Transcript Vocano (Lecture 2)

Chapter 7 Section 2
Volcanic Eruptions
Types Of Lava
 Lavas can be
grouped by the
different kinds of
rocks they produce,
generally based on
the amounts of
iron, magnesium,
and silica.
How can Volcanoes
be classified?
Volcanoes can be grouped by the
different kinds of rocks they produce.
Because of this large variation:
 Silica content is used to classify rocks.
The type of rock produced depends on
chemical composition.
When geologists analyzed
numerous types of volcanic rocks
from around the world, they found
that the silica (SiO2) content varied
by as much as 40 weight percent.
Classification system bases on
appearance
A simple classification scheme
based on silica content is used.
Mafic Lava
 Dark colored
 rich in magnesium and
iron
 usually of oceanic crust
origin
 Basalt - low SiO2=less
than 52%
Mafic Lava Flows
 Classified on
appearance.
Pahoehoe lava
 smooth, runny,
high temperature
lava with a ropy
appearance
aa lava
 rough jagged lava that forms from thicker non-runny lava
Blocky flow
 large angular pieces which flow like a slow motion
avalanche
Pillow lava
 forms when mafic lava cools
underwater
Felsic Lava
 High silica content -
lesser amounts of
iron and magnesium
 lighter color
 usually formed
from continental
crust
Rhyolite
Intermediate Lavas
 Have a range of
compositions that
fall between the
mafic and the
felsic varieties
Too Summarize…
Kinds of Eruptions
Quiet
 Oceanic volcanoes
 mafic lava
 gases can easily escape
 flow like a red-hot river
Explosive
 Continental volcanoes
 felsic lavas
 large amounts of trapped gases
 eruptions send molten and solid
particles shooting into the air
ERUPTIVE
PRODUCTS
Tephra (Pyroclastic Material)
 rock fragments of various sizes that are blown into the air,
generally from felsic lava because of rapidly expanding
gases.
Volcanic Dust
 Tephra particles less
than 0.25 mm in
diameter.
ash
 fine dust less
than 2 mm
in diameter
lapilli - little stones
(cinders)
 particles up to 64 mm in diameter
bombs
 chunks of red-hot rock which cool as they fly through the air
volcanic blocks
 Formed from
solid rock blasted
from the fissure
pumice
 light weight rock
 formed from air
passing through it
Lava flows
 molten rock that reaches the surface
 depends upon temperature and chemical composition
Pyroclastic flow
 Turbulent mixture of hot gas
and rock fragments that moves
like liquid / very fast
TYPES OF VOLCANOES
shape classification
Cinder cone
 made entirely from
tephera
 small with steep
sides
 short-lived (less
than 1000 years)
 not very explosive
Shield Volcano
 quite large and form from very fluid lava
 rarely explosive
 gently sloping sides
 are made of basaltic lava
Puu Oo Vent, Hawaii
 Crater
 Funnel-shaped pit at the top of a volcanic vent
Types of Volcanoes (classified by
shape)
 Shield volcano: characteristic of eruptions emitting low
http://www.cet.edu/ete/modules/volcanoes/vtypesvolcan1.html
viscosity lava. Hawaii is an example.
Olympus Mons is the largest volcano on Mars. This
shield volcano, similar to volcanoes in Hawaii,
measures 624 km (374 mi) in diameter by 25 km (16
mi) high. It is 100 times larger than Mauna Loa on
Earth. Located on the Tharsis Plateau near the
equator, Olympus Mons is bordered by an
escarpment. The caldera in the center is 80 km (50
mi) wide and contains multiple circular, overlapping
collapse craters created by different volcanic events.
The radial features on the slopes of the volcano were
formed by overflowing lava and debris.
http://pds.jpl.nasa.gov/planets/captions/mars/olym
pus.htm
Stratovolcano (composite)
 a succession of alternating
lava flows and tephra
deposits
 usually thick lava and are
some what explosive
 made mostly of andesite
(Mt. Rainier)
Mount Rainier
Mount Saint Helens
Lava Dome
 a single mass of
extruded lava that’s
too thick to move
away from the
volcano
 very explosive (Mt.
St. Helens)
Craters and Calderas
Caldera
 Large basin-shaped depression formed when an explosion
destroys the upper part of a volcanic cone
http://cimss.ssec.wisc.edu/sage/geology/lesson3/images/Figure%2012%20Aniakchak%20Caldera%20Alaska.jpg
Crater Lake
 (1,932 feet deep). The lake was formed by the explosion and
collapse of 12,000 foot Mt. Mazama
Crater Lake
Wizard Island – regrowing volcano
Can Be
Quite Large
 This happens
when the magma
chamber
empties and the
roof collapses.
Yellowstone
http://pu
bs.usgs.go
v/fs/2005/3
024/images
/fs20053024_fig_
15.jpg