Warm Up Why was the Mt. St. Helens Eruption so destructive? What about the explosion was not predicted by most scientists?

Volcanoes Earth Science

What is a Volcano? Volcano – A structure which moves magma onto the surface of the Earth Magma – liquid rock formed from heat inside the Earth Lava – Magma that erupts onto the Earth’s surface

Critical Thinking #1 Considering the prior definitions, explain how a volcano first forms.

How Volcanoes Form 1. Plate Boundaries – Subduction and Seafloor Spreading 2. Hot Spots – Extremely hot intraplate regions where mantle rises

Where are Volcanoes Located? Most located along the “Ring of Fire” - zone of volcanic activity along edge of Pacific Ocean

Critical Thinking #2 Why would oceanic-continental convergent boundaries produce a great number of volcanoes?

Types of Lava Mafic Lava – forms oceanic crust, dark colored, generally runny Felsic Lava – forms continental crust, light colored, generally thick

Two Forms of Mafic Lava Pahoehoe – means “ropy”, formed when mafic lava cools slowly

Two Forms of Mafic Lava AA – jagged chunks, formed when mafic lava cools rapidly

Critical Thinking #3 What factors would determine the speed at which mafic lava hardens?

Pyroclastics Pyroclastics – Rock fragments made of hardened felsic lava, types based on size Volcanic Dust – Less than 0.25mm in diameter Volcanic Ash – 0.25-2mm in diameter

Pyroclastics Lapilli – 2-64mm in diameter Volcanic Bombs - >64mm in diameter

Critical Thinking #4 What factors would determine the size of pyroclasts produced?

Viscosity and Gas Content Viscosity – Ability to resist flow Determined by amount of Silica Low Viscosity = Thin lava Medium Viscosity = Medium Thickness High Viscosity = Thick lava Gas Content – Determines Explosiveness High Gas Content = Most Explosive Low Gas Content = Least Explosive

Critical Thinking #5 What is the average viscosity of mafic and felsic lava? What does this tell you about the silica content of mafic and felsic lava?

Critical Thinking #6 What is the relationship between gas content and viscosity?

Three Types of Volcanoes 1. Shield Volcano 2. Cinder Cone 3. Composite Volcano (Stratovolcano)

Shield Volcano Broad, gently slopped (900)m High) Eruption of fluid low viscosity mafic lava through multiple vents Ex: Kilauea (Hawaii)

Cinder Cone Steep cone-shaped volcano (500m high) Medium-viscosity Lava fragments (Cinders) ejected from single vent Ex: Paricutin (Mexico)

Composite Volcano Large, often snow-capped peaks (3000m high) Most explosive eruptions, high viscosity lava and pyroclasts Ex: Mt. St. Helens (Washington)

Critical Thinking #7 Which volcano type would likely be the most dangerous? Explain why.

Craters and Calderas Crater – Funnel-shaped pit at top of volcanic vent Formed when material is blown out of a volcano by explosions Caldera – Large basin-shaped depression Formed when magma chamber is emptied and entire volcanic cone is collapsed

Critical Thinking #8 In what situation would a crater form? In what situation would a caldera form?

Mount St. Helens Mount St. Helens – Composite Volcano (Stratovolcano) in Washington State Erupted in 1980 after being dormant – “inactive” for 120 years North face of mountain experiences largest ever recorded debris landslide Pyroclastic flow from eruption destroyed 230 square miles 57 people killed from the blast

Critical Thinking #9 What factors would create the greatest landslide in a volcanic eruption?

Critical Thinking #10 What aspect of the eruption of Mount St. Helens caused the greatest damage and casualties?