CO: VOLCANOES LO: Describe the types of volcanoes, which type of plate boundaries create volcanoes and why volcanoes occur there.

DEFINITIONS VOLCANO - IS A MOUNTIAN FORMED WHEN MAGMA FROM THE MANTLE RISES TO THE EARTH’S SURFACE. MAGMA-IS MOLTEN ROCK THAT IS BENEATH THE EARTH’S SURFACE. LAVA- IS MOLTEN ROCK THAT IS ON OR ABOVE THE EARTH’S SURFACE.

HOW A VOLCANO FORMS 1. Magma starts to rise to the Earth’s surface because it is less dense than the surrounding rock. 2. The magma works its way to the surface through cracks (known as faults) in the Earth’s crust. 3. Once the magma reaches the surface a volcano is formed.

Where Do Volcanoes Form? 1. DIVERGENT PLATE BOUNDRIES 2. CONVERGENT PLATE BOUNDRIES 3. HOT SPOTS

DIVERGENT PLATE BOUNDARY VOLCANO At divergent boundaries crustal plates form deep cracks called rifts. Magma rises to the surface through these rifts. This occurs along mid-ocean ridges and within continents EXAMPLE: The Edfel Volcano on the island of Iceland .

Edfel Volcanic eruption in 1973

CONVERGENT PLATE BOUNDARY VOLCANOES At convergent boundaries magma that forms in the subduction zone rise to the surface through weaknesses or cracks the earth’s crust, forming a volcano. Mt. Shasta in California is a convergent boundary volcano. So is Mt. Saint Helens.

The Cascade Mountain Range in the Pacific Northwest features several active and dormant volcanoes. These volcanoes formed over the subduction zone where the Juan de Fuca plate meets the North American plate

HOT SPOT VOLCANOES Hot spots are place where the mantle is hotter than the rest of the mantle. Volcanoes form when magma from these spots rises to surface. Hot spots can occur in the middle of tectonic plates. Hawaiian islands formed over a hot spot. Yellowstone also is on top of a hot spot.

PACIFIC RING OF FIRE Most of the worlds active volcanoes occur at the plate boundaries that surround the pacific ocean. This forms a circle known as the pacific ring of fire.

VOLCANOES (PART 2)

TYPES OF ERUPTIONS Volcanic eruptions range from violent and explosive to relatively quiet. Two factors effect this: 1) Composition of the magma. ( how much silica is in the magma) 2) How much gas is trapped in the volcano

Composition of the Magma BASALTIC MAGMA (MAFIC)- the more basaltic the magma is the smoother it flows and the more quiet the eruption. GRANITIC MAGMA (FELSIC) - is thick and does not flow easily. Granitic magma plugs up the vent of the volcano allowing pressure to build. When the pressure becomes to great an explosive eruption results

TYPES OF VOLCANOES SHEILD VOLCANOES CINDER CONE VOLCANO COMPOSITE (Strato) VOLCANO

SHEILD VOLCANOES Broad base and gentle slopes Form as a result of quiet eruptions of basaltic (runny) magma. Hawaiian volcanoes are examples

CINDER CONE VOLCANOES Form from explosive eruptions. Steep slopes of tephra and ash.

COMPOSITE CONE VOLCANOES Form from combinations of explosive and quiet eruptions Steep slopes made of alternating layers of tephra and lava. Highest of all volcanoes

Colima in Mexico

Mount Hood in Oregon

Mount Rainier in Washington State

Mount Shasta in Northern California

Mount St. Helens and Spirit Lake before the 1980 Eruption Mount St Helens and Spirit Lake in 1982

Mt. St. Helens

Mt. Saint Helens

Mt. Saint Helens

Mt. Saint Helens

Volcanic Hazards More than 50 volcanoes in the United States have erupted one or more times in the past 200 years. The most volcanically active regions of the Nation are in Alaska, Hawaii, California, Oregon, and Washington. Volcanoes produce a wide variety of hazards that can kill people and destroy property. Large explosive eruptions can endanger people and property hundreds of miles away and even affect global climate. Some of the volcano hazards, such as landslides, can occur even when a volcano is not erupting. -- Myers, et.al., 1997