The Nature of Volcanic EruptionsThe Nature of Volcanic Eruptions
What determines, whether a volcano extrudes magma violently or gently?What determines, whether a volcano extrudes magma violently or gently?
1. Magma’s composition.( SiO2 content).
2. Magma's temperature.
3. The amount of dissolved gasses it
The moreThe more
viscous theviscous the
the greaterthe greater
to flowto flow ..
All of above points affect magma viscosity
Factor effecting viscosityFactor effecting viscosity
•SiO2 content; The more the SiO2 content,
the greater viscosity .
Materials Extruded During An EruptionMaterials Extruded During An Eruption
1. Lava flow.
3. Pyroclastic materials.
Lava FlowLava Flow
Basaltic: example: as in Hawaii Islands
the largest on the Earth and Kilauea)
- Flow rapidly 10-300 m/hr may reaches
- Travel about 150 km before congeal
Basaltic flows in Hawaii formBasaltic flows in Hawaii form
1. pahoehoe flows: A lava flow with smooth-
to- ropy surface. It resembles the twisting
braids in ropes.
2.aa flows: A lava flow that has a jagged
Lava tubesLava tubes
conduit that the molten lava continued to
move through it.
Pillow lavaPillow lava
basaltic lava that congeals under water
environment and develops a structure that
resembles a pile of pillows.
1. Andesitic: intermediate viscosity.
2.Rhyolitic: This is too slowing, thicker and
more viscous than andesitic or basaltic
Magma contains variable amount of dissolved gases
held in the molten rock by confining pressure.
These gases are:
70 % = water vapour.
5% = N2.
5% = S.
and amount of Cl2,H2,Ar.
The gas content is important in widening the volcanic
vent and the cracks by the expansion of the gases to
hundred or thousand times.
Pyroclastic materialsPyroclastic materials
Pyroclastic material is any ejected lava fragments of ash, dust, glass
fragments, ….. that range in size from microns to large pieces of metric size.
Ash: rise in the atmosphere that cause darkness and the drop of
temperature in the atmosphere.
Tuff: A rock formed from fine-grained pyroclastic particles
)ash and dust(.
Pumice: light, glassy, highly vesicular, it is as forth like lava.
Lapilli: size of walnut = little stone.
Cinder: pea-size= contain voids resulting from escaped gases.
Blocks: large grain
Bomb: A streamlined Pyroclastic fragment ejected from a volcano
Volcanoes And Volcanic EruptionVolcanoes And Volcanic Eruption
Based On Their Eruptive Patterns & Characteristic
Form, Three Groups of Volcanoes Are Recognize:
1. Shield volcanoes.
2. Cinder cones.
3. Composite cones )stratovolcano(.
Shield volcanoesShield volcanoes
1. Broad, gently sloping volcano built from fluid basaltic flow .
2. Have small percentage of pyroclastic material.
3. Ex: Mauna Loa, the largest volcano on the earth surface, is one of the
five shield volcanoes that together make up the island of Hawaii. Its
elevation reaches 9170 m )5000 m above ocean floor and 4170 m above
Cinder conesCinder cones
1.Build from ejected pyroclastic materials.
2.It has a high angle )30-40 degree(. Cinder
cones are small, usually less than 300 m
ExEx: Paricutin )west Mexico city( Cinder cones .
Composite coneComposite cone
Composed of alternatives of lava flows and
- Steep near the crater & more gentle a way.
- Andesitic lava )intermediate(.
- Encircle the Pacific Ocean )Ring Of FireRing Of Fire(.
Ex: Mt. Fujiyama )Japan(, Vesuvius )Italy,
extruded violently in A.D 79(, Mount Mayon
)Philippines(, Cascade Range in the northwestern
part of USA. It includes )Mt. St. Helens, Mt. Rainier
and Mt. Shasta(.
nuee ardentenuee ardente =glowing avalanche: it is deadly
pyroclastic flow that occur when hot gases infused
with incandescent ash are ejected, this fiery cloud
race down the volcano flank and travel a bout
200km/hr, and reach a bout 100 km away from the
source. )in Pompeii )Italy( during 3 days, 2000
person were killed(
LaharLahar: Ash + rain, which moves rapidly )about 30
km/hr( and carries the materials in its way.
Composite coneComposite cone
The most obvious volcanic landform is aa conecone ,
but there are other landforms:
1.Crater and caldera:
Caldera is a large crater
(must be >1 km in diameter)
caused by collapse or ejection of the summit
of a volcano.
An isolated, steep-sided, erosional remnant
consisting of lava that once occupied the vent of
3- Vent & Pipes :
Vent: conduit connects the magma chauper to a
Pipe: a vertical conduit reaches a bout 200-km or
more throw which magmatic materials have passed.
So it is consider as a window on the upper mantle
(Asthenosphere), mostly the magma of ultramafic
- Ex: diamond-bearing pipes of South Africa.
4- Fissure Eruptions and Lava Plateaus:
Fissure eruption: eruption where basaltic
lava is extruded via fractures or crack in the earth
The eruvative material cover an extensive area to
thickness of hundreds of meters,
form Flood Basalt.
Plateaus: flat topography result from
repeated eruptive phases of lava, it may reach 2-3
km in thickness and composed from flood basalt.
5- Pyroclastic flow:
Rich in SiO2, mainly of ash and pumice
fragments, when rest on the earth
surface, it resembles lava flow.
Ex: Yellowstone plateau, Wyoming,