Stages of a volcanoActive- when eruptions occur at frequent intervalsDormant-when eruptions are infrequent and one has

not occurred for some timeExtinct- when it is thought a volcano will never erupt

again.


Ways volcano affect humansLava flowsTephraPyroclastic flowsVolcanic gasesLaharsTsunamis


Lava flowsLava flows are streams of molten rock that pour or

ooze from an erupting vent. Lava is erupted during either nonexplosive activity or

explosive lava fountains. Lava flows destroy everything in their path, but most

move slowly enough that people can move out of the way.


Lava flowsThe speed at which lava moves across the ground

depends on several factors, including :(1) type of lava erupted and its viscosity; (2) steepness of the ground over which it travels;(3) whether the lava flows as a broad sheet, through a

confined channel, or down a lava tube; (4) rate of lava production at the vent.


Lava flows


Tephra When a volcano erupts it will sometimes eject material

such as rock fragments into the atmosphere. This material is known as tephra.

The largest pieces of tephra (greater than 64 mm) are called blocks and bombs. Blocks and bombs are normally shot ballistically from the volcano (refer to the gas thrust zone described in the direct blast section).


Tephra Because these fragments are so large they fall out near

their source. Blocks and bombs as large as 8-30 tons have fallen as far away as 1 km from their source (Bryant, 1991).

Small blocks and bombs have been known to travel as far away as 20-80 km !

Some of these blocks and bombs can have velocities of 75-200 m/s


Tephra


Pyroclastic flowsPyroclastic flows are fluidized masses of rock fragments

and gases that move rapidly in response to gravity. Pyroclastic flows can form in several different ways. They can form when an eruption column collapses, or as

the result of gravitational collapse or explosion on a lava dome or lava flow.

These flows are more dense than pyroclastic surges and can contain as much as 80 % unconsolidated material.


Pyroclastic flowsThe flow is fluidized because it contains water and gas

from the eruption, water vapor from melted snow and ice, and air from the flow overriding air as it moves downslope.


Pyroclastic flow


Volcanic gases Magma contains dissolved gases that are released into

the atmosphere during eruptions. Gases are also released from magma that either

remains below ground (for example, as an intrusion) or is rising toward the surface.

In such cases, gases may escape continuously into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems.


Volcanic gasesAt high pressures deep beneath the earth's surface,

volcanic gases are dissolved in molten rock. But as magma rises toward the surface where the

pressure is lower, gases held in the melt begin to form tiny bubbles.

The increasing volume taken up by gas bubbles makes the magma less dense than the surrounding rock, which may allow the magma to continue its upward journey.


Volcanic gases


Lahars Lahar is an Indonesian term that describes a hot or

cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys.

When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter.


Lahars Lahars vary in size and speed. Small lahars less than a

few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second--much too fast for people to outrun.


Lahars


Tsunamis A series of water waves caused by the displacement of

a large volume of a body of water, typically an ocean or a large lake.

Earthquakes, volcanic eruptions and other underwater explosions, landslides, glacier calvings, meteorite impacts and other disturbances above or below water all have the potential to generate a tsunami.


Tsunamis


Intrusive volcanic featuresDykeSillLaccolithBatholithPlug


Dyke When a mass of magma cuts across bedding planes, it

forms a wall-like feature called a dyke. Sometimes the rocks on either side of a dyke are more

resistant to erosion. When this happens, the dyke forms a depression. Dykes

sometimes occur in swarms, as in Arran and parts of western Scotland and northern Ireland.


Dyke


Sill When a sheet of magma lies along a bedding plane it

forms a structure called a sill. Some sills form ridge-like escarpments when exposed

by erosion.


Laccolith

A sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.

Laccoliths tend to form at relatively shallow depths and are typically formed by relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite.


Laccolith


Batholith This is a very large mass of magma which

accumulates in the crust. Sometimes it forms the root or core of a mountain.

Batholiths are made of granite and they form surface features only after they have been exposed by denudation.

Batholits are exposed at the suface in south-west England where they form Dartmoor, Bodmin Moor, Land's End and the Scilly Isles. These are the cores of an ancient mountain chain the tops of which have long since been removed by erosion.


Batholith


Extrusive volcanic featuresAcid lavaBasic lavaDome volcanoes Ash and cinder conesComposite conesShield volcanoesCalderasBasalt plateaus


Extrusive volcanic features Magma sometimes reaches the surface through a vent

(hole), or a fissure (crack) in the surface rocks. When magma emerges at the surface it is called lava.

If lava emerges via a vent, it usually builds up a volcano, which is a cone-shaped mound.

If it emerges from a fissure, it may build up a lava plain, or a lava plateau. Volcanic eruptions also take place on some ocean floors.


Acid lavaAcid lava comes from composite cone volcanoes, is

slow moving and viscousAcid lava is produced when there is subduction at

destructive plate boundaries. This lava is silica-rich and has temperatures of about

800°C. It cools and solidifies quickly and produces steep

sided volcanoes.


Acid lavaAs it solidifies quickly, it may solidify in the central

pipe, blocking the passageway and causing a buildup of pressure, which produces violent eruptions.

The solidified lava forms sheets of rough, jagged rocks called aá. Flows of boulders and rubble called block rubble and mounds of lava called domes are also formed.


Acid lava


Basic lavaBasic lava comes from shield volcanoes, is runny and

flows faster.Basic lava is produced at constructive plate

boundaries. This lava is rich in iron and magnesium and is rather

fluid. It has a temperature of about 1200°C. It flows for a distance before solidifying and forms

gently sloping volcanoes. The lava solidifies into smooth, folded sheets of rock called pahoehoe.


Basic lava


Dome Volcanoes A dome volcano is a round shaped mountain made of

viscous lava. The lava has a high silica content that prevents the

lava from flowing very far from its vent. Most domes are formed by dacite and rhyolite lavas.


Dome Volcanoes


Dome Volcanoes


Ash and cinder conesLava is blown to great heights when it is violently

ejected, and it breaks into small fragments which fall back to earth and build up a cone.

Good examples of ash and cinder cones are Volcano De Fuego (Guatemala) and Paricutin (Mexico).


Composite conesThis type of cone is formed of alternate layers of lava

and ash. The volcano begins each eruption with great violence

forming a layer of ash. As the eruption proceeds, the violence ceases and

lava pours out forming a layer on top of the ash. Lava often escapes from the sides of the cone where it builds up small conelets


Composite cone


Shield volcanoesA shield volcano is a type of volcano usually built

almost entirely of fluid lava flows. They are named for their large size and low profile,

resembling a warrior's shield. This is caused by the highly fluid lava they erupt,

which travels farther than lava erupted from more explosive volcanoes.

This results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form. Shield volcanoes contain low viscosity magma making it have flowing mafic lava.


Shield Volcanoes


Shield Volcano


Calderas These are ones where the diameter of the circular to

oval crater exceeds 1 mile. These form when so much lava is erupted (blown

out) so rapidly it partially empties the underlying magma chamber.

When this happens the summit of the volcanic structure collapses into the emptied magma chamber.

Typically the erupted material occurs as airfall or pyroclastic flows.


Calderas


Calderas


Basalt Plateau One or a succession of high-temperature basaltic lava

flows from fissure eruptions which accumulate to form a plateau. Also known as flood basalt.

Example the Indian Deccan


Basalt plateau


Volcano

Documents

type of lava

lava tube

magma chamber forces

explosive lava fountains

rate of lava production

smithways volcano

streams of molten rock

rock debris