Chapter 6, Lesson 1 Notes
*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.
Chapter 6, Lesson 2 Notes
*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 the Earth’s crust up through 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 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.
*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.