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
2. Gentle
eruptions: may pour out rivers
of lava that flow long distances, can be constant
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
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
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
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
2. Underwater
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
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
D. Calderas
1. Shield and composite volcanoes
2. Huge crater formed by the collapse of a
volcano into an empty magma chamber
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)
