Volcanic rock
A volcanic rock (also volcanic rock, volcanic rock, igneous rock, effusive or Extrusivgestein ) is a rock that is formed due to volcanic activity by rapid cooling of molten rock at the surface. Volcanics are present as lava flows or as pyroclastic and pyroclastic sediments. They form together with the plutonic rocks ( plutonic rocks ) consisting of slow -cooling in deeper areas of the crust melt ( magma ) occur, the group of igneous rocks ( igneous rocks ). Occasionally, rocks that have solidified in the transition zone of volcanic and plutonic rocks, referred to as Subvulkanite.
Structure
Igneous rocks are in contrast to the plutonic rocks often very fine-grained or even glassy solidified, resulting in a more even coloring. This is due to the rapid cooling, due to the insufficient time remains to the growth of large crystals. However, if crystals formed carried and included in the solidification of the melt as inclusions in the fine groundmass in the magma chamber, one speaks of a rock with porphyritic texture, which is characteristic of many volcanic rocks. As lava solidified igneous rocks many blowholes formed by volcanic gases can contain. While the outflow of lava flow can occur parallel to the flow direction to form a so-called flow texture or a trachytic structure by Einregelung of tabular or columnar crystals. On pyroclastic volcanic rocks distributed way ( tephra, about tuff, pumice ) are deposited in layers often now and so form a transition to the sediments and sedimentary rocks.
Composition
The mineral composition of volcanic rocks is very diverse and reflects a series of processes of Magmenentstehung and the cooling history of the magma ( fractional crystallization ) resist. Common minerals include, for example, quartz, feldspar, Foide, pyroxenes, olivine, amphiboles, magnetite and other oxides. Often, volcanic rocks also contain inclusions of country rock ( xenoliths ), which is plunged into the magma chamber or was swept away during the Magmenaufstieges of the chimney walls. A typical example is about to be found in the Basalttuffen the Eifel Olivinbomben here.
Classification and examples
Depending on the type of volcanic rock several classification methods can be applied. It is a widespread classification using the Streckeisen diagram from their mineral holdings. Fine-grained or glassy volcanic rocks are classified in the TAS diagram according to their chemical composition. Other classification schemes exist, for example, ultramafic or alkali-rich volcanic rocks and pyroclastic sediments for.
Some examples of volcanic rocks are:
- Andesite
- Basalt
- Dacite
- Foidit
- Ignimbrite
- Latit
- Melilithit
- Phonolite
- Pikrit
- Rhyolite
- Tephrite
- Tuffit
- Trachyte
- Obsidian
Occurrence
Volcanic rocks were formed in all periods and arise today in areas of active volcanic activity. Volcanics occur on all continents, especially in areas of recent or former continental plate margins, rifts and Mantle Plumes. The volumetrically most important group are the basalts of the ocean floor, which are formed at mid-ocean ridges ( MORB ). Occurrence of volcanic rocks are known not only on Earth but also on Earth-like planets and their moons. The Mary of the Moon are predominantly basaltic lava flows.