VOLCANOES AND VOLCANIC ACTIVITY Chapter 6

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

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+

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

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)

Typical Hawaiian eruption. Slow moving, blocky lava

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.

- 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)

(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)

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

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)

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)

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.

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

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.

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.

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)

Crater Lake in Oregon is a good example of a caldera. This particular caldera 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.

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 1-8. 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?