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?