Chapter 5 Volcanoes 5 1 Volcanoes and Plate

Chapter 5: Volcanoes 5. 1 Volcanoes and Plate Tectonics

Volcanoes �Volcano: mountain that forms in Earth’s crust when molten material (magma) reaches the surface. �Remember, magma is molten rock below ground; lava is molten rock at the surface.

Volcanoes �Volcanoes form along plate boundaries. At divergent boundaries, the crust cracks and allows molten rock to rise (mid-ocean ridge). At convergent boundaries, one plate can sink beneath the other, which helps bring magma to the surface (on ocean floor).

Island Arc �An island arc is a string of islands formed from volcanoes. �Some common ones are Japan, New Zealand, the Aleutians, and the Caribbean islands. �Some continental volcanoes include Mount St. Helens and Mount Rainier.

Hot Spots �Some volcanoes form from hot spots: an area where material from the mantle rises through the crust. �Hot spots are stationary beneath the crust, and the plates move over them.

Chapter 5: Volcanoes 5. 2 Volcanic Eruptions

Volcanic Structure �Magma chamber: pocket of magma beneath the volcano. �Pipe: connects the magma chamber to Earth’s surface. �Vent: the area where molten rock and has leave the volcano. �Lava flow: the spread of lava as it leaves the vent �Crater: bowl-shaped area around the central vent.

Volcanic Structure

Volcanic Eruption �Dissolved gases are built up in magma. �As they expand, pressure increases, and eventually forces the magma through the pipe and out of the vent.

Volcanic Eruption �Eruptions can be quiet or explosive; it depends on the magma’s silica content and the thickness of the magma.

Volcanic Eruptions �If magma is very hot and low in silica, the volcano erupts quietly. Lava can flow for long periods of time; built the Hawaiian islands. �Magma that is high in silica explodes from a volcano. Mount St. Helens in Washington erupts this way.

Rock Fragments �Volcanic Ash: Consists of particles ranging from the size of dust to the size of rice grains �Volcanic Cinders �Volcanic Bombs �Volcanic Blocks: Solid pieces of rock, as big as houses

Volcano Hazards �Quiet eruptions have large lava flows and burn and bury almost everything in its path. �Explosive eruptions release dangerous materials that form clouds around the volcano. Pyroclastic flow: mixture of hot gases, ash, cinders, and bombs that rush down a volcano.

Volcano Activity �An active volcano is one currently erupting or showing signs of erupting soon. �A dormant volcano is one that is “sleeping” but expected to turn active in the future. �An extinct volcano is one that is unlikely to ever erupt again.

Predicting Activity �Geologists use tiltmeters to measure small changes in the ground caused by magma movement. �They also monitor earthquakes, which can cause volcanic eruptions. �They measure gas from the volcano, and also keep track of temperatures.

Chapter 5: Volcanoes 5. 3 Volcanic Landforms

Calderas �A caldera is a hole left behind when a volcano collapses. �Lakes may form in this hole over time. �Usually associated with shield and composite volcanoes.

Cinder Cone Volcanoes �A steep, cone-shaped hill or small mountain �Formed by ash, cinders, and bombs building up around the volcano’s vent. �Usually magma high in silica

Composite Volcanoes �Tall, cone-shaped mountains with layers of lava and ash �Usually alternates between quiet and explosive

Shield Volcanoes �Wide, gently sloping mountain �Thin layers of lava pour out of a vent and cool in layers. �Hot spot volcanoes are usually this type.

The largest volcano on Earth, Mauna Loa.

Lava Plateaus �Lava plateaus are made from thin, runny lava floods that cool and solidify.

Magma Landforms �Volcanic neck: forms when magma hardens in a volcano’s pipe and the rock wears away.

Magma Landforms �Dike: magma that moves across rock layers �Sill: magma that squeezes between rock layers

Magma Landforms �Batholith: mass of rock formed when a large body of magma cools inside the crust.

Magma Landforms �Dome Mountains: bodies of hardened magma that are pushed upward toward the surface.