1. Types of Eruptions: Explosive and Nonexplosive
Geology 12
Ms. Bowie

2. Types of Lava There are 4 main types of lava: A’a (pronounced Ah-Ah)
Pahoehoe (pronounced pa-hoy-hoy)
Pillow
Blocky

3. 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.

4. Volcano Shapes & Eruption Types
The two most important properties of lava are:
its viscosity,
the amount of gases dissolved in the liquid rock.

5. Lava Viscosity
Viscosity is the “thickness” or “runniness” of the fluid
The explosivity of lava depends on several factors:
Composition - especially the amount of silica (SiO2) in the lava.
Amount of gases dissolved in the lava (H2O & CO2)
Pressure generated

6. Low Viscosity – Little Gas
Lava with a small amount of silica and little dissolved gases tend to be:
Runny
Mildly bubbly
Under Low pressure
This results in lava that flows fairly gently and smoothly out of the volcano.

7. 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.

8. Pillow Lava
Lower viscosity lava that erupts under water forms rounded mounds that look like pillows.

9. Moderate Viscosity Lava
Lava with a moderate amount of silica and little dissolved gases tend to be:
Lumpy
Moderately bubbly
Under Moderate pressure
This results in lava that flows less easily and somewhat bumpy out of the volcano.

10. 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.

11. Low Viscosity – High Gas
Lava with a little silica but lots of dissolved gases tend to be:
Frothy
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.

12. High Viscosity – High Gas
Lava with a large amounts of silica and dissolved gases tend to be:
Thick & Gooey
Pockets of trapped gas
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.

13. Pyroclastic Material from EXPLOSIVE ERUPTIONS
4 types of pyroclastic material:
a. Volcanic blocks
b. Volcanic bombs
c. Lapilli
d. Volcanic Ash

14. 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.

15. Lapilli Lapilli means “little stones” in Italian.
They are tiny pieces of magma that harden before they hit the ground.

16. 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.

17. 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.

18. Review 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.
This type of lava has a smooth surface and rounded edges.
This type of lava forms sharp edged chunks.
List the four types of pyroclastic material.
These are large blobs of magma that have cooled and hardened as they flew through the air.
These are tiny pieces of magma that have hardened in the air.
These are solid rock fragments and usually consist of pieces of the old volcano.
This is formed from when gases in stiff magma expand and explode into slivers.

19. Answers A’a, pahoehoe, pillow, blocky A’a Pillow Pahoehoe Blocky
Blocks, bombs, lapilli, ash
Bombs
Lapilli
Blocks
Ash