Much of this area was formed fewer than 3,000 years ago. There
are a number of local volcanoes that are known as shield
volcanoes, so named because they have regular lava eruptions
over the course of time, that eventually form a mountain in the
shape of an ancient shield. (Another description might be the shape
of a Vanilla Wafer!)
This is an EarthCache, which is not a physical cache in any way.
To get credit for this, you will need to travel to the coordinates.
read the information sign, read this page, look around at the
natural formations, and think about your answers a bit. This
section of Oregon is part of the Cascade range of mountains, one of
the most active volcanic regions in the world. For thousands of
years, eruptions have consistently reshaped the landscape. Lava
refers both to molten rock expelled by a volcano during an eruption
and the resulting rock after solidification and cooling. This
molten rock is formed in the interior of some planets, including
Earth, and some of their satellites. When first erupted from a
volcanic vent, lava is a liquid at temperatures from 700 °C to
1,200 °C (1,300 °F to 2,200 °F). Up to 100,000 times as viscous as
water, lava can flow great distances before cooling and
solidifying, due to the properties of the molten rock.
A lava flow is a moving outpouring of lava, which is created
during a non-explosive effusive (pouring out) eruption. When it has
stopped moving, lava cools and solidifies to form igneous rock. The
word "lava" is Italian, and is probably derived from the Latin word
labes, which means a fall or slide. The first use in
connection with extruded magma (molten rock from below the Earth's
surface) was apparently in an account written on the eruption of
Mt. Vesuvius in 1737, by the early geologist and scholar Francesco
Serao. Serao described "a flow of fiery lava" as an analogy to the
flow of boiling mud down the flanks of the volcano following heavy
rain.
Igneous rocks, which form lava flows when erupted, can be
classified into three basic chemical types; felsic, intermediate,
and mafic. The difference in these classes are primarily chemical;
but the chemistry of lava also tends to correlate with the magma
temperature, its viscosity and its mode of eruption. Here are the
three types:
Felsic lava
Felsic (or silicic) lavas (such as rhyolite and dacite) typically
form lava spines, lava domes or "coulees" (which are thick, short
lavas) and are associated with pyroclastic (fragmental) deposits.
Most Silicic lava flows are extremely viscous, and typically
fragment as they extrude, producing blocky jagged clumps known as
autobreccias. The high viscosity and strength are the result of
their chemistry, which is high in silica, aluminium, potassium,
sodium, and calcium, forming a polymerized liquid rich in feldspar
and quartz, which thus has a higher viscosity than other magma
types. Felsic magmas can erupt at temperatures as low as 650 to 750
°C. Unusually hot (>950 °C) rhyolite lavas, however, may flow
for distances of many tens of miles, such as in the Snake River
Plain of the northwestern United States. The felsic autobreccias
are among the most difficult types of terrain on Earth, with spires
and crevasses often more than 10 meters (33 feet) in height.
Intermediate lava
Intermediate or andesitic lavas are lower in aluminium and silica,
and usually somewhat richer in magnesium and iron. Intermediate
lavas form andesite domes and block lavas, and may occur on steep
composite volcanoes, such as in the Andes. Poorer in aluminium and
silica than felsic lavas, and also commonly hotter (in the range of
750 to 950 °C), they tend to be less viscous. Greater temperatures
tend to destroy polymerized bonds within the magma, promoting more
fluid behavior and also a greater tendency to form phenocrysts
(mixed or “spotty” rock formations). Intermediate lava
fields often resemble gravelly river beds, although the great lava
dome within the crater of Mt. St. Helens is comprised of
intermediate lava.
Mafic lava
Mafic or basaltic lavas are typified by their high content of
ferromagnesium (iron and magnesium), and generally erupt at
temperatures in excess of 950 °C. Basaltic magma also has a
relatively lower aluminium and silica content, which taken together
reduces the degree of polymerization within the melt. Owing to the
higher temperatures, viscosities can be relatively low, although
still thousands of times more viscous than water. Basalt lavas tend
to produce low-profile shield volcanoes or "flood basalt fields",
because the fluidal lava flows for long distances from the vent.
The thickness of a basalt lava, particularly on a low slope, may be
much greater than the thickness of the moving lava flow at any one
time, because basalt lavas may "inflate" by supply of lava beneath
a solidified crust. Most basalt lavas are of the ‘A’ä
or pähoehoe types, rather than block lavas. Underwater they can
form "pillow lavas", which resemble soft bed pillows. This is the
smoothest of the three lava types.
What you see before you are three different lava flows, all
within the last 2,000 years. There are elements of all three types
of lava here, but you will have to look and read carefully to
determine the answers.
To log this cache, you will need to email me the answers to
the following questions:
1. What type of lava are you looking at here?
2. How many of the lava flows can you make out?
3. What are the colors of lava that you can see?
4. You are actually standing on a rock outcropping that continues
north, one that predates the lava flows. Look at the direction that
the lava was flowing, and describe in your own words what effect
the outcropping would have on the lava.
5. What populated areas would be most at risk from another eruption
here?
And please be sure to include the name of the cache in your
email. After sending the email, go ahead and log the cache. I will
only contact you if there is a problem with your
answers.
