Lava composition and Volcanic rocks

In the previous section we described the basic types of volcanoes. Volcanic types are intricately connected with the types of mamga, lava and vocanic rock (also known as igneous rocks) composition which shape the volcano and the surrounding area. Minerals form in igneous rocks when molten magma or lava solidifies. The densest minerals, ferro-magnesian silicates, form at the highest temperatures, whereas less dense minerals form when the magma cools down. Mineral types forming in molten rock often grow unrestricted to a very large size, and can have a fine crystal form. At most basic, there are eight basic types of lava, which reflect the main types of volcanic rock which the lava is composed of. These types are:

Basalt - is a dark coloured extrusive rock characterised by grain so fine that it is difficult to see even with a magnifying glass. This is the most widespread of all igneous rocks. It is formed when lava cools rapidly. Basalt is composed primarily of calcium-rich plagioclase feldspar (gray), olivine and various pyroxenes (dark minerals). If basalt contains olivine it is often called an olivine basalt.

Andesite - is a fine-grained, gray to black extrusive igneous rock. It is an intermediate rock containing some minerals found in basalt and some common to rhyolite. Andesite can look very much like basalt to the unaided eye, but it is usually less dark and sometimes greenish in colour. Andesite is composed of 52 to 63%silica (SiO2). Crystals found in andesite are mainly plagioclase feldspar, pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende.

Dacite - is a quartz-rich extrusive rock which is a minor constituent of many arc volcanoes. Dacite lava consists of about 63 to 68% silica (SiO2). Dacite's common minerals include plagioclase feldspar, pyroxene, and amphibole.

Rhyolite - is silica (SiO2) rich (more then 68%), light-coloured rock. It also contains a high percentage of sodium and potassium oxides, which can reach up to 5%. Quartz, feldspar and biotite are the most common minerals found in rhyolite. In some cases, thick rhyolitic lavas will cool as glassy obsidian flows.

In addition to these four basic types of lava, there are also rare unusual lavas such as those described below:

Carbonatite - lava is defined by having more than 50% carbonate (CO3-bearing) minerals, which are typically composed of less than 10% SiO2. Carbonatite lavas generally have low eruption temperatures, between 500 and 600oC. (for comparison, basalt typically forms at a temperature of more than 1100oC). There are only 330 known carbonatite localities on Earth, mostly occurring in association with larger intrusions of alkali-rich silicate igneous rocks.

Natrocarbonatites - are carbonatites which are enriched in alkalies (Na and K).

Komatite - is named from its type locality along the Komati River in South Africa. It has a very low silica content - 40-45%, and very high MgO content (~18%).

Another way to classify volcanic rocks is by the proportion of light and dark minerals. Using this classification igneous rocks can be subdivided into four main types: felsic, intermediate, mafic and ultramafic. These types are defined in more detail in the table below.


Rock type Silica content Colour Examples
Felsic > 65% light rhyolite, granite
Intermediate 55-65% Intermediate Andesite, dacite, diorite, granodiorite
Mafic 45-55% dark basalt, gabbro
Ultramafic < 45% Dark Peridotite

Finally, volcanic rocks are also classified by the size, abundance, and type of crystals. If no crystals are visible, geologists call the rock aphanitic. If crystals are abundant, the rock is called porphyritic.

Something extra of interest:

The oldest volcanic rocks date back almost four billion years. They were discovered in Porpoise Cove on the shores of Hudson Bay in northern Quebec by chance, as part of a mapping project carried out under the direction of Martin Parent from the Ministry of Natural Resources of Quebec.


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