Extrusive Rock

The igneous rocks that are formed by crystallization of the magma to the surface of the earth are called extrusive rocks. These rocks are generally formed from a volcano, which are also called volcanic rocks. When the lava comes out to the surface of the earth, it cools quickly. When the lava cools in less than a day or two, there is no time for the elements to form minerals. , Items are well frozen in volcanic glass. Often, lava cools in a few days or weeks and minerals should be sufficient, but there is no time to grow large crystals.

Classification of igneous rocks / extrusive

Extrusive rock is an igneous rock, which can be further divided according to the circumstances of its formation.

Volcanic / extrusive rock

Extrusive rock or volcanic rock forms as a result of magma reaching the surface or in the form of lava or fragmentary ejection, so that rocks, such as pumice, basalt are formed. In other words, extrusive rocks are gradually crystallizing by time and therefore the crystals are very small or microscopic.

Plutonic / intrusive rock

Plutonic or intrusive rocks are formed when the magma cools in the crust and crystallizes. A common example is granite. In short, intrusive is formed underneath the ground.

Types of extrusive rocks

Komatite / ultramafic extrusive rocks

These are some of the less viscous lavas because of their low silica content, which is almost entirely composed of rectangular ferromagnetic minerals and has no feldspar or quartz. An ultramafic komatite rock is a typical extrusivive rock, which is mainly olivine and pyroxene and with less feldspar. They contain less than 45 percent silica and are believed to come from the mantle.


Is the main rock cider volcanoes cone. It does not have many uses, and his name is derived from a similar term means ‘waste’. The slag is dark red to black. It is less viscous than pumice, but as lava that is rich in gas. It is harder than pumice and does not float in the water.

Basalt / mafic / felsic rocks

Felsic are rich in silica, potassium, sodium and aluminum and contain only small amounts of iron, magnesium and calcium. Felsic magmas are more malignant due to their high content of silica. Typical felsic are dacite and rhyolite and about 50 percent silica and large amounts of iron, magnesium and calcium and are dark in color.


Dacite is an extrusive, enameled rock that is rich in iron and was first discovered in Dacia, a provision of the Roman Empire, where it gets its name. It’s colorful. It has a composition that is between those of andesite and rhyolite – it has a little less potassium feldspar and quartz and some forensic ferromagnetic minerals such as rhyolite. Dacite is generally light grayish green and often difficult to distinguish in rhyolite field.


Rhyolite crystals are very small, making it difficult, if not impossible to see. Rhyolite is often used in decoration and jewelry because of the integrated band of colors. Rhyolite is white to gray in color and is a very common rock. It is similar in composition to granite, although it is extrusive and while granite is intrusive.

Many rhyolites are formed from a granite magma, which has partially cooled down in the basement. When these magmas break out a rock with two grain sizes can be formed. Large crystals forming below the surface are called phenocrystals, and small crystals are formed in the surface called ground mass. Rhyolite consists of quartz, plagioclase and sanidine, with small amounts of amphibole and biotite. Trapped gas often occurs vacuoles in the rock. These often contain crystals, opal or glass material. The high gas content and high viscosity of these melts are perfect to produce an explosive eruption. The viscosity can be so high that the gas can escape the explosion of magma venting. Granitic magmas rich in silicic acid and often up to a gas weight percentage. Since these magmas cooled down, silica begins to connect in complex molecules. This gives the high viscosity and magma moves very slowly.


It is an amorphous material that is known as mineraloid. The result is a volcanic glass, with a smooth and uniform texture, breaking with conchoidalen bridge rupture. It is also known that volcanic glass is mainly formed of silica when the magma comes into contact with water. It varies in color of dark green and purple. Obsidian can also detect/capture air bubbles during the formation, the color effects produced from the strip spot. It can be formed in a variety of cooling environments: –

  • Along the edges of a washer this is extrusive
  • On the edge of a volcanic dome this is extrusive
  • Around the edges of a threshold or dam this is intrusive
  • When washing with water this is extrusive
  • It cools the lava while in the air this is extrusive


Pumice is light to gray and dark in color and forms from lava, which is rich in gas or air. When the lava forms a foamy texture, creates pumice. Pumice is so light and airy that many samples float in the water. The rough texture of pumice makes it ideal in the beauty industry to scrub dead and dry skin. It is used as an aggregate of lightweight concrete in landscaping and as an abrasive in a variety of industrial and consumer products. Many specimens have such porosity


It is an intermediate in the composition between lava rock rhyolite and basalt. Andesite is named after the Andes and is formed on continental borders of the ocean tectonic plates. It is composed of plagioclase, pyroxene, magnetite, quartz and sphene. It can be white, gray or white or gray. It can be used as aggregate, filler, etc., in industries, construction and labor, often ideal for concrete aggregates due to the high silica content.