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Types of Eruptions:
Explosive and Nonexplosive
Geology 12
Ms. Bowie
Types of Lava
There are 4 main types of lava:
1. A’a (pronounced Ah-Ah)
2. Pahoehoe (pronounced pa-hoy-hoy)
3. Pillow
4. Blocky
Volcano Shapes & Eruption Types
• The shape of volcanoes and the
violence of the eruptions depend on the
same rather thing:
• the physical properties of erupting lavas.
Volcano Shapes & Eruption Types
• The two most important properties
of lava are:
1. its viscosity,
2. the amount of gases dissolved
in the liquid rock.
Lava Viscosity
•
•
Viscosity is the “thickness” or “runniness”
of the fluid
The explosivity of lava depends on several
factors:
1. Composition - especially the amount of
silica (SiO2) in the lava.
2. Amount of gases dissolved in the lava
(H2O & CO2)
3. Pressure generated
Low Viscosity – Little Gas
•
Lava with a small amount of silica and little
dissolved gases tend to be:
1. Runny
2. Mildly bubbly
3. Under Low pressure
This results in lava that flows fairly gently
and smoothly out of the volcano.
Low Viscosity – Little Gas
This is called Pahoehoe
•
Pahoehoe (pa-hoy-hoy)
lava flows develop
surface crusts that form
thick plates with ropy
and/or gently
undulating surfaces.
Pillow Lava
• Lower viscosity lava
that erupts under
water forms
rounded mounds
that look like
pillows.
Moderate Viscosity Lava
•
Lava with a moderate amount of silica and
little dissolved gases tend to be:
1. Lumpy
2. Moderately bubbly
3. Under Moderate pressure
This results in lava that flows less easily
and somewhat bumpy out of the volcano.
A’a-Lava
• A’a (Ah-Ah) lava flows have
a very rough, rubbly
surface because of their
high eruption rates.
• Pieces of the rocky surface
are broken, rolled and
tumbled along as the lava
flow moves. When finally
cooled to a solid, a’a lava
flows look like a jagged
heap of loose rock.
Low Viscosity – High Gas
Lava with a little silica but lots of dissolved
gases tend to be:
1. Frothy
2. Tossing into the air
•
•
lava, cinders &
lava bombs
This results in spectacular lava fountains
that toss materials predictably and
impressively into the air above the vent.
High Viscosity – High Gas
Lava with a large amounts of silica and
dissolved gases tend to be:
1. Thick & Gooey
2. Pockets of trapped gas
3. Under High pressure
This results in lots of gas trapped under
thick lava. The pressure builds until it blows
the entire top of the volcano.
Pyroclastic Material from
EXPLOSIVE ERUPTIONS
4 types of pyroclastic material:
a. Volcanic blocks
b. Volcanic bombs
c. Lapilli
d. Volcanic Ash
Volcanic Bombs
• Volcanic bombs are
large pieces of magma
that harden in the air as
the erupt out of a
volcano.
• They can form in a
variety of sizes and
shapes.
Lapilli
• Lapilli means “little
stones” in Italian.
• They are tiny
pieces of magma
that harden before
they hit the
ground.
Volcanic Ash
• Volcanic ash forms
when gases in stiff
magma expand rapidly.
• The walls of the gas
bubbles explode into
tiny glasslike slivers.
• Ash makes up most of
a pyroclastic eruption.
High Viscosity – Low Gas
Blocky Lava
• Cool stiff lava that
does not travel far
from the erupting
vent.
• It cools and form
sharp edged
chunks.
Review
1.
2.
3.
List the 4 types of lava.
This type of lava is very rough with jagged edges.
This type of lava forms from underwater eruptions,
looks like rounded lumps.
4. This type of lava has a smooth surface and rounded
edges.
5. This type of lava forms sharp edged chunks.
6. List the four types of pyroclastic material.
7. These are large blobs of magma that have cooled and
hardened as they flew through the air.
8. These are tiny pieces of magma that have hardened in
the air.
9. These are solid rock fragments and usually consist of
pieces of the old volcano.
10. This is formed from when gases in stiff magma expand
and explode into slivers.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
A’a, pahoehoe, pillow, blocky
A’a
Pillow
Pahoehoe
Blocky
Blocks, bombs, lapilli, ash
Bombs
Lapilli
Blocks
Ash