Geology 309 - Lecture 9
Felsic lavas
Click here for a great resource on different composition lava flows
complete with pictures
A. Dacitic Lavas
Dacitic lavas are even more viscous than andesites and have even
higher yield strengths. Not just from increases in silica and
lower temperature, but also because they tend to have higher crystal
content.
Commonly associated with explosive events - often last gasp of magma.
More typically form domes than lava flows.
Lava Domes
- Endogenous - grow through internal expansion
- Exogenous - growth of lobes which break out to surface
Dome types:
1. low domes - flattish, round symmetrical extrusions. Also called
tortas.
2. Peleean domes - craggy topography and lava spines
3. Upheaved plugs (plug domes) - masses of rock pushed bodily upwards like
pistons. Yield strengths so high that they do not deform.
4. Cryptodome - Early dome growth where overlying sediments are uplifted,
but dome itself does not show through.
Dome collapse -
Click here for a view of collapsed dome deposits from Mt.
Unzen, Japan, and also for a diagram that shows a sequence of
events for dome collapse.
Coulees - Cross between domes and lava flows.
Thick extrusions erupted
on steep slopes so that shear stresses exceed yield strength
B. Rhyolitic Lavas
Very scarce because typically highly explosive. Light colored lava (white
or grey)
Obsidian - form of rhyolite that is pure glass. It is a supercooled
liquid. Typically jet black or red if oxidized.
Devitrification and Hydration in rhyolite flows (obsidian)
1. Devitrification - breakdown of volcanic glass into small crystals. Forms
what is known as
Snowflake Obsidian.
2. Hydration - superimposed on devitrification. Glasses
are hydroscopic and pick up water. Water diffused through edge
of glass
Cracks are effective paths of hydration.
Can actually date rock by looking
at thickness of hydration rind around crack.
x2 = Kt (x is squared, not times 2)
x = depth
k = constant
t = time in years
Perlite - very classic hydration effect. Concentric onion skin
cracks,
1-3 cm scale.
Apache's tears - round glass fragments that are the cores of perlite