Grade 8 Integrated Science Chapter 15 Lesson 2 on volcanoes. This lesson goes into detail about volcanoes, plate boundaries, lava chemistry, eruption types, and volcano types. The purpose of this lesson is for students to understand where and why volcanoes form and what factors cause differing volcanic features.

1. Volcanoes
Chapter 15 Lesson 2
Pages 544 - 552

2. Vocabulary
• Volcano – a vent in Earth’s crust through which
melted – or molten – rock flows
• Magma – Molten rock below Earth’s crust
• Lava – Molten rock that erupts onto Earth’s
surface
• Hot spot – Volcanoes that are not associated with
plate boundaries
• Shield volcano – common along divergent plate
boundaries and ocean hot spots, these volcanoes
are large with gentle slopes of basaltic lavas

3. Vocabulary
• Composite Volcano – large, steep-sided
volcanoes that result from explosive eruptions
of andesitic and rhyolitic lava and ash along
convergent plate boundaries
• Cinder Cone – small, steep-sided volcanoes
that erupt gas-rich, basaltic lava
• Volcanic Ash – tiny particles of pulverized
volcanic rock and glass
• Viscosity – a liquid’s resistance to flow

4. Famous Volcanoes
• Do you know the three famous volcanoes that
have erupted with the last 40 years?
– Mount St. Helens (WA, USA)
– Kilauea (HI, USA)
– Mount Pinatubo (Philippines)

5. What is a volcano?
• A volcano is a vent in Earth’s crust through
which melted – or molten – rock flows.
• Molten rock below Earth’s surface is called
magma.

6. How do volcanoes form?
• Volcanic eruptions constantly shape Earth
– They can form large mountains, create new crust,
and leave a path of destruction behind
• They are created and continue to erupt due to
tectonic plate movement
– Volcanoes form at:
• Convergent plate boundaries
• Divergent plate boundaries
• Hotspots

7. Convergent Boundaries
• Volcanoes form along convergent plate boundaries.
– When two plates collide the denser plate sinks, or subducts, into the
mantle.
– The thermal energy below the surface and fluids driven off the
subducting plate melt the mantle and form magma
– Magma is less dense than the surrounding mantle and rises through
cracks in the crust forming a volcano
– Lava is molten rock that erupts onto Earth’s surface

8. Divergent Boundaries
• Volcanoes can happen here too!
– Recall that two plates spread apart at divergent plate boundaries.
– As the plates separate, magma rises through the vent or opening in
Earth’s crust that forms between them
– This process commonly occurs at mid-ocean ridges and forms new
crust
– More than 60% of all volcanic activity occurs along mid-ocean ridges

9. Hot Spots
• Not all volcanoes form on or near plate
boundaries
– Volcanoes in the Hawaiian Island – Emperor
Seamount chain are far from plate boundaries.
• Volcanoes that are not associated with plate
boundaries are called hot spots.
– Geologist hypothesize that hot spots originate
above a rising convection current deep within
Earth’s mantle.

10. Hot Spots
• They use the word plume to
describe these rising currents
of hot mantle material.
– When the plate moves over the
plume, a new volcano forms
– When the plate moves away
from the plume the volcano
becomes dormant, or inactive
• Over time, a chain of
volcanoes forms as the plate
moves.
– The oldest volcano will be
farthest away from the hot spot.

12. Where do volcanoes form?

13. Ring of FIRE!!!
• The Ring of Fire represent an area of
earthquake and volcanic activity that
surrounds the Pacific Ocean.
• Volcanoes form mostly along convergent plate
boundaries (where plates collide), divergent
plate boundaries (where they separate), and
over hot spots (like Hawaii)

14. Volcanoes in the States
• There are 60 potentially active volcanoes
in the United States
– Most of these are part of the ring of fire
• In the United States, the United States
Geologic Society (USGS) has established
three volcano observatories to monitor the
potential for future volcanic eruptions
• Because of the populated areas
surrounding some potentially active
volcanoes the USGS has developed a
hazard assessment program.
– Scientists monitor earthquake activity,
changes in shape, gas emissions, and past
eruptive history to evaluate the possibility of
future eruptions

15. Types of Volcanoes
• Volcanoes are classified based on their shapes
and sizes.
• Magma composition and eruptive style of the
volcano contribute to the shape

16. Shield Volcanoes
• Shield volcanoes are common along divergent
plate boundaries and oceanic hot spots
• They are large with gentle slopes of basaltic
lavas

17. Composite volcanoes
• Composite Volcanoes are large, steep-sides
volcanoes that result from explosive eruptions
of andesitic and rhyolitic lava and ash along
convergent plate boundaries

18. Cinder Cones
• Small, steep-sided volcanoes that erupt gas-
rich, basaltic lavas

19. Supervolcanoes
• Some volcanoes are classifies as supervolcanoes –
volcanoes that have large and explosive eruptions
• Approximately 630,000 years ago, the Yellowstone
Caldera in Wyoming, USA ejected more than
1000km^3 of rhyoltic ash and rock in one eruption
• This eruption produced nearly 2500 times the volume
of material erupted from Mt. St. Helens in 1980.

