Extrusive Rocks: Impressive Facts On Types Of Igneous Rocks Explained
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Extrusive rocks are a type of igneous rocks.
Igneous rocks are formed after a period of cooling and solidification of magma. Extrusive igneous rocks form when magma erupts from the volcano and cools down above the Earth's surface.
Magma forms deep inside the Earth's surface near hot spots. Since Igneous rocks are formed from a volcanic eruption, they are also called volcanic rocks.
When the magma erupts from the volcano, it is known as lava. The lava undergoes rapid cooling when it is in contact with air. Extrusive igneous rocks have tiny crystals because there is no time for large crystals to form when the lava cools down rapidly.
Some extrusive rocks may have a large crystal set surrounded by tiny crystals. This different appearance happens when the magma cools enough to form a few large crystals before the eruption. Once the volcano erupts, the rest of the magma cools quickly. This type of texture is known as a porphyritic texture.
The textures of these rocks depend on the gas content and cooling rate. Quenched lava with many holes from gas bubbles creates a vesicular texture like pumice stone. When lava cools quickly, they have a glassy texture like in obsidian.
Types Of Rocks With Examples
Based on the process of their formation, rocks are divided into three major rock types. These are as follows:
Igneous Rock: Igneous rocks are formed from the solidification of magma. Magma is a mixture of molten, rock-forming minerals, gases, and steam. Igneous rocks form at very high temperatures since they are made from molten materials.
They are formed at depths of 31-124.2 mi (50-200 km) inside the Earth. These rocks are identified and divided into two categories; intrusive and extrusive rocks. Some common examples of these rocks are basalt, pumice, granite, and gabbro.
Sedimentary Rock: Sedimentary rocks are formed by sediment depositing, compacting, and cementing together on Earth's surface. They are formed with the help of wind, water, ice, and living beings.
They are mainly deposited from the surface of the Earth to the bottom of water bodies like oceans, rivers, and lakes. Sedimentary rocks have layers, which are different in particle size, color, internal arrangement, and type of cement. Shale, limestone, and sandstone are a few examples of sedimentary rocks.
Metamorphic Rock: Metamorphic rocks are created from pre-existing rocks. The pre-existing rock may be igneous, sedimentary, or existing metamorphic rock. High temperatures, pressure, and chemicals influence the change. These changes may be compositional or textural.
Processes deep within the planet's surface create metamorphic rocks. These processes create a new texture, minerals, and internal structure. During the process, the rock remains in solid form. Quartzite, gneiss, marble, phyllite, and schist are some common metamorphic rocks.
Ingenuous Rock And Their Types
Igneous rocks are created when magma is cooled down and then crystallized. The magma may cool either outside the surface of the Earth or inside the Earth's crust.
Magma, which is molten rock, is developed in the lower crust or the upper mantle because of the extreme heat inside. Igneous rock is very significant for geology. The minerals and their chemistry provide information about the composition of the lower crust, upper crust, and the temperature and pressure inside volcanos.
Igneous rock help by allowing calibration of the geological time scale because the absolute dating of these rocks can be obtained by radiometric dating and compared with adjacent geology strata. The rock features are characteristic of a specific tectonic environment and help in tectonic reconstructions. The ores inside these rocks are important.
Granites and diorites contain tungsten, tin, and uranium. Gabbros contain platinum and chromium.
Igneous rock differs in composition based on the magma and its cooling condition. Igneous rocks can be divided into two main categories; extrusive and intrusive igneous rocks.
During a volcanic eruption, magma comes out of the Earth's surface and cools down on the surface of the Earth. The rocks formed by the solidification of lava are called extrusive rocks.
The crystals inside volcanic rocks are very small because they don't get much time to form before the rock cools down, which stops the crystal formation. These small-grained extrusive igneous rocks are called aphanitic, which is derived from the Greek word 'aphanēs' meaning invisible.
Some rocks are formed so quickly that they don't have individual crystals and look glassy like obsidian. Examples of extrusive igneous rocks include dacite, rhyolite, scoria, andesite, and pumice.
Intrusive igneous rock forms inside the Earth's surface by cooling and solidifying magma. Intrusive igneous rock is also known as plutonic rock. They cool slowly inside the planet's crust without ever reaching the surface. Intrusive igneous rocks have easily visible mineral grains.
This is known as phaneritic texture. Examples of phaneritic rocks are granite and pegmatite, which have a huge variety of crystal shapes and sizes and are commonly found in the US state, Maine. Examples of intrusive rock include granite, diabase, diorite, peridotite, pegmatite, and gabbro.
To differentiate between an intrusive and extrusive igneous rock, we must look at the rock formation, cooling time, and crystal size and appearance. Extrusive rocks form on the surface of the Earth, but intrusive rocks are formed underground.
Extrusive rocks cool quickly, while intrusive rocks take a lot longer.
Extrusive igneous rocks have very small crystals, and intrusive igneous rocks are coarse-grained and have large mineral crystals. For example, granite is an intrusive rock and hence has coarse grains while obsidian, is an extrusive rock with a glassy appearance and almost no mineral grains.
Extrusive Rocks Features
Basalt, the most common type of extrusive rock, is formed by shield volcanoes which are large, slow forming volcanoes. It is composed of minerals like feldspar and pyroxenes, readily available in the Earth's crust.
Andesite is an extrusive igneous rock formed by composite or stratovolcanoes. It is composed of many gases and mantle rocks that are melted.
Basalt scoria is an extrusive rock formed when cinder or scoria cones violently throw out lava high in gas content. The vapor bubbles in the lava cause scoria to be vesicular.
Obsidian is created by high viscosity lava that piles up and forms a dome shape. This rock is rich in silica. Dacite is also formed in a similar fashion. It is found in Mt. St. Helens.
Refilled calderas make rhyolite. Calderas are depressions created after a volcano erupts and then collapses.
Pumice, an extrusive rock used for skincare purposes, is created when a submarine volcano erupts on the ocean floor. It is lightweight and has a vesicular form. Pumice can float on water.
Extrusive Rocks Uses And Textures
Extrusive rocks have fine-grained crystals which the human eye cannot see. These are known as aphanitic rocks.
The crystals are so tiny because they form very quickly after an eruption. The larger crystals visible to the human eye are formed by slowly cooling inside the volcano. This large crystal is known as phenocryst.
Extrusive rocks with two different grain sizes are said to be porphyritic, and the smaller crystals are called groundmass. Some extrusive rocks like pumice and scoria have bubble-like vesicular texture because air gets trapped in the lava.
Extrusive rocks have many uses in everyday life. For instance, basalt is an extrusive rock used for construction, making statues, and cobblestones.
Pumice is used in construction, was used in early medicine, and is used in the beauty industry.
Obsidian is a rock with volcanic glass used to make sharp knives and, for ornamental reasons, as a gemstone.
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