The type of Igneous rock that develops from lava is determined by the composition of the lava as it comes molten from the volcanic vent. The grain size depends on the speed at which the lava cools and to some extent, the composition of the lava. The finest grained lavas with few, if any, bubbles, are actually glassy and produce obsidian. If the lava is sticky and has more air bubbles, like the lava picture above, the resultant rocks are classified as pumice. A more fluid lava will produce scoria, which is primarily the composition of the cinders I talk about in previous posts.
As the silica (quartz) content of the lava increases the lava becomes a felsite. Felsite is fine grained but not glassy like obsidian, and may or may not have phenocrysts (large mineral grains). It is high in silica or felsic, and typically consists of the quartz, (plagioclase feldspar and alkali feldspar). Felsite is considered the extrusive equivalent of granite.
With the changes in the composition of the lava, depending on mineral content, the lava could end up a medium silica Andesite or a low silica basalt. Coarse grained rocks from lava can include porphyries, granites, syenites, diorites, gabbros, peridotites, pyroxenites, dunites and pegmatites. Some of the differences between these rock types is based entirely on grain size and not on any variations in mineral content.
The variety of rocks, is really fascinating.
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