Apr 24 2020


<a title="Volcanic" href="">Volcanic</a>-<a title="Volcanic" href="">Volcanic</a>
Volcanic rocks are also known as extrusive rocks. They're all formed out of magma, and they're all fascinating in their own way.

Volcanic and Extrusive Igneous Rocks

Igneous rocks — those which originate from magma — fall into two categories: extrusive and intrusive. Extrusive rocks erupt from volcanoes or seafloor fissures, or they freeze at shallow depths. This means that they cool relatively quickly and under low pressures. Therefore, they are typically fine-grained and gassy. The other category is intrusive rocks, which solidify slowly at depth and do not release gases.

Some of these rocks are clastic, meaning they are composed of rock and mineral fragments rather than solidified melt. Technically, that makes them sedimentary rocks. However, these volcaniclastic rocks have many differences from other sedimentary rocks — in their chemistry and the role of heat, especially. Geologists tend to lump them with the igneous rocks.

Massive Basalt

James St. John/Flickr/CC BY 2.0

This basalt from a former lava flow is fine-grained (aphanitic) and massive (without layers or structure).

Vesiculated Basalt

Jstuby at en.wikipedia/Wikimedia Commons/Public Domain

This basalt cobble has gas bubbles (vesicles) and large grains (phenocrysts) of olivine that formed early in the lava’s history.

Pahoehoe Lava

J.D. Griggs/Wikimedia Commons/Public Domain

Pahoehoe is a texture found in highly fluid, gas-charged lava due to the deformation of flow. Pahoehoe is typical in basaltic lava, low in silica.

James St. John/Flickr/CC BY 2.0

Andesite is more siliceous and less fluid than basalt. The large, light phenocrysts are potassium feldspar. Andesite can also be red.

Andesite from La Soufrière

James St. John/Flickr/CC BY 2.0

La Soufrière volcano, on St. Vincent island in the Caribbean, erupts porphyritic andesite lava with phenocrysts largely of plagioclase feldspar.

James St. John/Flickr/CC BY 2.0

Rhyolite is a high-silica rock, the extrusive counterpart of granite. It is typically banded and, unlike this specimen, full of large crystals (phenocrysts). Red volcanic rocks are usually altered from their original black by superheated steam.

Rhyolite with Quartz Phenocrysts

Rhyolite displays flow banding and large grains of quartz in the almost-glassy groundmass. Rhyolite can also be black, gray, or red.

volcanic glass, high in silica and so viscous that crystals do not form as it cools.

Obsidian or rhyolite flows that are rich in water often produce perlite, a lightweight, hydrated lava glass.

Ashley Dace/Wikimedia Commons/CC BY 2.0

Peperite is a rock formed where magma meets water-saturated sediments at relatively shallow depths, such as in a maar (a broad, shallow volcanic crater). The lava tends to shatter, producing a breccia, and the sediment is vigorously disrupted.

“Jonathan Zander (Digon3)”/Wikimedia Commons/CC BY 3.0

This bit of basaltic lava was puffed up by escaping gases to create scoria.


J.D. Griggs, USGS/Wikimedia Commons/Public Domain

The ultimate form of scoria, in which all the gas bubbles have burst and only a fine mesh of lava threads remains, is called reticulite (or thread-lace scoria).

Norbert Nagel, Mörfelden-Walldorf, Germany/Wikimedia Commons/CC BY 3.0

Pumice is also a gas-charged, lightweight volcanic rock like scoria, but it is lighter in color and higher in silica. Pumice comes from continental volcanic centers. Crushing this feather-light rock releases a sulfuric smell.

Ashfall Tuff

James St. John/Flickr/CC BY 2.0

Fine-grained volcanic ash fell upon the Napa Valley several million years ago, later hardening into this lightweight rock. Such ash is usually high in silica. Tuff forms from erupted ash. Tuff often has chunks of older rock, as well as freshly-erupted material.

Tuff Detail

This lapilli tuff includes reddish grains of old scoria, fragments of country rock, stretched grains of fresh gassy lava, and fine ash.

Tuff in Outcrop

Roy A. Bailey/Wikimedia Commons/Public Domain

Tierra blanca tuff underlies the metropolitan region of El Salvador’s capital, San Salvador. Tuff is formed by the accumulation of volcanic ash.

Tuff is a sedimentary rock formed by volcanic activity. It tends to form when erupting lavas are stiff and high in silica, which holds the volcanic gases in bubbles rather than letting them escape. The lava tends to fragment and explode into tiny pieces. After the ash falls, it may be reworked by rainfall and streams. That accounts for the crossbedding near the top of the lower part of the roadcut.

If tuff beds are thick enough, they can consolidate into a fairly strong, lightweight rock. In parts of San Salvador, the tierra blanca is thicker than 50 meters. A lot of old Italian stonework is made of tuff. In other places, the tuff must be carefully compacted before buildings can be constructed on it. Salvadoreans have learned this through centuries of rueful experience with major earthquakes. Residential and suburban buildings that short-change this step remain prone to landslides and washouts, whether from heavy rainfall or from earthquakes, like that which struck the area in 2001.




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