Volcanoes

What is a volcano? A volcano is an opening in the Earth’s crust through which molten rock, gases, ash, dust, and cinders can erupt

Where do volcanoes occur? 1. Divergent Plate Boundaries Examples: Iceland, Mid-Ocean Ridges 2. Subduction zones at Convergent Plate Boundaries Examples: Mt. St. Helens, the Ring of Fire 3. Hot Spots Examples: Hawaii

How does a Hot Spot form volcanic islands?

Ring of Fire

Eruptions and Magma

Two types of eruptions: 1. Explosive 2. Non-Explosive (Quiet) Pyroclastic Flow (clouds of gas and ash that race down the sides of a volcano) during an explosive volcanic eruptions

Two factors that determine whether an eruption will be explosive or non-explosive: The type of magma – felsic (granitic) or mafic (basaltic) The amount of water vapor and other trapped gases in the magma. more trapped gases = more explosive eruption

Two types of Magma

Felsic Magma Also known as granitic magma Light in color Composed primarily of Silicon and Oxygen Low density High viscosity (thick, slow moving) Explosive eruptions

Mafic Magma Also known as basaltic magma Dark in color Composed primarily of Iron and Magnesium High density Low viscosity (runny, flows easily) Quiet eruptions 2 types – pahoehoe and aa

Two types of Mafic Magma AA (spiny, rough) Pahoehoe (smoother)

Three Types of Volcanoes

Shield Volcano Shape – gentle slope, broad base Composition – successive layers of lava Features – quiet eruptions, lots of lava Locations – hot spots and mid-ocean ridges Examples – Hawaii and Iceland

Shield Volcano

Cinder Cone Shape – cone-shaped, very steep sides Composition – tephra (ash, dust, cinders) Features – very violent eruptions Locations – subduction zones Examples – Paracutin (Mexico) and Krakatoa (Indonesia)

Cinder Cone

Composite (Stratovolcano) Shape – cone-shaped, steep sides Composition – alternating layers (tephra and lava) Features – both explosive and quiet eruptions Locations – on or near continents, subduction zones Examples – Mt. St. Helens and Mt. Pinnatubo

Composite (Stratovolcano)

Intrusive Igneous Rock Structures (Plutons)

Igneous Intrusions Batholith – the largest intrusive igneous rock structures Sill – forms when magma squeezes into horizontal cracks in rocks and hardens Dike – forms when magma squeezes into vertical cracks in rocks and hardens Laccolith – forms when a sill pushes up on the rock layers above it to form a dome

Other volcanic structures: Volcanic neck – the magma core of an eroded volcano (very resistant to erosion)

Other volcanic structures: Caldera – forms when the top of a volcano collapses into a partially emptied magma chamber

Crater Lake in Oregon

Intrusive Formations Sill

Dike

Columnar Jointing

Laccolith

Batholiths

Caldera

Pyroclastics from an explosive volcanic eruption

Lapilli

Blocks and Lava Bombs