Chapter 9 - VolcanoesChapter 9 - VolcanoesVolcanoVolcano – An opening in the Earth’s surface through – An opening in the Earth’s surface through which magma and volcanic gases pass.which magma and volcanic gases pass.MagmaMagma – Molten rock, gases and water vapor inside – Molten rock, gases and water vapor inside the Earth.the Earth.LavaLava – Molten rock that flows out onto the surface of – Molten rock that flows out onto the surface of the Earth.the Earth.Eruption Types: Eruption Types: NonexplosiveNonexplosive – The most common eruption – The most common eruption where huge amounts of lava are released.where huge amounts of lava are released. ExplosiveExplosive – A much rarer but destructive type – A much rarer but destructive type of eruption with hot debris, ash and gases rapidly of eruption with hot debris, ash and gases rapidly shooting out from the volcano.shooting out from the volcano.AshAsh – Dust-sized particles of rock that get thrown into – Dust-sized particles of rock that get thrown into the atmosphere.the atmosphere.

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Magma Chamber – The underground body of magma that feeds the volcano.

Vent – The opening in the Earth’s surface through which magma is released.

The explosive potential of the volcano depends on the silica, water and gas content of the magma. The more water, the more explosive due to the underground pressure build up of the steam. Silica rich magma is thicker and can build up more pressure.

What erupts from a volcano: Lava – Liquid magma that flows out through the vent. Blocky Blocky – Flows slowly; oozes and doesn’t travel

very far. PahoehoePahoehoe – Flows slowly; has a glassy surface and

resembles dripping wax. aaaa –Flows quickly; forms a brittle crust. Pillow lavaPillow lava – Flows quickly; forms underwater. Pyroclastic Material – Magma that has been blasted into

the air and hardens before landing.

Types of Pyroclastic MaterialTypes of Pyroclastic Material

• BombsBombs – Blobs of magma that cool as they are thrown high in the air.

• LapilliLapilli – Pebble-sized bits of magma that harden before hitting the ground.

• AshAsh – Most of the material ejected during a volcanic eruption.

• BlocksBlocks – House-sized pieces of rock that are sheared from the vent throat and thrown into the air.

• Pyroclastic FlowsPyroclastic Flows – Dangerous eruptions in which huge amounts of hot ash, dust and gases blast from the volcano with temperatures of 1300°F and speeds of 130 m.p.h.

Types of VolcanoesTypes of Volcanoes

• The effects of a volcanic eruption are not only dangerous immediately but the enormous amounts of ash and gases ejected into the upper atmosphere can change the climate worldwide resulting in famine and disease.

• Shield ConesShield Cones - can be enormous but are usually built on layers of lava in non-explosive eruptions.

• Cinder ConesCinder Cones – smaller and formed from moderately explosive eruptions that last a short time and are composed of pyroclastic materials such as ash.

• Composite ConesComposite Cones – one of the most common types formed from alternating explosive/non-explosive eruptions and lava flows.

Volcanic LandformsVolcanic Landforms• Crater Crater – A funnel shaped pit at the top of many volcanoes.• CalderaCaldera – A large depression formed after the eruption and

much larger than the crater. A crater with collapsed walls.• Lava PlateausLava Plateaus – Formed by repeated eruptions with massive

outpourings of lava spreading out over a large area. These are usually formed by rift zones (huge cracks in the surface of the Earth).

• Causes of Volcanic Eruptions:Causes of Volcanic Eruptions: The rock of the mantle is hot enough to melt but the pressure prevents the molecules from liquefying. However, when the pressure is decreased, magma forms and begins to rise because it is less dense than the surrounding rock.

• Where Volcanoes form:Where Volcanoes form: 80% of the worlds volcanoes form at convergent plate boundaries and 15% form at divergent boundaries.

• The Ring of FireThe Ring of Fire – The area of the Pacific plate perimeter where many volcanoes are found.

Tectonic Plate BoundariesTectonic Plate Boundaries• Divergent BoundaryDivergent Boundary – Areas where tectonic plates separate

forming rift zones (deep cracks). The pressure rapidly decreases allowing the rock to melt and form magma which rapidly rises to fill in the rift zones.

• Mid-Oceanic RidgesMid-Oceanic Ridges – Undersea rift zones that produce volcanoes and mountains by producing new crust from erupted lava.

• Convergent BoundaryConvergent Boundary – An area where tectonic plates collide.• Subduction ZonesSubduction Zones – Areas where one tectonic plate is pulled

underneath (subducted) under another. Oceanic crust will subduct under continental crust because it is denser than continental crust. This process generates tremendous heat and pressure that helps in the formation of magma and new volcanoes.

• Hot SpotsHot Spots – Volcanically active places that are far from plate boundaries. These usually will form volcanic island chains.

A. Mantle PlumesMantle Plumes – Areas with rising magma that form hot spots.

B. Cracks in the Earth’s crust that permit magma to rise upward.

Volcanic Activity: Volcanic Activity: A. Extinct Extinct – Volcano has not erupted within recorded history

and most likely never will. B. DormantDormant – Volcano is currently not erupting but may again. C. ActiveActive – Volcano is currently erupting or showing signs of

erupting in the near future.Eruption PredictionEruption Prediction 1. Watch for an increase in earthquakes. 2. Observe the ratio of certain gases. 3. Tiltmeters and GPS detect changes in the volcano's slope. 4. Infrared satellite images watch for an increase in

temperature.