TYPES OF LAVA
There are three categories of lava, classified on the basis of its
silica (SiO2) content, which affects how readily it flows.
1. BASALTIC LAVA
a. Relatively low SiO2 content (less than 52%)
b. Relatively low viscosity (flows readily when hot)
Basalt is a hard, black volcanic rock with less than about 52
weight percent silica (SiO2). Because of basalt's low silica
content, it has a low viscosity (resistance to flow). Therefore,
basaltic lava can flow quickly and easily move >20 km from a
vent. The low viscosity typically allows volcanic gases to escape
without generating enormous eruption columns. Basaltic lava
fountains and fissure eruptions, however, still form explosive
fountains hundreds of meters tall. Common minerals in basalt
include olivine, pyroxene, and plagioclase. Basalt is erupted at
temperatures between 1100 to 1250° C.
2. ANDESITIC LAVA
a. Intermediate SiO2 content (between 52 and 63%)
b. Intermediate viscosity (does not flow readily)
Andesite is a gray to black volcanic rock with between about 52 and
63 weight percent silica (SiO2). Andesites contain crystals
composed primarily of plagioclase feldspar and one or more of the
minerals pyroxene (clinopyroxene and orthopyroxene) and lesser
amounts of hornblende. At the lower end of the silica range,
andesite lava may also contain olivine. Andesite magma commonly
erupts from stratovolcanoes as thick lava flows, some reaching
several km in length. Andesite magma can also generate strong
explosive eruptions to form pyroclastic flows and surges and
enormous eruption columns. Andesites erupt at temperatures between
900 and 1100° C.
3. RHYOLITIC LAVA
a. Relatively high SiO2 content (above 68%)
b. Relatively high viscosity (flows like wet concrete)
Rhyolite is a light-colored rock with silica (SiO2) content greater
than about 68 weight percent. Sodium and potassium oxides both can
reach about 5 weight percent. Common mineral types include quartz,
feldspar and biotite and are often found in a glassy matrix.
Rhyolite is erupted at temperatures of 700 to 850° C.
Yakima Valley has many layers of lava. This layers were created
by floods of lava which poured from large fissures in the
southeastern Washington between 17 and 10 millions years ago. At
lease 10 flows underlie Yakima but stream deposit as thick as 1,000
feet hide them from view in most places along the Greenway. Other
views visit Harlan Land and Century Landing in Union Gap.
To log this earthcache, please email me (through my gc.com
profile) the answers to the following questions. You will be able
to find the information at the roadside marker. Once you've sent
the email, go ahead and log your find without waiting for a reply
from me. I will contact you if there are any problems with your log
entry.
1. What type of lava is this layers and is known by?
2. what City was the large fissures near?
3. In some places, what is the reported thickness of this lava
field under Yakima?
4. Optional (but appreciated): Post photos of yourself with the
view, being sure to not reveal any answers to the questions in your
photographs.