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Chapter 5 Key Concepts
Lesson 1: Volcanoes and Plate Tectonics
Where are volcanoes found on Earth’s surface?
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A volcano is a mountain that forms in Earth’s crust when molten material, or magma, reaches the
surface. Magma is a molten mixture of rock-forming substances, gases, and water from the mantle.
When magma reaches the surface, it is called lava. After magma and lava cool, they form solid rock.
In general, volcanoes form a regular pattern on Earth. They occur in many great, long belts. Volcanic
belts form along the boundaries of Earth’s plates.
Volcanoes can occur where two plates pull apart, or diverge. They can also occur where two plates
push together, or converge. The Ring of Fire is one major belt of volcanoes. It includes the many
volcanoes that rim the Pacific Ocean.
Volcanoes form along the mid-ocean ridges, where two plates move apart. Along the rift valley, lava
pours out of cracks in the ocean floor. This process gradually builds new mountains. Volcanoes also
form along diverging plate boundaries on land.
Many volcanoes form near converging plate boundaries where two oceanic plates collide. The
resulting volcanoes sometimes create a string of islands called an island arc. Volcanoes also occur
where an oceanic plate is subducted beneath a continental plate.
A hot spot is an area where material from deep within Earth’s mantle rises to the crust and melts to
form magma. A volcano forms above a hot spot when magma erupts through the crust and reaches
the surface. Hot spots stay in one place for many millions of years while the plate moves over them.
Some hot spot volcanoes lie close to plate boundaries. Others lie in the middle of plates.
Lesson 2: Volcanic Eruptions
What happens when a volcano Erupts?
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All volcanoes have a pocket of magma beneath the surface, called a magma chamber, where the
magma collects. Magma moves upward through a pipe, a long tube that extends from Earth’s crust up
though the top of the volcano, connecting the magma chamber to Earth’s surface. Molten rock and gas
leave the volcano through an opening called a vent. A lava flow is the spread of lava as it pours out of
a vent. A crater is a bowl-shaped area that may form at the top of a volcano around the central vent.
During an eruption, dissolved gases trapped in the magma expand, form bubbles, and exert great
force. When a volcano erupts, the force of the expanding gases pushes magma from the magma
chamber through the pipe until it flows or explodes out of the vent.
Geologists classify volcanic eruptions as quiet or explosive. Whether an eruption is quiet or explosive
depends in part on the magma’s silica content and whether the magma is thin and runny or thick and
sticky.
Silica is a material found in magma that forms from the elements oxygen and silicon. A volcano erupts
quietly if its magma is hot or low in silica. The gases in the magma bubble out gently. The lava oozes
quietly from the vent and can flow for many kilometers. A volcano erupts explosively if its magma is
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high in silica. Trapped gases build up pressure until they explode. The erupting gases and steam push
the magma out with incredible force. Both kinds of eruptions can cause damage far from a crater’s rim.
A pyroclastic flow is a mixture of hot gases, ash, cinders, and bombs that flow down the sides of a
volcano when it erupts explosively.
What are the stages of volcanic activity?
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Geologists often use the terms active, dormant, or extinct to describe a volcano’s stages of activity. An
active, or live, volcano is one that is erupting or has shown signs that it may erupt in the near future. A
dormant, or sleeping, volcano is a volcano that scientists expect to awaken in the future and become
active. An extinct, or dead, volcano is a volcano that is unlikely to ever erupt again.
Lesson 3: Volcanic Landforms
What landforms do lava and ash create?
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Volcanic eruptions create landforms made of lava, ash, and other materials. These landforms include
shield volcanoes, cinder cone volcanoes, composite volcanoes, and lava plateaus. Other landforms
include calderas, which are the huge holes left by the collapse of volcanoes. Inside a caldera, a lake
may form.
In an explosive eruption, ash, cinders, and bombs can build up around the vent in a steep, cone-shaped
hill or small mountain that is called a cinder cone. A cinder cone volcano may be hundreds of meters
tall.
Composite volcanoes are tall, cone-shaped mountains in which layers of lava alternate with layers of
ash. Composite volcanoes can be more than 4,800 meters tall.
Thin layers of lava that pour out of a vent and harden on top of previous layers build a wide, gently
sloping mountain called a shield volcano. Hot spot volcanoes on the ocean floor are usually shield
volcanoes.
Repeated floods of lava can form high, level plateaus called lava plateaus.
What landforms does magma create?
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Sometimes magma cools and hardens into rock before reaching the surface. Over time, forces such as
flowing water, ice, or wind may strip away the layers above the hardened magma and expose it.
Features formed by magma include volcanic necks, dikes, and sills, as well as dome mountains and
batholiths.
A volcanic neck forms when magma hardens in a volcano’s pipe and the surrounding rock later wears
away. Magma that forces itself across rock layers hardens into a dike. Magma that squeezes between
horizontal rock layers hardens to form a sill. A dome mountain forms when uplift pushes a large body
of hardened magma toward the surface, which eventually becomes exposed. A batholith is a mass of
rock formed when a large body of magma cools inside the crust.