Lava Flows/Igneous Rocks/Pyroclastic Material

Lava Flows

When a volcano erupts, molten rock (lava) flows across Earth’s surface. Not all lava flows look the same—scientists recognize three main types: pahoehoe, aa, and pillow lava.

Pahoehoe Lava Flow

Pahoehoe lava has a smooth, rope-like surface. It forms when very hot, fluid lava flows slowly. Because it stays runny, the surface wrinkles into smooth, wavy patterns as it cools.

Aa Lava Flow

Aa lava is rough, jagged, and sharp. It forms when lava is cooler and thicker, so it breaks into chunks as it moves. Cooled Aa flows are hard to walk across because of their sharp, broken rock.

Pillow Lava Flow

Pillow lava forms under water, such as at mid-ocean ridges. When hot lava erupts into cold seawater, it cools instantly, creating rounded, pillow-shaped formations stacked together.

These three types of lava flows show how differences in temperature, viscosity, and environment affect the way lava moves and solidifies.

Lava Tubes

Lava tubes are natural tunnels formed by flowing lava during a volcanic eruption. They act like underground rivers that carry molten rock away from the eruption site.

Lava tubes form when the surface of a lava flow cools and hardens into a solid crust, while hot, liquid lava continues to move underneath. The crust insulates the flowing lava, keeping it hot and allowing it to travel long distances. When the eruption ends and the remaining lava drains out, it leaves behind a hollow tunnel.

Some lava tubes can be very large—big enough for people to walk through. Famous examples can be found in Hawaii and Iceland.

Igneous Rocks

Igneous rocks are formed from magma or lava that cools and solidifies. The word “igneous” comes from the Latin word ignis, which means fire, because these rocks originate from molten material inside the Earth.

There are two main types of igneous rocks:

Intrusive (or plutonic) rocks form inside the Earth when magma cools slowly. Slow cooling allows large crystals to grow in the rock.
Extrusive (or volcanic) rocks form on the Earth’s surface when lava cools quickly. Rapid cooling produces small crystals or no crystals at all within the rock.

Igneous Rock Texture

The texture of an igneous rock describes the size, shape, and arrangement of the crystals (or lack of crystals) in the rock. Texture tells us how the rock formed—specifically, how fast the magma or lava cooled. There are 4 main igneous rock textures:

Coarse-grained

Forms when magma cools slowly underground.

Crystals have time to grow large, so they are easy to see with the naked eye.

Example: Granite

2. Fine-grained

Forms when lava cools quickly at the surface.

Crystals are tiny and hard to see, giving the rock a smooth appearance.

Example: Basalt

3. Glassy

Forms when lava cools almost instantly, so no crystals have time to form.

The rock looks shiny and smooth, like glass.

Example: Obsidian

4. Frothy (or vesicular)

Forms when lava traps gas bubbles as it cools.

The rock has a spongy, porous texture full of holes.

Example: Pumice

Igneous Rock Mineral Composition

Igneous rocks are formed when magma or lava cools and solidifies. Their appearance—whether light or dark colored—depends on the minerals they contain. Minerals are the building blocks of rocks, and different minerals give igneous rocks their unique colors.

Light-colored igneous rocks (like granite and rhyolite) are called felsic rocks. They are rich in minerals such as quartz and feldspar, which contain more silica (SiO₂). These minerals are usually lighter shades of white, pink, or gray. Because of their high silica content, felsic rocks are often less dense.

Dark-colored igneous rocks (like basalt and gabbro) are called mafic rocks. They contain more magnesium and iron-rich minerals such as pyroxene and olivine. These minerals give the rocks darker colors—black, green, or dark gray—and make them denser than felsic rocks.

In between, there are intermediate rocks (like andesite and diorite) that contain a balance of light and dark minerals, leading to medium gray colors.

Igneous Rock Classification

By combining texture and composition, geologists can accurately classify igneous rocks. For example:

Granite = coarse-grained and felsic
Basalt = fine-grained and mafic
Andesite = fine-grained and intermediate
Gabbro = coarse-grained and mafic

Pyroclastic Material

When a volcano erupts explosively, it doesn’t just release lava—it can also blast out broken pieces of rock, ash, and molten fragments. These materials, thrown into the air during an eruption, are called pyroclastic materials. The word comes from Greek: pyro (fire) and clastic (broken).

There are several types of pyroclastic materials, grouped mainly by their size:

Ash

The smallest particles (less than 2 mm), like fine dust, made of tiny shards of volcanic glass and rock. Ash can travel long distances in the air and even circle the globe.

Cinders (or lapilli)

Small rock fragments about the size of peas to golf balls (2–64 mm). These often fall around the volcano to form cinder cones.

Blocks

Large, solid pieces of rock (> 64 mm) ejected during an eruption. They can be as big as a car and are often torn from the sides of the volcano itself.

Bombs

Large (> 64 mm) blobs of molten rock (lava) that are hurled into the air. They cool and harden as they fall, sometimes shaping into round or twisted forms.