Here are some images of volcanic rocks that I have taken over
several decades of interest in these phenomena.
iamges are copyrighted, but publishable high resolution image files are
I will be adding to these collections continually.
Oceanic islands erupt low viscosity basaltic lavas at
relatively high temperatures. The eruptions are seldom explosive and
the lava flows can extend for tens of kilometers.
This is a night view of a lava flow above Kalapana in 1989.
Here we are using a sensitive temperature probe to test the lava
temperature. The probe here isa marshmallow and, yup, the lava is hot.
Here an aa flow front is advancing on an older pahoehoe flow and the
geologist is sampling the lava.
Black sand beaches occur where lava flows enter the sea. The sand is
quenched lava that is mostly glass. The finely divided glass and
crystals weather quickly so the beaches don't usually last for long
This beach is on the east coast of the Big Island of Hawaii.
The columnar fractures in this basalt flow develop on cooling. This
flow is in the bottom of the Grand Canyon, Arizona, USA, near Lava
Here is a classic columnar basalt texture at the Giant's Causeway,
County Antrim, Ireland.
Continental alkali basalts commonly bring nodules called xenoliths to
the surface. These green nodules are peridotite composed largely of the
These nodules are mined for gem peridote on the San Carlos Apache
Reservation in Arizona.
Andesites are more silicic than basalts which makes
the lavas much more
viscous, hydrous, and explosive. Andesite volcanoes are common near
subduction zones where hydrated oceanic plates plunge back into the
mantle. The wate released from the subducted plate causes melting of
the slab and overlying mantle. The resulting volcanoes are composite
cones made of viscous lava flows and volcaniclastic deposits.
This beautiful symmetric cone is Mt. Fuji in Japan.
More silicic volcanoes are more explosive. This is Crater Lake in
Oregon, USA and erupted a rhyolite to rhyo-dacite composition magma.
The mountain exploded and left this wide crater.
Some rhyolite volcano craters can be 40 km or more across. There are
three relatively young (<1 Million years) rhyolite volcanic centers
in USA, Long Valley, CA, Yellowstone, WY, and Valles, NM.
Here are some volcanic deposits typical of silicic centers. The light
colored material at the lower right is an air-fall pumice deposit made
of pebble-sized pieces of a glass foam called pumice.
The dense, black blocky material overlying the pumice is an obsidian
(glass) lava flow that was the last material erupted here. This deposit
is in the Valle Grande in the Jemez Mountains, NM, USA.
Mono Lake in California lies in part of the Long Valley crater
(caldera). These deposits are tufa calcite and gypsum deposited from
hot ground water entering the cold lake water. Lake level has been
dropping because much of the water draining east out of the Sierras is
taken to LA leaving these once-submerged columns out of the water.
Joe Smyth's Home