Extrusive rock

Hot liquid rock flows out of a volcano.

This liquid cools very fast in the air. Because it cools fast, it cannot grow big crystals. The rocks often look very smooth. Some rocks have tiny holes from gas bubbles.

These bubbles stay in the rock as it hardens. One light rock is called pumice. It is so light that it can float! Other rocks like basalt form from slow flows. These rocks make up the Earth's crust.

Extrusive rocks form on the Earth's surface. This happens when hot magma flows out of a volcano. We call this liquid lava.

Magma cools very fast in the air or seawater. Because it cools fast, crystals do not have time to grow. Most extrusive rocks have tiny crystals. We call this texture aphanitic. Sometimes, a few large crystals do grow. We call these phenocrysts.

Some rocks have holes from gas bubbles. These holes are called vesicles. Rocks with many holes are called vesicular. For example, pumice and scoria have this texture. Pumice is a light glass. It can even float on water!

Different volcanoes make different rocks. Shield volcanoes make basalt. This rock is made of minerals like feldspars. Cinder cones make scoria. Some volcanoes make lava domes. These domes can turn into obsidian. Obsidian is a natural glass.

Other volcanoes make andesite. Large areas called calderas can also form rocks. The Yellowstone Caldera can make rhyolite. This is a type of extrusive rock.

Extrusive rocks are special types of igneous rocks. These rocks form on the Earth's surface. This happens when hot magma flows out of a volcano. We call this liquid lava once it reaches the outside. Sometimes, the magma explodes into the air. It can fall back down as pieces called pyroclastics or tuff. This is different from intrusive rocks. Those rocks form when magma stays below the surface and cools slowly.

The way these rocks form depends on how fast they cool. When magma is in the open air, it cools very quickly. It can also cool fast under seawater. Because it cools so fast, crystals have very little time to grow. This creates a texture called aphanitic. This means the crystals are too small to see with your eyes. Sometimes, some crystals grow larger before the eruption. We call these large crystals phenocrysts. If a rock has both large and small crystals, it has a porphyritic texture.

Nature also makes rocks with many holes. These holes are called vesicles. They are shaped like bubbles. These bubbles come from gas or vapor trapped in the magma. When the magma cools, the bubbles stay behind. Some rocks like pumice or scoria have this vesicular texture. Pumice is a very light glass. It is made of silicic material. Because it is so light, pumice can actually float on water.

Different volcanoes create many different kinds of rocks. Shield volcanoes erupt fluid basaltic magma. This forms the extrusive rock known as basalt. Basalt contains minerals like feldspars and pyroxenes. Fissure volcanoes also pour out basalt from long vents. Other volcanoes, called composite or stratovolcanoes, make andesite. This magma contains many gases and melted mantle rocks. Cinder or scoria cones expel lava with high gas content. This creates the rock called scoria.

Some volcanic shapes create unique rocks too. Lava domes form when thick lava piles up. These can turn into obsidian, which is a natural glass. They can also form dacite. Mount St. Helens is a place where dacite domes form. Large depressions called calderas can also form. A resurgent caldera might refill with rhyolitic magma. The Yellowstone Caldera is a famous example. It can produce the extrusive rock called rhyolite.

Extrusive rocks are a specific category of igneous rocks formed on the Earth's surface. They result from the process of extrusion, where hot magma moves from inside the planet to the outside. Once magma reaches the surface, it is called lava. This material can flow out smoothly or explode violently into the atmosphere. In the case of explosions, the material falls back to the ground as pyroclastics or tuff. This process is the opposite of intrusive rock formation, where magma cools deep below the surface.

The primary factor determining the appearance of these rocks is the cooling rate. When magma is exposed to open air or seawater, it loses heat very rapidly. This rapid cooling leaves very little time for mineral crystals to grow. Most extrusive rocks have an aphanitic texture, meaning the crystals are too small to be seen by the human eye. However, some crystals may begin forming slowly in a magma reservoir before the eruption occurs. These larger, visible crystals are called phenocrysts. When a rock contains both these large phenocrysts and a fine-grained groundmass, it has a porphyritic texture.

Sometimes, the cooling process is so fast that a portion of the rock matrix does not crystallize at all. Instead, it becomes a natural glass, such as obsidian. Other extrusive rocks contain many small, bubble-shaped cavities called vesicles. These vesicles form when the magma contains abundant volatile components, which are released as free gas. As the lava cools, these gas bubbles become trapped inside the solidifying rock. Rocks like scoria and pumice are defined by this vesicular texture. While they look similar, pumice is a light-weight glass with a silicic composition that allows it to float.

Different volcanic structures produce specific types of extrusive rocks. Shield volcanoes are large, slow-forming structures that erupt fluid basaltic magma. This magma cools to form basalt, a rock composed of minerals like feldspars and pyroxenes. Fissure volcanoes also produce basalt by pouring low-viscosity magma from long vents. In contrast, composite volcanoes, also known as stratovolcanoes, often erupt andesitic magma. This magma is made of melted mantle rocks and contains many gases. These eruptions typically result in the formation of the extrusive rock known as andesite.

Other volcanic features create distinct rock types through different mechanical processes. Cinder or scoria cones expel lava that has a very high gas content. This process creates scoria, which is a mafic, vesicular rock. Lava domes form when high-viscosity lava piles up into a mound shape. These domes can solidify into obsidian or sometimes dacite, as seen at Mount St. Helens. Large volcanic depressions called calderas can form after a volcano collapses. A resurgent caldera, such as the Yellowstone Caldera, may refill with rhyolitic magma to produce rhyolite.

The variety of extrusive rocks is closely linked to the chemical makeup of the magma. For example, submarine volcanoes erupting on the ocean floor produce pumice. The specific weight contents of silica and alkali elements determine where a rock falls on a classification scale. This relationship between chemistry and texture helps scientists understand the history of volcanic activity. By studying whether a rock is aphanitic, porphyritic, or vesicular, researchers can determine how fast the lava cooled and how much gas was present during its formation.