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VOLCANOES
Earth Science Fall 2012
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Vocabulary:
aa – jagged chunks of lava formed by rapid cooling on the surface of the lava flow
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Vocabulary:
Caldera- large basin-shaped depression formed when an explosion destroys the upper part of a volcanic cone or when the magma chamber below a volcano is emptied during an eruption
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Vocabulary:
Cinder cone- steep sloped deposit of solid fragments ejected from a volcano
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Vocabulary:
Composite volcano- (strato volcano) steep sloped volcanic deposit with alternating layers of hardened lava flows and pyroclastic materials
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Vocabulary:
Crater- funnel-shaped pit at the top of a volcanic cone; bowl shaped depression on the surface of a planetary body
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Vocabulary:
Felsic lava- silica rich lava
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Vocabulary:
Fissure- crack in a rock surface through which lava flows
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Vocabulary:
Hot spot- area of volcanism within a lithospheric plate
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Vocabulary:
Lapilli- pyroclastic particles between 2mm & 64 mm in diamter
Lava- magma that reaches the earth’s surface
Mafic lava- dark colored rich in magnesium and iron
Magma- liquid rock produced deep within the earth
Pahoehoe- solidified mafic lava with a wrinkled surface
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Vocabulary:
Pillow lava- lava that flows out of fissures in the ocean floor and cools rapidly in round shapes
Pyroclastic material- the rock fragments ejected from a volcano
Sheild volcano- has a cone of hardened lava that forms a broad base and slopes gently
Stratovolcano- also called composite volcano; steep sloped volcanic deposit with alternating layers of hardened lava flows and pyroclastic material
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Vocabulary:
Vent- opening through which the molten rock flows onto the surface
Volcanic ash- pyroclastic particles between 0.25mm and 2 mm in diameter
Volcanic block- the largest possible pyroclastic material that is formed from solid rock blasted from a volcanic fissure
Volcanic bomb- large, spindle-shaped clot of lava thrown out of a volcano
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Vocabulary:
Volcanic dust- pyroclastic particles that are less than 2 mm in diameter
Volcanism-any activity that includes the movement of magma toward or onto the earth’s surface
Volcano- lava and pyroclastic material built up on the earth’s surface around a vent
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Volcanoes & Plate Tectonics
Scientists have no direct way to measure temperatures within the earth Analysis of seismic waves & hear flow near
the earth’s surface & computer modeling allow scientists to estimate those temperatures
Areas of solid rock melt to form magma Forms when mantle material flows to areas
of lower pressure faster than it can lower its temperature by hear flow or when fluids such as water are added to the mantle material
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Volcanoes & Plate Tectonics
Volcanism Any activity that includes the movement of
magma toward or onto the surface of the earth
Pockets of magma form due to melting of some of the surrounding rock More rock melts, magma pockets get large
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Volcanoes & Plate Tectonics
most magma forms at plate boundaries, where one plate is subducted beneath another plate
Magma that erupts onto the Earth’s surface is lava
The opening through which magma flows to the surface is called a vent
The structured formed by the vent and the material that builds up on the earth’s surface around the vent is called a volcano
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Volcanoes & Plate Tectonics
Major Volcanic Zones Most active volcanoes occur in zones near
both convergent and divergent boundaries of lithospheric plates
Subduction zones Deep trench forms on the ocean floor where
the continental crust is being subducted The plate of continental crust buckles and
folds, forming a line of mountains along the edge of the continent
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Volcanoes & Plate Tectonics
Magma erupts to the surface at the trench, forming volcanic mountains near the edge of the continent
Major zone of active volcanoes is the Pacific Ring of Fire
If 2 oceanic plates collide one plate is subducted, forming a deep trench
After time a string of volcanic islands forms, called an island arc
Island arcs start as small islands and grow to large islands
Ex: Aleutian Islands- small islands Ex: Japan – large islands
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Volcanoes & Plate Tectonics
Mid-Ocean Ridges The largest amount of magma comes to the
surface at mid-ocean ridges Major zone of volcanic activity Upwelling magma adds material to the mid-
ocean ridge & creates new lithosphere along the rift Can also form underwater volcanoes
Iceland is located on top of the mid-ocean ridge, it is separated by a fissure
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Volcanoes & Plate Tectonics
Hot spots Areas of volcanism within the interior of the
lithosphere Appear to remain stationary, but actually
drift slowly; as a result the volcano actually is carried away When the volcano is carried it away, it is moved
away from the hot spot and the activity of the volcano comes to an end
Hawaiian Islands are an example of a chain of volcanic islands formed over a hot spot
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Volcanic Eruptions
Geologists have concluded that there are two general types of lava: Mafic lava- dark colored when hard and is
rich in magnesium and iron Forms much of the ocean crust
Felsic lava- contains lots of silica, less magnesium and iron, has a lighter color when hardened Common in continental crust
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Volcanic Eruptions
When mafic lava cools quickly, the surface of the lava flow forms a crust The flow continues move and wrinkles up
forming pahoehoe, which means ropey in Hawaiian
If the crust is deformed rapidly or grows too thick to form wrinkles, it breaks into jagged chunks to form aa Sharp blocky texture of the volcanic rock
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Volcanic Eruptions
Lava cools so rapidly that the outside hardens leaving a liquid core in the middle, called a lava tube
Lava can also flow out of fissures on the ocean floor, it cools rapidly separating into rounded blobs called pillow lava
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Volcanic Eruptions
Kind of Eruptions The composition of the lava that reaches
the surface largely determines the force with which a volcano will erupt.
