1. VOLCANOES
AND VOLCANIC ACTIVITY
Chapter 6

2. TOPIC OUTLINE
History of volcanic eruptions
Nature of volcanic eruptions
- types of volcanic eruptions
Types of volcanoes
- Shield
- Strato (Composite)
- Cinder Cone
Continental Flood Basalts and Mass Extinctions?
Bentonites and tonsteins
Associated volcanic problems

3. HISTORY OF VOLCANIC ERUPTIONS
There are volcanic eruptions weekly on the earth, and they are all different.
Below is a list of the some of largest eruptions in the history of the planet………
Year
Location
Deaths
1815
1883
1912
1991
1900 79
Tambora, Indonesia
Krakatoa, Indonesia
Mt. Katmai, Alaska
Pinatubo, Mexico
Mt. St. Helen’s, Washington
Vesuvius, Italy
50,000-90,000
36,000
900+

4. NATURE OF VOLCANIC ERUPTIONS
No two volcanic eruptions are the same, some slower, some more explosive, etc.
Several factors control the nature of these volcanic eruptions and whether or
not they will be explosive.
Factors controlling volcanic eruptions
Temperature
Composition
Amount of dissolved gases
Viscosity (resistance to flow)
Variations in one or more of these factors will result in a different
type of volcanic eruption

5. NATURE OF VOLCANIC ERUPTIONS
All volcanic eruprtions on the planet can be classified into one
of four catagories. They are Hawaiian, Strombolian, Vulcanian,
and Plinian.
Hawaiian Eruptions – consist of very runny lava that flows from a volcanic
vent, usually on the side of the volcano (Shield Volcanoes)
(Fig. 6.2F)
Strombolian Eruptions – consist of eruptions more explosive then
Hawaiian eruptions & can eject lava or rock 100s of feet into
the air (Cinder Cones) (Fig. 6.2D)
Vulcanian Eruptions – consist of much more explosive eruptions and can
eject ash 1000s of km into the atmosphere & also contain
pyroclastic flows (Fig. 6.2B)
Pyroclastic Flow – hot volcanic fragments buoyed by heat &
volcanic gases, which travel very fast
Plinian Eruptions – represent the violent eruptions in history (named after
Pliny the Elder, 79AD) & they produce ash that reaches 20km into
the stratosphere & violent pyroclastic flows (stratovolcanoes) (6.2C)

6. Typical Hawaiian eruption. Slow moving, blocky lava

7. This eruption is indicative of a
Strombolian type eruption.
They can be more explosive than
Hawaiian eruptions and can eject
lava hundreds of kilometers
into the air.

8. - Contain a very broad base with gentle slopes
TYPES OF VOLCANOES
Shield Volcanoes
- Contain a very broad base with gentle slopes
- Contain a very low percentage of pyroclastic flows
Are usually basaltic in composition
Examples: Mauna Loa and Mauna Kea (Fig. 6.2i)
Basalt – a mafic, aphanitic igneous rock that is composed of dense
oceanic crust minerals
(this will discussed in more depth when igneous rocks are
addressed)

9. (Composite volcanoes)
TYPES OF VOLCANOES
Stratovolcanoes
(Composite volcanoes)
Formed in the Ring of Fire from subduction
Interbedded lava and pyroclastic flows
Andesitic in composition
Examples: Mt. Fujiyama, Mt. Hood (Fig. 6.2A)
Ring of Fire – the boundary around the Pacific Ocean that is lined with
large stratovolcanoes; these result from subduction
Andesitic – an intermediate, aphanitic igneous rock that forms in
subduction zones; (this will be discussed in more depth
when igneous rocks are addressed)

10. Strato-volcano in the Pacific northwest. This structure results from
the subduction of the Juan de Fuca Plate.

11. Built primarily from ejected material
TYPES OF VOLCANOES
Cinder Cones
Built primarily from ejected material
30° to 40° steep – beyond the angle of repose
Usually less than 300m high
Example: Paracutin in Mexico & Sunset Crater, AZ
(Fig. 6.2e)

12. Continental Flood Basalts
TYPES OF VOLCANOES
Continental Flood Basalts
At several times in the geologic past, large cracks open in the
crust and high amounts of basalt flows over the surface
These deposits can cover 1,000,000s of square miles
Examples of these deposits are the Deccaan Traps & the
Siberian Traps (intertrappen deposits)
Several of these deposits coincide with the five mass extinction levels
in the geological past
Are the Continental Flood Basalts responsible? Did they at least have
a hand in the extinctions?
(Fig. 6.2h)

13. associated with the Columbia River basalt flows.
Part of a small flood basalt in the western part of Wyoming. This flow may be
associated with the Columbia River basalt flows.

14. BENTONITIES & TONSTEINS
Both bentonites and tonsteins are volcanic ash layers that are used
For long distance correlation between different geographic areas
Bentonites
Volcanic ash layers that have been altered & are interbedded with
rock sequences
Each has a specific chemical fingerprint and this fingerprint can be used
to correlate layers of rocks over long distances, even between continents
Composed primarily of smectite clays
Since volcanic ash falls from the sky geologically instantaneously, these
layers can be used for time markers

15. BENTONITIES & TONSTEINS
Altered volcanic ash layer found in coal producing regions
Composed of montmorillinite clays
The ash fall was altered as it fell into the acid environment of
A coal swamp
These layers can be used for long distance corrleations
We have examples of both bentonites and tonsteins here in Ohio.
The bentonites can be observed in the Cincinnati area and the
tonsteins can be seen in the area of Hocking Hills and eastern Ohio.

16. hammer) is about 454 million years old.
Diecke K-bentonite on Manitoulin Island, Canada. This ash layer (base of the
hammer) is about 454 million years old.

17. ASSOCIATED VOLCANIC TERMS
Crater – the basin-like, rim shaped depression at the summit of a volcano
Caldera – a basin-like, rim shaped depression that is several times
larger than a crater, usually resulting from a volcano
experiencing a massive eruption
Lahar – a flood of debris associated with a volcanic eruption, usually
associated with stratovolcanoes due to the snow melting near
the summit of the mountain
Nuée Ardente – a glowing cloud of ash and glass that results from a
volcanic eruption
(This is what happened to the people of Pompeii)

18. Crater Lake in Oregon is a good example of a caldera. This particular
caldera happens to be filled with water.

19. This photo shows a nuee ardente travelling down the slope of a volcano.
The glowing cloud of ash and glass is extremely dangerous to people in its path.

20. At the end of the chapter on page 180, please do the self test
QUESTIONS
At the end of the chapter on page 180, please do the self test
questions Once completed, please consider the following questions.
Please consider why the eruptions on the Hawaiian Islands are
not very explosive.
Why are there stratovolcanoes located around the Pacific Rim
in the Ring of Fire?
Why would an early warning system for volcanic eruptions
be useful?