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VOLCANOES
CHAPTER 7
7.1 VOLCANOES AND PLATE
TECTONICS
Section Objectives:
• Describe the formation and movement of magma
• Define volcanism
• List three locations where volcanism occurs
I. VOLCANISM
1. Volcanism is any activity that includes the movement of
magma toward or onto the surface of the earth.
2. Asthenosphere is very hot but stays mostly solid
because of the great pressure of the surrounding rock.
3. Magma forms where decreased pressure allows the rock
to melt
4. Lower density of magma compared to the surrounding
rock and allows it to rise toward the surface.
I. VOLCANISM
• Lava:
Magma that reaches Earth’s surface
• Vent:
The opening through which the molten rock
flows onto the surface
• Volcano:
The structure that is formed by the vent
and volcanic material that builds up on Earth’s
surface around the vent
volcano
lava
vent
cone
II. MAJOR VOLCANIC ZONES
Three major locations
where volcanism
occurs:
1. Subduction Zone
2. Mid-ocean Ridge
3. Hot Spot
Subduction Zone
Mid Ocean Ridge
Hot Spot
VOLCANIC ERUPTIONS
Section Objectives:
• Summarize the relationship between lava types and
the force of volcanic eruptions.
• Describe the major types of pyroclastic material.
• Identify the three main types of volcanic cones.
• Summarize the events that may signal a volcanic
eruption.
VOLCANIC ERUPTIONS
Type of Lava
Color:
Rich in:
Commonly found in:
mafic
dark
iron and
magnesium
oceanic crust
felsic
light
silica
continental crust
VOLCANIC ERUPTIONS
Mafic lava textures:
pahoehoe: “ropy”; forms as lava first forms
a crust
aa: jagged chunks; forms when crust is
too thick
pillow lava: lava that cools very quickly,
usually underwater.
Pahoehoe lava
Aa lava
Pillow lava
KINDS OF ERUPTIONS
Type of Eruption
Lava
Characteristics
Where Found
Type of Lava
"quiet"
hot, thin, flows
easily; gases
escape easily
mid-ocean
ridges; hotspots
mafic
cooler, thicker, lots
subduction zones
of trapped gases
felsic
explosive
KINDS OF ERUPTIONS FOR EACH
TYPE OF LAVA
“Quiet” eruptions:
 Mafic lavas flow more easily
 gases escape easily
 lava flows out onto the surface
Explosive eruptions:
 Felsic lava is thicker
 gasses cannot escape easily
 gas pressure builds up and creates a more violent
eruption
 pyroclastic material is thrown into the air.
VOLCANIC ROCK FRAGMENTS
Pyroclastic Material
• Associated mainly with felsic eruptions because they are
explosive.
• Dust: less than .25 mm in diameter; can travel completely
around the Earth in the upper atmosphere
• Ash: less than 2 mm diameter
• Lapilli: less than 64 mm diameter
• Blocks: larger than 64 mm; some as big as a house
• Bombs: formed when lava is thrown into the air and solidifies
while in the air
VOLCANIC CONES
NAME
SIZE
LAVA
TYPE
MADE UP
OF
SLOPE
TYPE OF
ERUPTION
LOCATION
shield
largest
mafic
lava
unsteepest
"quiet"
hot spot
smallest
mafic
and
felsic
pyroclastic
fragments
steepest
explosive
hot spot,
subduction
zone, midocean ridge
felsic
lava and
pyroclastic
fragments
medium
explosive
subduction
zone
cinder
cone
composite
middle
Shield Volcano
Here are 4 of the volcanoes that make up the big island of Hawai'i. They are Mauna Kea (MK), Mauna Loa
(ML), Hualalai (H), and Kohala (K). The photo was taken from near the summit of East Maui volcano (EM).
These are the largest volcanoes on Earth
Cinder Cone
Composite Cone
Popocatepetl
Mt. Merapi, Indonesia
Fujiyama, Japan
Mt.
Mt. St
St.Mt.
Helens,
Helens,
Shasta,
Washington
Washington
California
Mt. Shasta, California
VOLCANIC FEATURES
craters and calderas
•
crater = the funnel shaped pit at the top
of a volcanic vents
•
caldera = when the magma chamber
below a volcano is emptied, the volcanic
cone may collapse leaving, a large basin
shaped depression
VOLCANIC FEATURES
crater
VOLCANIC FEATURES
caldera
Caldera Formation
What controls eruptions?
The amount of trapped gas
 When more gas is trapped, pressure builds up
 MORE PRESSURE MAKES A MORE VIOLENT ERUPTION
The amount of silica in the magma
 Silica makes the magma thicker
 Thicker magma does not flow easily so can get
stuck underground, allowing pressure to build up
 THICKER MAGMA MAKES A MORE VIOLENT ERUPTION
Predicting Volcanic Eruptions
• Increase in the number and strength of small
earthquakes (magma working its way upward)
• Slight bulging of the surface of a volcano
(upward movement of magma just below the
surface)
• Information from previous eruptions
7.3 EXTRATERRESTRIAL
VOLCANISM
• Mars
– volcanic cones on Mars
– most famous is Olympus Mons
– Mars seems to be seismically active
• Io
– one of Jupiter’s moons
– has active volcanoes
Fig. 22.9
Fig. 22.10c
Fig. 22.10a
Fig. 22.11b
Fig. 22.1_4
Fig. 22.17
Olympus Mons
Io
Io
Fig. 22.1_2
Fig. 22.14b
FAMOUS ERUPTIONS
•
•
•
•
•
•
ELDFELL
MOUNT ST. HELENS
KILAUEA
VESUVIUS
MOUNT PINATUBO
CRATER LAKE