Unit: Earth’s Changing Interior
Section: Where and Why Volcanoes Form?
Key Vocabulary: magma, lava, Ring of Fire, plate boundary, convergent plate boundary,
divergent plate boundary, transform plate boundary, volcano, hot spot, magma chamber, vent,
fissure, felsic, mafic, lithosphere, asthenosphere, cinder cone, composite volcano, shield volcano
Essential Questions:
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Where do volcanic eruptions most often occur?
What causes a volcanic eruption?
What is the difference between magma and lava?
What controls the type of volcanic eruption that will occur?
What causes Volcanoes?
Volcano – an opening in Earth’s surface
- can form a mountain from layers of lava and ash
- can erupt smoke, steam, ash, cinders, and flows of lava
- intensity of eruptions depends upon composition of magma/lava
- Active volcanoes
o Pacific ocean – Ring of Fire
o Kilauea volcano – Hawaii – erupting for centuries
o World’s most active volcano
- Big eruptions
o Largest of 20th century (Alaskan Penisula)
 Mt. Katmai
 Mt. Pinatubo – Philippines
 Mt. St. Helen’s
 Mt. Unzen – Japan
How do volcanoes form?
- Magma forced upward in the interior
o Convergent boundaries create subduction zones
 One plate forced beneath another
 The subducted plate melts into the mantle
o Less dense than rock around it so it rises
- Lava flows out from a vent
o cools quickly
o forms layers of igneous rocks around the vent
o crater is formed – steep-walled depression
Where do they occur?
- Mostly at Earth’s plate boundaries
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Plates moving apart
o Iceland
o Sits on top of the mid-atlantic ocean ridge
o Rift valleys form here and allow magma to come up
o Rise above sea level (deposits) forms an island
Plates moving together
o Lesser Antilles Islands of Caribbean – Soufriere Hills
o North & South American plates are forced under the Caribbean
plate
Hot spots
o Hawaiian Islands
o Some areas on mantle are hotter than others
o Pacific plate hot spot
o Islands in a line – plate continues to move
o Plate activity formed the islands over 5 million years
Section: Types of Volcanoes
Key Vocabulary: magma, lava, vent, magma chamber, volcano, cinder cone, shield volcano,
composite volcano, viscosity, ash, pyroclasts, cinders
Essential Questions:
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What is a volcano and what is its internal structure?
What are the three main types of volcanoes?
How is volcano type related to lava viscosity?
Types of Volcanoes
Styles of Eruptions
- Explosive or Quiet
- Factors: How much water or gases are trapped in magma, and how much
silica is present in the magma
- Gas build up can create explosive eruptions
Magma Composition
- Low on silica, more fluid and quiet eruptions
- Mafic magma
o High iron content
o Gasses can escape easily
o Eruptions are less violent
- Felsic magma
o Silica–rich magma, explosive eruptions
o Plates are moving and one is forced to subduct
o Plate under eventually melts
o Rich in silica
o Trapped gas results
o Thick and viscous
Magma Water Content
- Converging zones can contain a lot of water vapor
- May cause trapped gas (water vapor) – causes an explosive eruption
Forms of Volcanoes
- three types
o Shield
 Quiet eruptions
 Gently sloping sides
 Hawaiian Islands
o Cinder Cone
 Explosive eruptions
 Various tephra released
 Forms steep-sided, loosely packed tephra shaped into a
cinder cone
o Composite
 Quiet or explosive
 Alternating layers of lava and tephra
 Common at subduction zones
Igneous Rock Features
Intrusive features
- When magma cools and hardens underground
o Batholiths
 Largest igneous rock
 Hundreds of km wide and long
 Most are underground
 Some exposure after erosion to areas
o Dikes & Sills
 Magma squeezes into cracks vertically
 Hardens
 Formed from layers - Dike
 Magma squeezes into cracks horizontally
 Hardens
 Formed from layers – Sill
Other features
- Volcanic neck
o Magma cools and hardens in the vent
o Erosion takes away the softer rock around the vent
o Leaves a volcanic neck
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Caldera
o After an eruption, the top of a volcano falls/collapses
o Water collects in the depression
o Can form lakes
Section: Effect from Volcanoes
Key Vocabulary: volcano, lava, magma, volcanic ash, pyroclastic flow, ecosystem, habitat,
atmosphere, lahar
Essential Questions:
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What are the effects that a volcanic eruption can have on Earth’s surface?
What are the effects of a volcanic eruption on ecosystems and the atmosphere?
Volcanic eruptions
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destructive
o pressure builds in the magma chamber
o flanks of a volcano or caldera can bulge
o volcanic cone can be damaged by the explosion
o Mt. St. Helen’s
 erupted on May 18th, 1980
 top of the cone was blown off
constructive
o lava flowing on the surface cools
o adds new rock to the crust
o builds up and adds elevation
o shoreline
 add new crust to the shore
 expands shoreline or island outward
Impact on Environment
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Atmospheric influence
o Ash in atmosphere blocks and reflects solar energy
o Lowers global temps
o Pyroclastic flows & ash displaces or causes asphyxiation of fauna
o Cinders and hot ash create wildfires
Ecosystem impact
o Drop of solar energy affects photosynthesis
o Pyroclastic flows & ash suffocates/asphyxiates fauna
o Wildfires impact flora and fauna
o Lahars (mudflows) destroy landscapes
o Heat
 Causes rocks to loosen
 Mixes with rain or groundwater creates mudflows
o Volcanic ash – very fertile
 Great for growing crops/plants