Chapter 7
Sections 3 & 4
Volcanic Eruptions
 As
magma heats up it
becomes LESS dense and
flows upwards.
 If there is an opening in the
crust, it will reach the
earth’s surface – a volcano
is born!
Inside a volcano
ERUPTIONS

As magma rises the pressure surrounding
it decreases. (It is not as deep anymore)
This allows gases in and around the
magma to expand. These expanding
gases push the magma out of the
volcano.
Eruptions can spew out
Ash – fine dust specks
Cinders – pebble-sized particles
Bombs – baseball to car sized
pieces!

ASH
CINDERS
BOMBS!
2 types of eruptions
1)
2)
QUIET: magma low in silica; magma flows
easily, gases bubble out slowly and gently.
EXPLOSIVE: magma high in silica;
thick/sticky; doesn’t always flow out of the
crater; can plug it like a cork; gases get
trapped and build up pressure until they
“explode”.
PYROCLASTIC FLOW: when the volcano
hurls out a mixture of hot gases, ash,
cinders, and bombs.
QUIET
EXPLOSIVE
SCARY PYROCLASTIC
FLOW!
Mt. Pinatubo
Philippines
1991
eruption
More pyroclastic flows
Edge of a pyroclastic flow
Pyroclastic flows cause
tremendous damage!
Volcano Hazards





Fire
Ash can bury towns
Landslides
Mudslides
Flooding (melted
snow)
Stages of Volcanic Activity
Volcanoes are classified
as:
1) ACTIVE – a live
volcano that is
erupting or will in the
near future
2) DORMANT –
“sleeping” may erupt
again
3) EXTINCT – a dead
volcano; unlikely to
ever erupt again.
TYPES OF VOLCANOES
Wide, gently
sloping,
usually form
at a hot
spot, lava
pours out
gently
Lava has
high
viscosity,
steep, cone
shaped hill,
more
explosive
eruptions
Combination of
both; sometimes
gently flowing
lava, sometimes
explosive,
alternating
layers of
ash/cinders &
lava
Click for the video clip!
Other volcanic landforms

Lava plateau – lava flows out of several
long cracks in an area and eventually
over hundreds of years build up to form
a plateau
Calderas
– huge hole left by the
collapse of volcano
 Caldera
Soils from Lava & Ash
When hardened lava erodes and forms soil,
phosphorous, potassium and other
important minerals are released. These
make the soil very fertile.
Dikes and Sills
When magma forces itself across rock
layers it hardens into a DIKE.
When magma squeezes between
horizontal layers of rock it forms a
SILL.
Dikes & Sills in New Zealand
Volcanic Necks
Forms when magma hardens in the
volcano’s pipe and the softer rock
surrounding it erodes away.
Batholiths & Dome Mountains
Batholith: large rock masses form
the core of many mtn ranges;
formed when a large body of
magma cools inside the crust.
Dome Mountain: smaller bodies of
hardened magma
Batholiths
Sierra Batholith (Yosemite Nat’l Park)
Dome Mountains
Geothermal Activity
– means “earth”
 Therme – means “heat”
 Geothermal is heat produced
by the Earth (actually magma!)
 Hot springs & Geysers are
types of geothermal activity
found in volcanic areas.
 Geo
In ARKANSAS
In IDAHO
Hot Springs
GEYSERS (New Zealand)