Chapter 7 Section 2 Volcanic Eruptions

Types Of Lava Lavas can be grouped by the different kinds of rocks they produce, generally based on the amounts of iron, magnesium, and silica.

How can Volcanoes be classified?

Volcanoes can be grouped by the different kinds of rocks they produce.

Because of this large variation: Silica content is used to classify rocks.

The type of rock produced depends on chemical composition.

When geologists analyzed numerous types of volcanic rocks from around the world, they found that the silica (SiO2) content varied by as much as 40 weight percent.

Classification system bases on appearance

A simple classification scheme based on silica content is used.

Mafic Lava Dark colored rich in magnesium and iron usually of oceanic crust origin Basalt - low SiO2=less than 52%

Mafic Lava Flows Classified on appearance.

Pahoehoe lava smooth, runny, high temperature lava with a ropy appearance

aa lava rough jagged lava that forms from thicker non-runny lava

Blocky flow large angular pieces which flow like a slow motion avalanche

Pillow lava forms when mafic lava cools underwater

Felsic Lava High silica content - lesser amounts of iron and magnesium lighter color usually formed from continental crust Rhyolite

Intermediate Lavas Have a range of compositions that fall between the mafic and the felsic varieties

Too Summarize…

Kinds of Eruptions

Quiet Oceanic volcanoes mafic lava gases can easily escape flow like a red-hot river

Explosive Continental volcanoes felsic lavas large amounts of trapped gases eruptions send molten and solid particles shooting into the air

ERUPTIVE PRODUCTS

Tephra (Pyroclastic Material) rock fragments of various sizes that are blown into the air, generally from felsic lava because of rapidly expanding gases.

Volcanic Dust Tephra particles less than 0.25 mm in diameter.

ash fine dust less than 2 mm in diameter

lapilli - little stones (cinders) particles up to 64 mm in diameter

bombs chunks of red-hot rock which cool as they fly through the air

volcanic blocks Formed from solid rock blasted from the fissure

pumice light weight rock formed from air passing through it

Lava flows molten rock that reaches the surface depends upon temperature and chemical composition

Pyroclastic flow Turbulent mixture of hot gas and rock fragments that moves like liquid / very fast

TYPES OF VOLCANOES shape classification

Cinder cone made entirely from tephera small with steep sides short-lived (less than 1000 years) not very explosive

Shield Volcano quite large and form from very fluid lava rarely explosive gently sloping sides are made of basaltic lava

Puu Oo Vent, Hawaii Crater Funnel-shaped pit at the top of a volcanic vent

Types of Volcanoes (classified by shape) Shield volcano: characteristic of eruptions emitting low viscosity lava. Hawaii is an example. http://www.cet.edu/ete/modules/volcanoes/vtypesvolcan1.html Olympus Mons is the largest volcano on Mars. This shield volcano, similar to volcanoes in Hawaii, measures 624 km (374 mi) in diameter by 25 km (16 mi) high. It is 100 times larger than Mauna Loa on Earth. Located on the Tharsis Plateau near the equator, Olympus Mons is bordered by an escarpment. The caldera in the center is 80 km (50 mi) wide and contains multiple circular, overlapping collapse craters created by different volcanic events. The radial features on the slopes of the volcano were formed by overflowing lava and debris. http://pds.jpl.nasa.gov/planets/captions/mars/olympus.htm

Stratovolcano (composite) a succession of alternating lava flows and tephra deposits usually thick lava and are some what explosive made mostly of andesite (Mt. Rainier)

Mount Rainier

Mount Saint Helens

Lava Dome a single mass of extruded lava that’s too thick to move away from the volcano very explosive (Mt. St. Helens)

Craters and Calderas

Caldera Large basin-shaped depression formed when an explosion destroys the upper part of a volcanic cone http://cimss.ssec.wisc.edu/sage/geology/lesson3/images/Figure%2012%20Aniakchak%20Caldera%20Alaska.jpg

Crater Lake (1,932 feet deep). The lake was formed by the explosion and collapse of 12,000 foot Mt. Mazama

Wizard Island – regrowing volcano Crater Lake Wizard Island – regrowing volcano

Can Be Quite Large This happens when the magma chamber empties and the roof collapses.

Yellowstone

http://pubs.usgs.gov/fs/2005/3024/images/fs2005-3024_fig_15.jpg