Volcanic Eruptions

Chapter 7 Section 2

Types Of LavaLavas 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 LavaDark coloredrich 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 flowlarge angular pieces which flow like a

slow motion avalanche

Pillow lava

forms when mafic lava cools underwater

Felsic LavaHigh silica content - lesser amounts of iron and magnesium

lighter colorusually 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

QuietOceanic volcanoesmafic lavagases can easily escapeflow like a red-hot river

ExplosiveContinental volcanoesfelsic lavaslarge amounts of

trapped gaseseruptions send molten

and solid particles shooting into the air

ERUPTIVEPRODUCTS

Tephra (Pyroclastic Material)

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

Volcanic DustTephra 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

bombschunks 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 flowsmolten rock that reaches the surfacedepends upon temperature and chemical

composition

Pyroclastic flowTurbulent mixture of

hot gas and rock fragments that moves like liquid / very fast

shape classificationTYPES OF VOLCANOES

Cinder conemade 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 explosivegently sloping sidesare made of basaltic lava

Puu Oo Vent, Hawaii

CraterFunnel-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

CalderaLarge 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

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