Volcanoes
and Plate Tectonics (part 2)
“an opening in the Earth’s crust
through which an eruption takes place”
Volcanoes
Three types
 Cinder
Cones
Cinder Cone
Inside a Cinder Cone
Cinder Cone
Steep sloped sides (angles close to 40°)
 Relative to other volcanoes – small few
hundreds of meters high
 Small explosive eruptions
 Made up of pyroclastic material (ash and
tephra)

Shield
Montserrat Volcano eruption
Shield Volcanoes
The volcano have a very broad base, with
gently sloping side (like a ‘shield’)
 Quiet eruptions
 Made up of layers of hot, mafic (basaltic)
lava
 Hawaii is a good example of a shield
volcano

Composite
Mount Saint Helens
May 15, 1980
Mount Saint Helens
May 18 1980
Mount Saint
Helens
May 18,
1980
Eruption
Mount Saint Helens
May 18 1980
Mt. St. Helens devastation
Mount Saint Helens
September 10, 1980
Mount Saint Helens
September 24, 1984
Mount Saint Helens
(from Spirit Lake) May 15, 1980
Mount Saint Helens
(from Spirit Lake) May 19,1982
Popocatepetl composite volcano in Mexico is on
the Ring of Fire
Composite Volcanoes
Steep sloped sides
 Very tall, 1000s of meters
 Very explosive eruptions, sometimes quiet
eruptions (alternating for the most part)
 Made up of alternating layers of lava flows
and silica rich (granitic) pyroclastic
material.
 Mt. St. Helens is a good example of a
composite volcano

Volcanoes (cont.)
Kinds of Eruptions:
Quiet
Shield Volcanoes –
broad base, low angle
slope
• Pillow lava – lava that
occurs in mid-ocean
ridges
• Basalt Plateaus – lava
spreading evenly over
a large area
Volcanoes (cont.)
Kinds of Eruptions:
•
Rift Eruptions – Opening in the crust
“spreading centers”:
• sea floor spreading (ocean)
• rift valley (continent)
Lava ‘oozes’ out because of its mafic
or basaltic composition
Areas of Volcanic Activity
(kinds of eruptions)
Same regions as Earthquakes, which often
serve as warning signs that a volcanic
eruption might occur.
1. Ring of Fire – around the rim of Pacific
Ocean, subduction zone
2. Ocean Ridge system
3. Hotspots
Volcanoes (cont.)
Kinds of Eruptions:
Ring of Fire –
Subduction boundary Eruptions –
Cinder cones and Composite Volcanoes
Explosive eruptions, usually young mountain
chains – like around the Ring of Fire
Hot Spots
Hot Spots – a place in the mantle where
great amounts of heat are rising through
the lithosphere.
 Causes of Hot Spots are still unknown –
the source of the heat or ‘spot’ remains in
the same location while the plate moves
over it.
 For example: Hawaii
Hot Spots
Hot Spots
Thermal signature of the
Hawaiian Islands
Plutonic – Igneous Rocks
When masses of magma cools beneath the
surface, it forms the cores of mountains.
These
‘igneous intrusions’ are called
Plutons.
Plutons have different names depending on
their size and shape
Plutonic – Igneous Rocks
Small intrusive igneous formations are:
• Dikes
• Sills
• Volcanic Necks
• Laccoliths…
Plutonic – Igneous Rocks
Laccolith – magma that buldges upward
and formed dome mountains. (Henry
Mountains, Utah and Black Hills, South
Dakota)
 Batholith – a LARGE body (> 100 km3) of
intrusive igneous rock. Forms the core of
most mojor mountain ranges (Sierra
Nevada)
 Stock – small batholith, less than100 km3

Volcanoes
Ruby Mountains Batholith
La Paz Batholith
East side of Sierra Batholith
Granite Dome - Texas
The End
Volcano Essential Questions
What are the 3 types of volcanoes? How do they
differ in eruptive force, composition of magma,
ejected material and cone structure?
2. Identify 3 tectonic settings where volcanoes
form; which type of volcano forms at these
locations?
3. Explain how the composition of magma affects
volcanic eruptions.
4. What are the 5 types of pyroclastic material?
5. List 3 events that MAY signal a volcanic eruption.
6. What are the different igneous intrusions
associated with volcanoes?
1.
For a Jolly Rancher of your choice…
Send me an eMail with “Earth Science rocks
my socks” in the title before the end of
the volcano unit
Only the first few will get this….
So don’t tell!!!!!!!!