Ocean volcanoes, those that erupt on the ocean floor and on oceanic islands, usually are produced by mafic lava Mafic lava is very hot and thin and flows easy Because gases escape quickly these eruptions
are very quiet
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Volcanic Eruptions
Felsic lavas of continental volcanoes tend to be cooler and thicker Contain large amounts of trapped gases and
when the vent or fissure opens up, the dissolved gases within the lava boil out explosively, sending molten and solid particles shooting into the air
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Volcanic Eruptions
Volcanic Rock Fragments Felsic lava explodes through pyroclastic material
into the air Rock fragments ejected from a volcano Some forms when cooling magma breaks into
fragments, other forms when a spray of lava cools and solidifies
Volcanic ash and dust generally land close the volcano Lapilli are larger than ash and dust and generally land
near the volcano Volcanic bombs are formed when large clots of lava
harden in the air Volcanic blocks can be a large as a house
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Volcanic Eruptions
Volcanic Features Piles of volcanic material are volcanic cones Types of volcanoes
Volcanic cones that are broad at the base and have gently sloping sides are called shield volcanoes
Covers a wide area & generally results from lava eruptions
Slowly build up Ex: Hawaiian islands
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Volcanic Eruptions
Cinder cones- cause by explosive eruptions Made of solid fragments ejected from the volcano Usually have very steep slopes Rarely more than a few hundred meters tall
Composite volcanoes (stratovolcanoes) Are formed by both quiet and explosive eruptions Cone usually formed from lava flows Explosive eruptions deposit lots of material around
the vent Cone is made of alternating layers of lava and
pyroclastic material Ex: Mount Fuji , Mount Raineer, Mount Hood
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Volcanic Eruptions
Craters & Calderas Craters
Formed when material is blown out of the volcano by explosions
Usually gradually becomes wider A small cone can form within a crater
Caldera When the magma chamber below a volcano is
emptied, the cone may collapse, leaving a large, basin-shaped depression
Can also be caused by eruptions that discharge large amounts of magma
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Volcanic Eruptions
Predicting Volcanic Eruptions Scientist use measuring devices to detect
geologic events that may signal the beginning of an eruption Small earthquakes
Result from growing pressure on the surrounding rocks as magma works it way upward
Temperature changes within the rock Slight bulging of the surface of the volcano Change in composition of the gases given off
by the volcano Study previous eruptions
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Extraterrestrial Volcanism
Many of the planets and moons of the solar system were volcanically active at one time.
The Moon Near side is covered with basaltic lava flows Craters on the moon’s lunar surface come
from meteorite bombardment Maria – large impacts early in the moon’s
history created deep basins that later flooded with lava
We don’t know how magma was formed on the moon
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Extraterrestrial Volcanism
Mars There are numerous volcanoes and volcanic
features on the surface of Mars Largest volcano on Mars is Olympus Mons.
Rise nearly 28 km above the surface 600 km across Caldera is 70 km across
We don’t know if the Martian volcanoes are still active
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Extraterrestrial Volcanism
Io Moon of Jupiter 1st planetary body other than Earth where
there have been active volcanoes found Probably the most volcanically active body
in the solar system The material that erupts from these
volcanoes is a bright yellow-red Scietists think that it is primarily sulfur and
sulfur dioxide
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Review
Temperature and pressure generally keep the rocks in the athenosphere below melting point. Pink True Yellow False
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Review
Solid rock located deep in the mantle is called magma.
Pink- TrueYellow- False
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Review
Magma may sometimes break through to the earth’s surface.
Pink- trueYellow- false
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Review
Magma is able to rise upward through the earth’s crust because it is less dense than the surrounding rocks.
Pink- trueYellow- false