Volcanoes

VOLCANOESEarth Science Fall 2012

Vocabulary: aa – jagged chunks of lava formed by

rapid cooling on the surface of the lava flow

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

Vocabulary: Cinder cone- steep sloped deposit of

solid fragments ejected from a volcano

Vocabulary: Composite volcano- (strato volcano)

steep sloped volcanic deposit with alternating layers of hardened lava flows and pyroclastic materials

Vocabulary: Crater- funnel-shaped pit at the top of a

volcanic cone; bowl shaped depression on the surface of a planetary body

Vocabulary: Felsic lava- silica rich lava

Vocabulary: Fissure- crack in a rock surface through

which lava flows

Vocabulary: Hot spot- area of volcanism within a

lithospheric plate

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Review Temperature and pressure generally

keep the rocks in the athenosphere below melting point. Pink True Yellow False

Review Solid rock located deep in the mantle is

called magma.

Pink- TrueYellow- False

Review Magma may sometimes break through to

the earth’s surface.

Pink- trueYellow- false

ReviewMagma is able to rise upward through the

earth’s crust because it is less dense than the surrounding rocks.

Pink- trueYellow- false