QUESTION:  What happens when a volcano erupts?

BIG IDEA:  Mountains and volcanoes form as tectonic plates move.

3.2  Volcanoes form as molten rock erupts

I.  Vocabulary

A.  volcano- an opening in Earth’s crust through which molten rock, rock fragments, and hot gases erupt

B.  lava- magma that has reached the Earth’s surface

C.  pyroclastic flow- a dense cloud of superhot gases and rock fragments that races downhill

II.  Volcanoes erupt many types of material

A.  Earth’s crust is made of cool rock, but most of the Earth is made of extremely hot rock and molten metal

B.  Some of the heat escapes to the surface through volcanoes

C.  Volcanoes erupt violently or gently depending on the type of magma feeding them

    1.  Violent eruptions:  tremendous destruction (even without much lava), huge rock fragments causing fires, thick ash on roofs causing collapses; often referred to as gray eruptions

    2.  Gentle eruptions:  may pour out rivers of lava that flow long distances, can be constant; often referred to as

         red eruptions

D.  Magma

    1.  Magma is made mostly of silica (silicon, oxygen)

a.  High silica content: thick, sticky magma that resists flowing, traps gases, and builds up high pressures that cause big blasts

b.  Mount Saint Helens, Washington

    2.  Some magma has a lower silica content

a.  Low silica content:  flows easily, so gases move up and escape through it; can create lava fountains that can be observed safely from a distance

      b.  Kilauea, Hawaii

3.  Magma chambers are areas where magma collects when it stops rising (it stops rising when it is denser than the rock around it)

      a.  Magma remains in chambers until it cools, forming igneous rock

b.  If the chamber is full and more magma is being forced in, the volcano may erupt

­­­­­E.  Rock Fragments

    1.  Rock fragments form as

         a.  escaping gas bubbles pop, tearing magma apart

         b.  larger pieces of lava are thrown into the air, cooling during flight

         c.  rocks of all sizes rip from the inside of the volcano

    2.  Volcanic ash

         a.  Dust to rice-sized rock particles

         b.  When inhaled, may mix with water in the lungs to form a suffocating concrete

         c.  Falls like heavy snow, collapses rooftops

3.  Cinders, bombs and blocks

F.  Volcanic Gases

1.  Looks like smoke coming from a volcano, but is actually a mixture of ash and gases

2.  Mostly water vapor and carbon dioxide

3.  Pyroclastic flow

      a.  Cloud of superheated gases and debris (1500 F/800 C)

      b.  Flows down the landscape at 100mph (160km/h) +

      c.  Most dangerous type of eruption

II.  Most volcanoes form along plate boundaries

         A.  Subduction zones- where oceanic crust is being dragged back into the Earth

1.  As crust subducts, it is heated and melts

             2.  If it reaches the surface, a volcano forms   

         B.  Divergent boundaries- where plates are pulling apart

             1.  Great Rift Valley, Africa

             2.  Underwater (mid-ocean ridges)

         C.  Over 400 volcanoes (80% of all on Earth) are in the Ring of Fire

III.  Volcanoes can have many shapes and sizes    Page 90

         A.  Shield Volcano        (Haleakala, Hawaii)

    1.  Shaped like a broad, flat dome

    2.  Built by many eruptions of low-silica lava, which flows easily and spreads out

    3.  Kilauea, Hawaii

         a.  World’s largest volcano, one of the most active

         b.  10.5 miles (17 km) tall, but only 2.5 miles (5 km) is above sea level

         c.  The top crater, or caldera, is 3 miles across at its widest

B.  Cinder Cone

    1.  Steep, cone-shaped hill

    2.  Formed by eruptions of cinders that pile up in the crater

    3.  Magma is gas-rich, and throws small chunks of rock into the air

    4.  Tens to hundreds of meters tall, usually forming on the sides of other volcanoes

C.  Composite Volcano  (new terminology:  stratovolcano)

    1.  Cone-shaped, containing layers of lava and layers of rock fragments

    2.  Magma is high-silica, pasty

    3.  Grows much larger than a cinder cone

    4.  Violent eruptions due to high pressures of gases trapped in the magma, and rock being blasted away

    5.  Mount Vesuvius, Italy

D.  Calderas

    1.  Shield and composite volcanoes

    2.  Huge crater formed by the collapse of a volcano into an empty magma chamber

    3.  Yellowstone caldera

    4.  Crater Lake, Oregon

IV.  Scientists monitor volcanoes

         A.  Volcanoes can go hundreds and thousands of years without an eruption

         B.  Scientists monitor volcanoes around the world for signs of eruptions

             1.  Uplifting or tilting of the ground

             2.  Gas levels (carbon dioxide, sulfur dioxide)

             3.  Rising temperatures of gases and water (springs, lakes)