volcanoes and volcanic activity chapter 6. topic outline 1.history of volcanic eruptions 2.nature of...
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VOLCANOES AND VOLCANIC ACTIVITY
Chapter 6
TOPIC OUTLINE
1. History of volcanic eruptions
2. Nature of volcanic eruptions- types of volcanic eruptions
3. Types of volcanoes- Shield- Strato (Composite)- Cinder Cone
4. Continental Flood Basalts and Mass Extinctions?
5. Bentonites and tonsteins
6. Associated volcanic problems
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………
Tambora, Indonesia
Krakatoa, Indonesia
Mt. Katmai, Alaska
Pinatubo, Mexico
Mt. St. Helen’s, Washington
Vesuvius, Italy
1815
1883
1912
1991
1900
79
50,000-90,000
36,000
900+
Year Location Deaths
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 ornot they will be explosive.
Factors controlling volcanic eruptions
1. Temperature
2. Composition
3. Amount of dissolved gases
4. Viscosity (resistance to flow)
Variations in one or more of these factors will result in a different type of volcanic eruption
NATURE OF VOLCANIC ERUPTIONS
All volcanic eruprtions on the planet can be classified into oneof 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 intothe stratosphere & violent pyroclastic flows (stratovolcanoes) (6.2C)
Typical Hawaiian eruption. Slow moving, blocky lava
This eruption is indicative of a Strombolian type eruption.They can be more explosive thanHawaiian eruptions and can ejectlava hundreds of kilometers into the air.
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)
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 withlarge stratovolcanoes; these result from subduction
Andesitic – an intermediate, aphanitic igneous rock that forms in subduction zones; (this will be discussed in more depthwhen igneous rocks are addressed)
Strato-volcano in the Pacific northwest. This structure results fromthe subduction of the Juan de Fuca Plate.
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)
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 & theSiberian Traps (intertrappen deposits)
Several of these deposits coincide with the five mass extinction levelsin the geological past
Are the Continental Flood Basalts responsible? Did they at least have a hand in the extinctions?
(Fig. 6.2h)
Part of a small flood basalt in the western part of Wyoming. This flow may beassociated with the Columbia River basalt flows.
BENTONITIES & TONSTEINS
Both bentonites and tonsteins are volcanic ash layers that are used For long distance correlation between different geographic areas
BentonitesVolcanic ash layers that have been altered & are interbedded withrock sequences
Each has a specific chemical fingerprint and this fingerprint can be usedto 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
BENTONITIES & TONSTEINS
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.
Diecke K-bentonite on Manitoulin Island, Canada. This ash layer (base of the hammer) is about 454 million years old.
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 nearthe 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)
Crater Lake in Oregon is a good example of a caldera. This particularcaldera happens to be filled with water.
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.
QUESTIONS
At the end of the chapter on page 180, please do the self test questions 1-8. Once completed, please consider the following questions.
Please consider why the eruptions on the Hawaiian Islands arenot very explosive.
Why are there stratovolcanoes located around the Pacific Rim in the Ring of Fire?
Why would an early warning system for volcanic eruptionsbe useful?