Igneous rock

Igneous rock is one of the three main rock types, along with sedimentary and metamorphic rocks. It forms when magma or lava cools and hardens. Magma comes from melted parts of rocks deep inside planets, usually because of higher temperatures, lower pressure, or changes in rock makeup. When this melted material cools, it can form rocks either underground as intrusive rocks or on the surface as extrusive rocks. Igneous rocks can be made of crystals or, if they cool very quickly, they can form natural glasses. These rocks are found in many places on Earth, such as shields, platforms, mountain ranges, and ocean floors.

igneous igneus rock types sedimentary metamorphic magma lava partial melts terrestrial planet mantle crust pressure intrusive rocks extrusive crystallization natural glasses

Geological significance

Igneous and metamorphic rocks make up most of the Earth's outer layer. Igneous rocks cover about 15% of the land we see today and form the bottom of the oceans.

These rocks are important because they tell us about the deep parts of the Earth where their liquid material came from. We can also find out how old they are and use this to understand the history of our planet. They even help us find useful minerals like tungsten, tin, uranium, chromium, and platinum.

Main article: Plate tectonics

Geological setting

Igneous rocks can be intrusive or extrusive.

Intrusive

Main article: Intrusive rock

Intrusive igneous rocks form when magma cools and hardens inside the Earth’s crust. These rocks are usually coarse-grained because they cool slowly. Common intrusive rocks include granite, gabbro, and diorite. Large areas of mountains often have intrusive rocks in their cores.

Intrusive rocks that form deep inside the crust are called plutonic rocks. Those that form closer to the surface are called subvolcanic or hypabyssal rocks and are usually finer-grained.

Extrusive

Main article: Extrusive rock

Extrusive igneous rocks, also called volcanic rocks, form when magma cools on the Earth’s surface. This happens during volcanic eruptions. These rocks are usually fine-grained or even glassy because they cool quickly. Basalt is the most common type of extrusive rock and forms lava flows.

When magma reaches the surface, it is called lava. Volcanoes can erupt above water or on land. The way lava behaves depends on its thickness, which is affected by its temperature, makeup, and crystal content. Some lava flows are smooth, while others can be very thick and erupt explosively.

Classification

Igneous rocks are grouped based on how they form, their texture, mineral makeup, and chemical makeup.

The way igneous rocks are classified helps scientists understand the conditions when they formed. Two key factors in classifying igneous rocks are particle size, which depends on how fast they cooled, and the minerals they contain. Important minerals in igneous rocks include feldspars, quartz, olivines, pyroxenes, amphiboles, and micas. Rocks with mostly quartz, plagioclase, alkali feldspar, and muscovite are called felsic, while those with biotite, hornblende, pyroxene, and olivine are called mafic. Felsic rocks are usually light-colored, and mafic rocks are darker.

Texture describes the size and shape of the minerals in the rock. Rocks with large crystals visible to the naked eye are called phaneritic, while those with very small crystals are called aphanitic. A rock with both large and small crystals is called porphyry.

Origin of magmas

The Earth's crust, the outer layer of our planet, is thicker under continents and thinner under the oceans. Rocks can melt due to changes in pressure, temperature, or the addition of water. These changes can happen when rocks move or when hot material rises from deeper inside the Earth.

When pressure decreases, rocks can melt. This is called decompression melting and happens when rocks rise from deep inside the Earth. It creates new ocean crust and causes volcanic activity in places like Europe, Africa, and the Pacific Ocean floor. Water also plays a big role in melting rocks. When water is added, rocks melt at lower temperatures. This happens when ocean crust moves deep into the Earth and releases water, creating new magmas that form island chains like those in the Pacific Ring of Fire.

Main article: Igneous differentiation

As magma cools, different minerals form at different temperatures. This changes the makeup of the remaining liquid. For example, a magma that starts as one type of rock can change into a different type as it cools. This process helps create many different kinds of igneous rocks. Magmas can also mix with other rocks or with each other, leading to even more variety in the rocks they form.

Etymology

The word igneous means "composed of fire" and comes from Latin words meaning fire. The term volcanic rock comes from the name Vulcan, the Roman god of fire. The term plutonic rock comes from Pluto, the Roman god of the underworld.