Igneous rock

Hot melted rock makes new stone.

This rock can form in two ways. It can cool deep underground. This makes rocks with big grains.

It can also come out of a volcano. This is called lava.

Lava cools very fast on the surface. This makes rocks with tiny grains.

Some rocks look like smooth glass. Other rocks form long, tall columns.

These rocks make up much of the Earth. They are very important to our world.

Igneous rocks are made from hot, melted rock.

This melted rock is called magma. It forms deep inside the Earth. Magma rises because it is less dense than the rock around it. When magma reaches the surface, we call it lava.

There are two main ways these rocks form. The first way is called intrusive. This happens when magma cools slowly underground. Because it cools slowly, it forms large grains. You can often see these grains with your eyes.

Granite is a common example.

The second way is called extrusive. This happens when lava cools quickly on the surface. This fast cooling makes very tiny grains. Some rocks cool so fast they become natural glass. Basalt is a common extrusive rock. Some basalt even forms long, tall columns.

Scientists study these rocks to learn about Earth. They use them to find metals like tin or gold. They also use them to find out how old the Earth is.

Igneous rocks are one of the three main types of rocks on Earth.

They form when hot, melted rock cools down and becomes solid. This type of rock is very important to our planet. In fact, igneous and metamorphic rocks make up 90% to 95% of the Earth's crust by volume. Most of the oceanic crust is made of these rocks. They also make up about 15% of the Earth's current land surface.

There are two main ways these rocks form. The first way is called intrusive formation. This happens when magma cools slowly deep inside the Earth's crust. Because the surrounding rock acts like an insulator, the magma stays warm for a long time. This slow cooling allows large mineral grains to grow. These rocks are called phaneritic, which means you can see the grains with your eyes.

Common examples include granite, gabbro, and diorite.

The second way is called extrusive formation. This happens when magma reaches the surface and is called lava. When lava is released by volcanoes, it cools very quickly. This fast cooling creates fine-grained rocks called aphanitic. Some lava cools so fast it turns into natural glass. Basalt is the most common extrusive rock. Sometimes, basalt cools into long, tall columns.

You can see these at the Giant's Causeway in Northern Ireland.

Scientists use these rocks to learn many secrets about our world. By looking at the minerals, they can learn about the temperature and pressure deep underground. They can even find out how old the rocks are using radiometric dating. This helps them build a timeline for the Earth. Igneous rocks also hold important metal deposits. For example, granite and diorite can hold tungsten, tin, and uranium.

Other rocks like gabbro can hold chromium and platinum.

You can think of igneous rocks as the building blocks of the Earth's skin. Just as ice forms from water, these rocks form from heat. The way they cool changes how they look. Slow cooling makes big, chunky crystals. Fast cooling makes tiny, smooth textures.

Whether they are light-colored felsic rocks or dark mafic rocks, they tell a story of fire and cooling.

Igneous rocks, also called magmatic rocks, are one of the three primary rock types on Earth.

They form through the cooling and solidification of molten material. This material starts as magma, which is molten rock found beneath the surface. When this magma reaches the surface, it is called lava. Igneous rocks are essential to understanding our planet. They and metamorphic rocks make up 90% to 95% of the Earth's crust by volume. Most of the oceanic crust is composed of igneous rock. Furthermore, these rocks cover about 15% of the current land surface.

The process begins when existing rocks in the mantle or crust undergo partial melting. This melting happens due to three specific causes: an increase in temperature, a decrease in pressure, or a change in chemical composition. Once the rock melts, the resulting magma rises because it is less dense than the surrounding solid rock.

As the magma loses heat, it begins to solidify into a solid state. If this happens deep underground, it creates intrusive rocks. If it happens on the surface, it creates extrusive rocks. The speed of this cooling process determines the final texture of the rock.

Intrusive igneous rocks form when magma cools slowly within the Earth's crust. These bodies of rock are called intrusions and are surrounded by pre-existing country rock. The country rock acts as a thermal insulator, which keeps the magma warm for a long time. This slow cooling allows large mineral crystals to grow.

Scientists call this coarse-grained texture phaneritic, meaning the crystals are large enough to see with the naked eye. Common examples include granite, gabbro, and diorite. Some massive intrusive bodies, known as batholiths, can occupy huge areas of the surface when exposed by erosion.

Extrusive igneous rocks, or volcanic rocks, form from lava that cools rapidly on the surface. This rapid cooling prevents large crystals from growing. The resulting texture is called aphanitic, which means the grains are too fine to see without help. In some cases, the lava cools so quickly that it forms natural glass without any crystals at all. Basalt is the most common extrusive rock and often forms large lava plateaus. Some basalt flows even solidify into long, polygonal columns, like those found at the Giant's Causeway.

Geologists classify these rocks using several different methods. One way is by texture, which refers to the size and arrangement of the crystals. A special texture called porphyritic occurs when magma undergoes two distinct cooling phases. This results in large crystals, called phenocrysts, embedded in a much finer mass called groundmass.

Another method is chemical or mineralogical classification. Rocks are often grouped as felsic or mafic. Felsic rocks are light-colored and rich in minerals like quartz and feldspar. Mafic rocks are darker and contain more iron and magnesium.

These rocks are incredibly important for scientific discovery. By studying their minerals, scientists can learn about the temperature and pressure of the deep mantle.

Using radiometric dating, researchers can find the absolute age of the rocks. This helps calibrate the entire geological time scale. Additionally, the features of igneous rocks help scientists reconstruct the history of plate tectonics. They also serve as important sources of valuable minerals. For instance, granites often host tungsten, tin, and uranium, while gabbros can contain chromium and platinum.

The behavior of the lava also depends on its viscosity, which is its resistance to flowing. High-temperature basaltic magma flows easily, like thick oil. In contrast, felsic magma, such as rhyolite, is extremely viscous and can be 10,000 times thicker than basalt.

This high viscosity often leads to violent, explosive eruptions. These explosions are driven by the release of dissolved gases like water vapor and carbon dioxide. Such events produce tephra, which includes materials like volcanic ash and tuff. Understanding these processes helps us map the complex geological history of our world.