Volcanic Rocks

Here are some images of volcanic rocks that I have taken over several decades of interest in these phenomena. The iamges are copyrighted, but publishable high resolution image files are available. 

I will be adding to these collections continually.  

Hawaiian Basalts
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.
.Hawaiian lava

This is a night view of a lava flow above Kalapana in 1989.

Toasting marshmallows on the pahoehoe.

Here we are using a sensitive temperature probe to test the lava temperature. The probe here isa marshmallow and, yup, the lava is hot.

  Aa flow

Here an aa flow front is advancing on an older pahoehoe flow and the geologist is sampling the lava.

Blacksand beach

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 periods.
This beach is on the east coast of the Big Island of Hawaii.

Continental Basalts

Columnar basalt

The columnar fractures in this basalt flow develop on cooling. This flow is in the bottom of the Grand Canyon, Arizona, USA, near Lava Falls Rapid.

Columnar Basalts Giant's Causeway

Here is a classic columnar basalt texture at the Giant's Causeway, County Antrim, Ireland.

San Carlos Olivine Nodules

Continental alkali basalts commonly bring nodules called xenoliths to the surface. These green nodules are peridotite composed largely of the mineral olivine.
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.

Mt Fuji, Japan

This beautiful symmetric cone is Mt. Fuji in Japan. 

Crater Lake, OR

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.

Banco Bonito

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 Tufa Columns

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.

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