Tuesday, October 19, 2010

Magma, Lava, Felsic, Mafic, Ah-Ah, Oh My!

You just finished walking through an old lava tube (remember, the Ape Cave!!!). And you even did your best impression of a lava flow. Isn't it about time your learned about all the different kinds of lava there are? The correct answer: YES OF COURSE!!! Lava! Lava! Lava! Lava!

Here's a quick review:
  • Magma = an underground molten mixture of rock-forming substances, gases and water vapor that originates from the asthenosphere.
  • Lava = magma that has erupted onto the surface of the Earth.
There are a few different ways to classify lava that measure the following characteristics:
  • Chemical composition - mainly how much silica ( SiO2 ) is found in the lava.
  • Eruption temperature - the temperature range at which the lava comes out of the ground.
  • Viscosity - a liquid's resistance to flow (honey=high viscosity and water = low viscosity)
  • Gas content - the amount of gases dissolved in the lava.
There are three main types of magma/lava that produce four different types of volcanic rock. Notice that depending on how much silica (SiO2) felsic lava contains, in can either produce dacite or rhyolite. (Select the Volcanic Rock types to get more information)

SiO2 CONTENT
MAGMA TYPE
VOLCANIC ROCK
~50%
Mafic
Basalt
~60%
Intermediate
Andesite
~65%
Felsic (low Si)
Dacite
~70%
Felsic (high Si)
Rhyolite

Take a look at the chart below and answer the questions below:

Summary Table

Magma Type

Solidified Rock

Chemical Composition

Temperature

Viscosity

Gas Content

Mafic

Basalt

45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na

1000 - 1200 oC

Low

Low

Intermediate

Andesite

55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K

800 - 1000 oC

Intermediate

Intermediate

Felsic

Rhyolite

65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na.

650 - 800 oC

High

High


  1. Rank the magma types from highest silica content to lowest silica content.
  2. Rank the magma types from highest temperature to lowest temperature.
  3. Other than honey and water, what are some three liquids you know with different viscosity? Try ranking these liquids from highest viscosity to lowest viscosity.
  4. Rank the magma types from highest viscosity to lowest viscosity.
  5. Rank the magma types from highest gas content to lowest gas content.
  6. Which kind of lava formed the Ape Cave? What kind of viscosity did this lava have?

And because earth science videos are amazing, check these out:
  • The Hawaiians have more specific names for the mafic/basaltic lava flows they routinely see erupt from Hawaiian shield volcanoes. CHECK IT OUT!
  • Here's a look forward at the Rock cycle that we'll cover second semester: CHECK IT OUT!
  • And since Leah asked about different kinds of caves, here's an awesome video about caves. CHECK IT OUT!

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