CHAPTER TWELEVE
Volcanoes
If green dots are earthquakes and red triangles are volcanoes,
What are two things that you notice about their locations??
Section 1: Volcanoes and Earth’s Moving Plates
A volcano is an openingin Earth that eruptsgases, ash and lava.
Volcanic mountains formwhen layers of lava, ashand other material buildup around theseopenings.
What Are Volcanoes?
Section 1: Volcanoes and Earth’s Moving Plates
Kilauea (kee low AY ah),located in Hawaii, is theworld's most activevolcano.
The most recent series oferuptions began in 1983and still continues.
What Are Volcanoes?
Section 1: Volcanoes and Earth’s Moving Plates
Magma is forced upwardbecause it is less dense thanthe rock around it.
After many thousands oreven millions of years,magma reaches Earth'ssurfaces and flows outthrough an opening called avent.
How Do Volcanoes Form?
Section 1: Volcanoes and Earth’s Moving Plates
As lava flows out, it cools quickly and becomessolid, forming layers of igneous rock around thevent.
The steep walled depression around a volcano'svent is called a crater.
How Do Volcanoes Form?
Section 1: Volcanoes and Earth’s Moving Plates
Divergent Plate Boundaries
The mid-Atlantic ridge is anarea where the plates aremoving apart.
When plates separate, theyform long, deep cracks calledrifts.
Lava flows from these rifts and iscooled quickly by sea water.
Where Do Volcanoes Occur?
Section 1: Volcanoes and Earth’s Moving Plates
Sometimes, the volcanoes andrift eruptions rise above sealevel, forming new islands suchas Iceland.
In 1963, the new islandSurtsey was formed during avolcanic eruption.
Where Do Volcanoes Occur?
Section 1: Volcanoes and Earth’s Moving Plates
Convergent Plate Boundaries
Forms volcanoes where an oceanicplate slides below a continentalplate.
Magma forms when the platesliding below another plate getsdeep enough and hot enough tomelt partially and the magma isthen forced upward to the surface.
Volcanoes that form at convergentplate boundaries erupt moreviolently than other volcanoes do.
Where Do Volcanoes Occur?
Section 1: Volcanoes and Earth’s Moving Plates
Hot spots
Form when areas between theEarth's mantle and core areunusually hot and rock at theseareas is forced toward the crustwhere it melts partially.
Magma breaks through the crust toform several volcanoes.
Not at a plate boundary
Formed Hawaiian islands
Where Do Volcanoes Occur?
Section 2: Types of Volcanoes
Trapped gases like water vapor and carbondioxide build up in volcanoes.
If the gas escapes easily, it is a quiet eruption.
If gas and water vapor build up, eruptions canbe explosive
What Controls Eruptions?
Section 2: Types of Volcanoes
Quiet eruptions: magma that is low in silica andis basaltic.
Has lava that pours from volcanic vents and runsdown the sides of a volcano
Composition of Magma
Section 2: Types of Volcanoes
Explosive eruptions: magma that is high in silicaand is andesitic.
Has magma that is thick and when pressure buildsup an explosion occurs. It often carries pieces oflava.
Composition of Magma
Section 2: Types of Volcanoes
Shield Volcano
Broad, gently sloping volcano formed by quieteruptions of basaltic lava.
The basaltic lava flows over Earth's surface coveringlarge areas with deposits of basaltic igneous rockswhen it cools.
Forms of Volcanoes
Section 2: Types of Volcanoes
Shield Volcano
Accounts for the greatest volume of erupted volcanicmaterial.
Much of the new seafloor that originates at the mid-ocean ridge forms as underwater flood basalts.
Forms of Volcanoes
Section 2: Types of Volcanoes
Cinder Cone Volcano
Steep-sided, loosely packedvolcano formed when tephra fallsto the ground.
Tephra: bits of rock or solidifiedlava dropped from the air after anexplosive eruption.
Vary in sizes from volcanic ash,cinders, large rocks and blocks.
Forms of Volcanoes
Section 2: Types of Volcanoes
Composite Volcano
Volcano built by alternating explosive and quieteruptions that produce layers of tephra and lava.
Found mostly where Earth's plates come togetherand one plate sinks below the other.
Forms of Volcanoes
Section 3: Igneous Rock Features
Batholiths: large intrusive rock body that formswhen magma being forced upward towardEarth's crust cools slowly and solidifiesunderground.
Dikes: magma that is forced into a crack thatcuts across rock layers and hardens.
Sills: magma that is forced into a crack parallelto rock layers and hardens.
Intrusive Features
Section 3: Igneous Rock Features
Forms of Volcanoes
Section 3: Igneous Rock Features
Volcanic neck: solid igneouscore of a volcano leftbehind after the softercone has been eroded.
When a volcano stopserupting, the magma hardensinside the vent.
Erosion, usually by water andwind, begins to wear away atthe volcano.
Other Features
Section 3: Igneous Rock Features
Caldera: large circular-shaped opening formedwhen the top of a volcano collapses.
Crater Lake in OR is a caldera that filled with waterand is now a lake.
Other Features
Section 3: Igneous Rock Features
Examples of destruction:
Lava flows destroy everythingin their path.
Falling volcanic ash cancollapse buildings, block roadsand can cause lung disease inpeople and animals.
Volcanic ash and debris rushdown the side of a volcano:pyroclastic flow.
Effects of Eruptions
Section 3: Igneous Rock Features
Examples of destruction:
Temperature inside a pyroclastic flow can be highenough to ignite wood.
Sulfurous gases from volcanoes can mix with watervapor in the atmosphere to form acid rain.
Effects of Eruptions
In your notes, create a diagram of a volcano. It must includethe following features (with correct lables):
• Ash cloud
• Pyroclastic flow
• Tephra
• Layers of ash
• Layers of lava
• Crater
• Main vent
• Dike
• Sill
• Magma chamber
