Volcanic Eruptions
Chapter 7.2
Volcanic Eruptions
1.Through analysis of the chemical
composition of volcanic rock,
geologists have concluded that
there are two general types of
lava.
a.Mafic lava
i.Dark colored when hardened.
Mafic Lava
ii.Rich in magnesium and iron.
iii. Forms much of the oceanic crust.
iv. Tends to quietly flow.
Mafic Lava
Felsic Lava
b. Felsic lava
i.High in silica with lesser amounts of
magnesium and iron.
ii.Lighter color when it hardens.
iii.Common in the continental crust.
iv.Tends to be explosive.
Felsic Lava
Pahoehoe
c. Pahoehoe (puh-hoy-hoy)
i.When mafic lava cools rapidly, the
surface of the flow forms a crust that is
wrinkled by movement. This is solidified
mafic lava with a wrinkled surface.
ii.Pahoehoe means “ropy” in Hawaiian.
Pahoehoe
AA
d. AA (ah-ah)
i.Lava that is deformed rapidly or grows
too thick to form wrinkles, and the
surface breaks into jagged chunks. Jagged
chunks of lava formed by rapid cooling
on the surface of a lava flow.
ii.AA refers to the sharp, blocky texture of
the volcanic rock.
AA
Lava Tubes
e. Lava Tubes
i.These tubes are created when the
outer part of the mafic lava flow
cools rapidly and forms a hardened
shell around a liquid interior which
flows out leaving the hardened shell.
Lava Tube
Pillow Lava
f. Pillow Lava
i.This is when lava flows out of
fissures on the ocean floor, cools
rapidly, and separated into
rounded blobs.
Pillow Lava
Volcanic Rock Fragments
2. Volcanic Rock Fragments
a.Pyroclastic materials are thrown into the air
when felsic lava explodes.
i. Pyroclastic materials are rock fragments ejected
from a volcano.
ii.Volcanic ash is pyroclastic material less than
2mm in diameter.
iii.Volcanic dust is material less than .25mm in
diameter.
Volcanic Rock Fragments
Volcanic Rock Fragments
iv. Lapilli (luh-pil-ie) is material that is less
than 64mm in diameter.
1. Means “little stones” in Latin.
v. Volcanic bombs are large, spindle shaped
clots of lava thrown out of the volcano.
vi. Volcanic blocks are the largest possible
pyroclastic material that is formed from
solid rock blasted from a volcanic fissure.
Bombs & Blocks
Volcanic Cones
3. Volcanic cones are piles of ejected
material that build up around the
vent and there are three main types.
a.Shield volcanos are broad at the base
and have gently sloping sides.
i.These types of volcanos cover a large
area.
Shield Volcano
Volcanic Cones
b. Cinder cones are made of solid fragments
ejected from the volcano, are very steep,
and are rarely more than a few hundred
meters high.
c. Composite or Stratovolcano is a steep
sloped volcanic material deposited with
alternating layers of hardened lava flows
and pyroclastic materials.
Cinder Cone
Stratovolcano
Craters & Calderas
4.
Craters & Calderas
a. A crater is a funnel shaped pit at the top of a
volcanic vent.
i.
Craters are formed when material is blown out of
the volcano by explosion.
b. A caldera forms when the magma chamber
below a volcano is emptied and the volcano
collapses leaving a large basin shaped
depression.
Crater
Caldera
Label