THE IMAGES HERE RELATE TO THE PHS 120 LAB DEALING WITH VOLCANIC ROCKS.
The above picture is a black sand beach in Hawaii.
This black sand sample is from Black Sand Beach on the big island of
Hawaii. The composition of the sand is obsidian.
Pyroclastics in this specimen include a bomb and cinders. As these
materials were ejected from the volcano (in northern AZ) the they accumulated
along the flanks of the volcano (cinder cone). Due to the high temperature
of the pyroclasitics, they were able to plastically flow as well as weld
together.
Both rocks are fine grained (volcanic) and light colored (felsic). This
combination strongly points to RHYOLITE as the rock type. Crystals are
garnet (left) and topaz (right).
Topaz in Rhyolite
A close-up view of the garnet in rhyolite showing the sharp crystal faces.
Obsidians
all cool rapidly and are glassy. The snowflake obsidian, however, has
crystals of cristobalite.
Pele's Hair is formed when small gas pockets rise to the top of a lava
flow. When the bubble reach the surface, they "pop" sending
streamers of lava into the air. This lava cools quickly to form long,
slender, golden strands of obsidian - Pele's Hair.
A microscopic view of Pele's Hair.
Similar to Pele's Hair are Pele's Tears. These are often associated with
the hair and form in a similar manner. The tears are just larger blobs of
molten material instead of the slender strands. A complete gradation can
be seen from tears to hair. These samples attest to the fluidity (low
viscosity) of the Hawaiian flows.
Pumice/Obsidian (Close up view on right) This
sample simply shows how closely related these two forms of volcanic glass are.
A vial of pumice. This is the typical rock type erupted from the big
eruption of Mt. St. Helens in 1980. Pumice covered many square miles of
the mountainous terrain.
Individual fragments of the pumice contained within the vial from above.
Note how porous the material is.
Reticulated pumice. This is a microscopic view (on right) of the sample
that is in the box with Pele's Tears and Hair. The extremely porous
material attests to a highly fluid lava that was charged with gas - but of
course, not explosive.
This is a chunk of basalt from the eruption of Puuohoh from Hawaii. This
was part of a gas blister on the flow. The blister was created when a
large gas bubble moved towards the top of the flow but the lava had already
formed a crust. The large gas bubble domed the crust up forming a
blister. Then your instructor came along and stepped on it, broke it and
brought part of it back for you to look at in lab.
Pele's Hair on the top of this specimen give it a golden hue.
Yakima Washington
(Thumbnail provided for a larger file size.)
The above picture is a panoramic view of the green sand beach near South Point, Hawaii.
This closer
view shows the eroded volcanic vent.
The beach
sand is dominated by olivine. You can also see that the source of the
olivine (the ash deposits) are sending a constant supply to the beach.
South Point sand
This sand has abundant grains of OLIVINE which is unstable in the beach environment. The constant agitation of the beach sands quickly erodes the olivine. But...
Notice also the abundance of shell fragments.
These photos are of granodiorite with a xenolith. The boundary of the xenolith and the granodiorite is shown in the close up picture on the right.
Sulfur crystals. 8.35 is the GCC mineral catalog number.
The Big Island of Hawaii is still active. Here a road has been covered by a pahoehoe flow making the speed limit sign somewhat comical.
This is a
fumerole - a volcanic vent from which gases escape. This fumerole was
lined with sulfur crystals. By moving some of the
rocks around I was able to get some good photos of the sulfur crystals.
These
sulfur crystals were formed by PNEUMATOLITIC action. i.e., they formed from
volcanic gases rich in sulfur. As the gases cooled, the sulfur was
deposited.