Volcanic rock

Volcanic rocks come from a volcano.

Hot, melted rock flows out. This is called lava. When lava cools, it turns hard.

Some lava is smooth. Other lava is rough and jagged.

Sometimes, a volcano blows up. It shoots pieces into the air. These pieces are called tephra. Some pieces are large bombs. Other pieces are tiny bits of ash.

Some rocks have many small holes. This happens because of gas bubbles. One such rock is called pumice. It is very light. It can even float! These rocks are everywhere on Earth.

Volcanic rocks form from lava. Lava is melted rock that erupts from a volcano.

When this lava cools, it turns into solid rock. Some lava flows are smooth and ropy. We call this pahoehoe. Other flows are rough and jagged. This is called ʻaʻā.

Sometimes, volcanoes erupt with a big blast. They shoot pieces of rock into the air. This material is called tephra. Large, hot pieces are called volcanic bombs. Smaller bits are called lapilli. Very tiny pieces are called ash.

Many volcanic rocks have small holes. These holes are called vesicles. They form when gas bubbles get trapped in the cooling lava. One rock with many holes is pumice. Pumice is so light it can float on water.

Scientists name these rocks by their chemistry. For example, basalt has low silica. Rhyolite has high silica.

Some rocks also have large crystals. These are called phenocrysts. They sit inside a finer part of the rock.

Volcanic rocks are some of the most common rocks on our planet. They form when molten rock erupts from a volcano and cools down.

These rocks cover about 8% of the Earth's current land surface. They are very common in the oceans too. You can often find them near plate boundaries. They also appear in huge areas called flood basalt provinces. Even though we call them volcanic rocks, they sometimes blend into other types of rock.

There are two main ways these rocks form. First, liquid lava flows onto the surface and hardens. This creates rocks like basalt, andesite, or rhyolite. Some lava looks smooth and ropy, which is called pahoehoe. Other lava is rough and jagged, known as ʻaʻā.

Second, a volcano can erupt explosively. This shoots fragmented material into the air called tephra. Large, hot pieces are called volcanic bombs. Smaller bits are called lapilli, and tiny particles are called ash.

Scientists use chemistry to name and classify these rocks. They look at how much silica and alkali metals are inside. Silica is a very important part of the mix. For example, basalt has a low amount of silica. Rhyolite is different because it has a high amount of silica.

Some rocks are called intermediate, like andesite or dacite. This chemical makeup depends on the original magma and how it changes. As magma changes, it often gains more silica through a process called differentiation.

Volcanic rocks often have very interesting textures. Many have tiny holes called vesicles. These holes are left behind by gas bubbles trapped in the cooling lava.

One famous rock with many holes is pumice. Pumice is so light that it can actually float on water.

You might also see large crystals inside a fine-grained rock. These large crystals are called phenocrysts. They are easy to see without a microscope. Sometimes, magma even picks up crystals from other rocks. These special pieces are called xenocrysts.

Learning about these rocks helps us understand how the Earth works. Just like salt dissolving in water, lava behaves in a special way. It can drop large crystals first, then a cloud of smaller ones. This helps us predict which minerals will form.

When lava cools very fast, it can even turn into glass. This creates a dark, shiny rock called obsidian. By studying these rocks, we can see the history of past eruptions. Every piece of rock tells a story about heat and pressure.

Volcanic rocks are a major component of the Earth's crust. They form when molten rock erupts from a volcano and solidifies on the surface.

While we use the term "volcanic" to describe these rocks, they often grade into other types. They can transition into hypabyssal rocks or metamorphic rocks. In some geological areas, like Precambrian shields, scientists call them metavolcanic rocks. These rocks are incredibly common, especially in the oceans. On land, they cover approximately 8% of the current surface. They appear frequently at plate boundaries and in large areas known as flood basalt provinces.

There are two primary ways these rocks are created. The first method involves lava flows. When molten rock erupts and cools, it forms coherent rocks like basalt, andesite, or rhyolite. The texture of these flows can vary greatly. For example, pahoehoe lava is smooth and ropy. In contrast, ʻaʻā lava is rough and jagged.

The second method is through explosive eruptions. This process ejects fragmented material called tephra into the air. Tephra includes large volcanic bombs that are semi-molten when ejected. It also includes lapilli, which are fragments between 2 and 64 mm in diameter. Finally, there is volcanic ash, which consists of fine particles smaller than 2 mm.

Geologists classify these rocks based on their chemical composition. Modern petrologists rely on chemistry to understand a rock's origin. This is because different textures can grow from the same initial magma. Scientists use the Total Alkali Silicate (TAS) classification system. This system looks at the weight fraction of silica and alkali oxides, which include sodium and potassium.

Based on these levels, rocks are divided into subalkaline, alkaline, or peralkaline groups. Peralkaline rocks are specifically defined by having more alkali oxides than aluminum oxide. This chemical makeup is determined by the original magma and a process called differentiation. Differentiation usually increases the silica content through crystal fractionation.

Mineralogy also helps define these rocks. As volcanic rocks undergo differentiation, they become richer in silica-heavy minerals. These include feldspars, quartz, and muscovite. Primitive rocks, however, contain more minerals like olivine and pyroxene. The order in which these minerals form is predicted by Bowen's reaction series. Sometimes, a magma will pick up crystals from other rocks during its journey. These are called xenocrysts. A famous example is finding diamonds inside kimberlite rocks. The kimberlite does not create the diamond; it simply transports it to the surface.

Volcanic rocks possess unique physical textures. Many are aphanitic, meaning they have a very fine-grained texture. Some rocks are even glassy, like obsidian, which forms when rhyolite lava cools extremely quickly.

Another common feature is a vesicular texture. These are small holes or voids left by gas bubbles trapped in the lava.

If the bubbles are very numerous, the rock becomes highly vesicular. Pumice is a famous example of this. It is so light and airy that it can float on water.

You might also see phenocrysts, which are large crystals embedded in a finer matrix.

Differentiation also creates specific structural patterns in the rock. As lava moves, it may create a fluxion or fluidal structure. This appears as subparallel winding lines in the ground-mass. This happens because the minerals form while the lava is still creeping forward. The cooling process is similar to how salt behaves in a hot water solution. First, large, well-formed crystals deposit during a labile stage. Later, a cloud of smaller particles precipitates during a metastable stage. This explains why many volcanic rocks show large phenocrysts surrounded by a fine-grained matrix.

Understanding volcanic rocks connects us to the broader systems of our planet. They show how heat and chemical changes move material from the deep Earth to the surface. By studying the size of clasts in pyroclastic rocks, like tuff or ignimbrite, we learn about past eruptions.

We can trace the movement of plates and the history of the atmosphere. Every piece of volcanic rock, from a tiny ash grain to a massive basalt flow, provides a record of Earth's thermal history.