Chapter 9
Volcanoes
9.1: How and Where Volcanoes Form
Key Idea: Volcanoes form where magma reaches Earth’s surface
Vocabulary:
Volcano: refers to both the opening in Earth’s crust through which molten
rock, gases, and ash erupt and to the landform that develops around this
opening.
Hot Spot: refers to areas of volcanic activity that result from plumes of hot
solid material that have risen from deep within Earth’s mantle.
Magma is
↓
Molten rock that has formed deep inside Earth
↓
3 conditions for magma formation
Decrease in Pressure
Increase in temperature Increase in the amount
which
which
of water in the
↓
↓
asthenosphere
Which
↓
Lowers melting
Melts rock
Lowers melting
temperature
occurs at
temperatures of
occurs at
↓
materials and
↓
occurs at
↓
asthenosphere
Hot spots
Subduction boundaries
9.2: Magma and Erupted Materials
Key Idea: The composition of magma largely determines how explosive an
eruption will be.
Vocabulary:
Viscosity: A substance’s resistance to flow.
Lava: Magma that reaches Earth’s surface
Pahoehoe: Solidified basaltic lava flow on land that has formed with
smooth, ropelike surfaces.
Aa: A solidified basaltic lava flow on land that has formed rough, jagged
surfaces.
Pyroclastic materials: Solid rock fragments that are ejected during a
volcanic eruption.
How and why does the silica content of magma affect the explosiveness of
an eruption?
The higher the silica content, the more explosive the eruption, because more
viscous magmas (those with higher silica content) don not allow gases to
escape as easily as less-viscous magmas do.
Magmas form at rifts, subduction boundaries, and continental hot spots.
Which type of magma would you expect to be formed at each location?
Rifts: basaltic magma which has the highest melting temperature, the least
amount of silica, low gas content, low viscosity, and is rarely explosive.
Subduction boundaries: andesitic magma which has a moderate (not high,
not low) melting temperature, moderate amount of silica, moderate gas
content, moderate viscosity, and is sometimes explosive.
Hot Spots: rhyolitic magma which is has the lowest melting temperature, the
greatest amount of silica, the highest gas content, high viscosity, and is
usually explosive.
Melting
Temperature
Viscosity
Silica Content
Gas Content
Explosiveness
Greatest
Intermediate
Basaltic
Andesitic
Least
Rhyolitic
Rhyolitic
Rhyolitic
Rhyolitic
Basaltic
Basaltic
Basaltic
Andesitic
Andesitic
Andesitic
Pahoehoe and Aa are both basaltic.
Pahoehoe forms from high-temperature basaltic lava and cools into ropelike
surfaces.
Aa forms from cooler, slower-moving basaltic lava and cools into rough,
jagged surfaces.
9.3: Volcanic Landforms
Key Idea: The shape of volcanic landform is determined by the nature of the
erupted materials
Vocabulary:
Shield Volcano: A shield-shaped volcano with a broad base and gently
sloping sides that is made of basaltic lava.
Cinder Cone: A cone-shaped volcano formed from lava fragments that have
been ejected from a volcanic vent.
Composite Volcano: A volcano made layers of hardened lava flows and
pyroclastic materials.
Caldera: A large, crater-shaped basin formed after the top of a volcano
collapses.
Lava Plateau: A plateau formed from basaltic lava pouring from a crack or
fissure in Earth’s surface.
Broad base, gently sloping sides
Oval base, made of ash and other
pyroclastic materials
Shield Volcano
Formed of layers of materials
Cinder Cone
A large, crater-shaped basin on top of
a volcano
Composite Volcano
Caldera
Lava Plateaus form after a long crack appears in the earth’s surface, and
basaltic lava pours up through it and spreads out over the land.