Causes of Volcanoes:
1) Convergence (continental & island arcs)
2) Divergence (mid-ocean ridges and continental rift valleys)
3) Hotspot / plume (oceanic island chains)
Volcano Flavors
Mmmmmm…. Volcanoes….
Eyjafjallajökull
Magma vs. Lava?
Magma is melted rock with gasses inside
that is BELOW the surface.
Lava is what we call the melted rock when
it escapes into the AIR. Gas is usually
released from the lava while it cools into
a rock.
Viscosity
 Viscosity = Thickness
 High Viscosity = Very Thick, Chunky
 Low Viscosity = Very Thin, Runny
High
Viscosity
Low
Viscosity
Flow Rate
How fast lava flows some distance over some time
depends on its viscosity.
High flow rate = fast, fluid, thin = Low Viscosity
Low flow rate = slow, chunky, thick = High Viscosity
Flow Rate = Distance Lava Flows / Time
Silica Content
 The Elements Silicon and Oxygen combine to make
Silica
 Silica is the most common element in the Earth’s Crust
 Silica is lightweight and makes magma and lava thicker
Explosivity
In pairs, discuss the answers to the following questions.
Be ready to share with the class.
1. Why do some volcanoes explode more violently than
others?
2. If a volcano has thick lava (high viscosity) will it be
more or less explosive than a volcano with thin lava
(low viscosity)?
1. Different volcanoes will have different
viscosities depending on where they form.
2. Thicker, more viscous lava will hold gasses
back, causing it to build up pressure and
explode more violently.
Volcano Anatomy
Ash
Caldera
Lava
Tephra
Fissures
Side Vent
Main Vent
Dike
Magma Chamber
3 Main Types of Volcanoes
 Shield Volcano
 Composite / Stratovolcano
 Cinder Cone
Shield Volcano Diagram
Properties of a Shield Volcano
 Low Viscosity Lava
 Low Silica Content in Lava
 Not very explosive – actually quiet and gentle.
Because the Lava is not very viscous, gasses
escape easily from the lava.
 Lava flows very far and fast. Flows
continuously for a long time.
 Gently Sloping sides, very wide base
Example: Mauna Loa, Hawaii
Largest active volcano in the world
Stands 5.5 miles high
Video of a Shield Volcano
Composite / Stratovolcano
Diagram
Properties of a Composite or
Stratovolcano
 Medium Viscosity Lava
 Medium Silica Content in Lava
 VERY explosive and dangerous. Unpredictable
bursts of activity. Gasses are trapped by the
viscous lava and build up pressure.
 Lava flows slow and not very far
 Steep sides because the lava doesn’t flow far.
Layers of lava and ash built upon each other.
Examples: Mt. St. Helens, Mt. Rainier
Makes up the largest percentage of Earth’s
volcanoes (60%)
Video of Composite / Stratovolcano
Cinder Cone Diagram
Properties of a Cinder Cone
 High Viscosity magma
 Very high Silica Content
 Medium-High explosivity, massive amounts of
ash. Develops very quickly and is
unpredictable.
 Lava does not flow, but builds up in one spot
 Most of the volcano is formed by ash and small
lava chunks. Very steep sides.
Example: Paricutin, Mexico
1943, Grew 300 ft in 5 days
Video of a Cinder Cone
Basalt
Andesite
Rhyolite
• Dark Color
• Intermediate lava
• Light Color
• Low Silica Content (less than
• Med Silica Content (57%)
• High Silica Content
52%)
(more than 68%)
• Low Viscosity Lava
• Med Viscosity Lava
• High Flow Rate (can travel far)
• Intermediate Flow Rate
• Hottest eruptive temp (1700+
°F)
(flows more than rhyolite, but not as
easily as basalt)
•Medium Temperature
(1400-1700°F)
• High Viscosity Lava
• Low Flow Rate
(barely flows)
• Low eruptive temp
(1,200-1,400 °F)
Which lava rocks form at each
type of volcano?
 Basalt = Shield Volcanoes
 Andesite =Composite/Stratovolcano
 Rhyolite = Cinder Cones
What kind of Volcano is this?
Eyjafjallajökull
What type of volcano is this?
Olympus Mons
Biggest Volcano in
our Solar System