20. Volcanic Eruptions
• When magma comes to the surface, it might
erupt as lava slows flows out.
• Other times, magma might erupt explosively,
sending volcanic ash – tiny particles of pulverized
volcanic rock and glass – high into the
atmosphere
Quiet Eruption
Violent Eruption

21. Eruption Style
• Magma chemistry determines a volcano’s
eruptive style.
• The explosive behavior of a volcano is affected
by the amount of dissolved gases, specifically
the amount of water vapor, a magma contains
• It is also affected by the silica, SiO2, content of
the magma

22. Magma Chemistry
• Silica is the main chemical compound in all
magmas.
• Differences in the amount of silica affect
magma thickness and its viscosity – a liquid’s
resistance to flow

23. • High silica = high viscosity
– Flows like sticky tooth paste
– This type is magma is formed from melt rocks rich
in silica or from the mixture of magma from the
mantle and continental crust
– Volcanic andesite and rhyolite rocks form when
intermediate and high silica magma erupt from
subduction zone volcanoes and continental hot
spots
• Low silica = low viscosity
• Flows like warm maple syrup
• When the magma erupts, it flows as fluid lava that
cools, crystallizes, and forms volcanic basalt.
• Erupts along mid-ocean ridges and hot spots

24. Dissolved Gases
• The presence of dissolved gases in magma contributed to
how explosive a volcano can be
– Like shaking a bottle of soda, the CO2 in the liquid creates the
bubbles. When the bottle is opened pressure decreases rapidly.
The trapped bubbles increase in size and escape the liquid
• All magma contains dissolved gas including water vapor and
small amounts of CO2 and sulfur dioxide.
• As magma moves towards the surface, the pressure from
the weight of the rock above decreases.
• As pressure decreases the gases can no longer remain
dissolved and bubbles begin to form.
• Because gases cannot easily escape from high-viscosity
lavas, this results in explosive eruptions

25. Effects of Volcanic Eruptions
• On average, about 60 different volcanoes erupt
each year.
– http://www.theatlantic.com/infocus/2013/12/2013-
the-year-in-volcanic-activity/100645/
• Volcanoes enrich rock and soil with valuable
nutrients and help to regulate climate
• Unfortunately, they also can be destructive and
sometimes even deadly

26. • Mount Etna is one of the world’s most active
volcanoes. People that live near the volcano
are accustomed to frequent eruptions of both
lava and ash.

27. Lava Flows
• Because lava flows are relatively
slow moving, they are rarely deadly
• However they can be extremely
damaging
• Mount Etna in Sicily, Italy, is
Europe’s most active volcano.
– In May 2008, the volcano began
spewing lava and ash in an eruption
lasting over 6 months
• Although lava is slow moving it
threatens nearby communities
– People that live on the slopes of Mt.
Etna are use to frequent evacuations

28. Ash Fall
• During an explosive eruption, volcanoes can erupt large
volumes of volcanic ash
– Ash columns can reach heights of more than 40km
– The ash is a mixture of pulverized rock and glass
• Ash can disrupt traffic and cause airplane engines to stop
mid-flight as the ash fuses onto hot engine blades
• Ash can also affect air quality and cause serious breathing
problems
• Large quantities of ash in the atmosphere can also affect
climate by blocking out sunlight and cooling the atmosphere

30. Pyroclastic Flow
• Explosive volcanoes can produce fast-moving avalanches
of hot gas, ash, and rock called pyroclastic flows.
– “pyro” = hot ; “clast” = volcanic particles
• Pyroclastic flows travel at speeds of more than 100 km/hr
and with temperatures greater than 1000°C.
• In 1980, Mount St. Helens produced a pyroclastic flow that
killed 58 people and destroyed 1 billion km³ of forest.

32. Predicting Volcanic Eruptions
• Unlike earthquakes, volcanic eruptions can be
predicted.
• Moving magma can cause ground deformation, a
change in shape of the volcano, and a series of
earthquakes called an earthquake swarm.
• Volcanic emissions can increase.
• Ground and surface water near the volcano can
become more acidic
• Additionally, geologists will study satellite and
aerial photographs, to assess volcanic hazards

34. Volcanic Eruptions and Climate Change
• Volcanic eruptions affect climate when volcanic ash in the
atmosphere blocks sunlight
• High-altitude wind can move ash around the world.
• In addition, sulfur dioxide gases released from a volcano
form sulfuric acid droplets in the upper atmosphere
• These droplets reflect sunlight into space, resulting in lower
temperatures as less sunlight reaches Earth’s surface