1. INTRUSIVE AND EXTRUSIVE FEATURES
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2. What is a Volcano?
A volcano is a vent, or opening in the crust from
which pour molten rock, rock debris, gases and steam
The name comes from the Roman god of fire Vulcan.
When magma penetrates the surface it is known as
lava.
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3. What is a Volcano?
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5. Global distribution of volcanoes
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6. Global distribution of volcanoes
Most 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.
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7. Global distribution of volcanoes
At 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 Islands
Some hot spots can also be found beneath continents
such as Yellowstone Basin in the USA
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8. Hot spots
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9. 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.
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10. Stages of a volcano
Active- when eruptions occur at frequent intervals
Dormant-when eruptions are infrequent and one has
not occurred for some time
Extinct- when it is thought a volcano will never erupt
again.
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11. Ways volcano affect humans
Lava flows
Tephra
Pyroclastic flows
Volcanic gases
Lahars
Tsunamis
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12. Lava flows
Lava 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.
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13. Lava flows
The 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.
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14. Lava flows
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15. 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).
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16. 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
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17. Tephra
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18. Pyroclastic flows
Pyroclastic 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.
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19. Pyroclastic flows
The 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.
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20. Pyroclastic flow
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21. 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.
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22. Volcanic gases
At 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.
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23. Volcanic gases
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24. 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.
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25. 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.
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26. Lahars
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27. 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.
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28. Tsunamis
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29. Intrusive volcanic features
Dyke
Sill
Laccolith
Batholith
Plug
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30. 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.
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31. Dyke
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32. 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.

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33. 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.
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34. Laccolith
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35. 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.
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36. Batholith
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37. Extrusive volcanic features
Acid lava
Basic lava
Dome volcanoes
Ash and cinder cones
Composite cones
Shield volcanoes
Calderas
Basalt plateaus
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38. 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.
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39. Acid lava
Acid lava comes from composite cone volcanoes, is
slow moving and viscous
Acid 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.
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40. Acid lava
As 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.
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41. Acid lava
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42. Basic lava
Basic 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.
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43. Basic lava
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44. 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.
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45. Dome Volcanoes
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46. Dome Volcanoes
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47. Ash and cinder cones
Lava 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).
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48. Composite cones
This 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
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49. Composite cone
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50. Shield volcanoes
A 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.
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51. Shield Volcanoes
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52. Shield Volcano
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53. 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.
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54. Calderas
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55. Calderas
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56. 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
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57. Basalt plateau
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