Magma is molten rock trapped beneath the Earth’s surface within the crust or mantle. When this superheated material, which can reach temperatures between 700 and 1,200 degrees Celsius, erupts onto the surface, it is called lava. This distinction determines the resulting rock type. When lava cools and solidifies, the dried material is broadly classified as a type of igneous rock.
The General Term for Solidified Lava
The technical term for dried lava is Extrusive Igneous Rock, sometimes called Volcanic Igneous Rock. This classification reflects the material’s origin, as it was extruded onto the planet’s surface before solidifying. The rapid cooling process that occurs when lava meets the atmosphere or water is the defining characteristic of this rock group.
This quick solidification prevents the formation of large, visible mineral crystals. Instead, extrusive rocks typically develop a fine-grained texture, described as aphanitic, where individual crystals are too small to see without magnification. This contrasts with intrusive igneous rock, which cools slowly deep underground, allowing crystals to grow to a much larger size. In some cases, cooling is so instant that no crystals form, resulting in a type of natural volcanic glass.
Key Types of Volcanic Rock Based on Formation
The specific rock that forms from dried lava depends on its chemical composition, temperature, and the presence of dissolved gasses. Basalt is the most common extrusive igneous rock, forming the majority of the world’s oceanic crust. This dark-colored, fine-grained rock solidifies from mafic lava, which is rich in magnesium and iron but low in silica content, allowing it to flow easily and cool quickly into a dense rock.
When lava has a higher silica content, it is highly viscous, making it thick and stiff. If this lava cools extremely rapidly, the atoms do not have time to arrange into a crystalline structure. The resulting material is Obsidian, a naturally occurring volcanic glass that is amorphous, meaning it lacks an ordered internal structure. Obsidian is typically black and fractures with a characteristic conchoidal, or shell-like, pattern.
Another common type is Pumice, a light-colored rock with a highly porous, frothy appearance. This distinctive texture, called vesicular, is a result of gas bubbles trapped within the lava as it solidifies. As the lava erupts, the pressure drops rapidly, causing dissolved gasses to expand like foam. If the lava cools quickly while these gas bubbles, or vesicles, are still present, the rock ends up being extremely lightweight, sometimes even light enough to float on water. A similar, darker, and denser version of this vesicular rock is known as Scoria.
How Lava Flow Texture Affects the Resulting Rock
Beyond the internal structure and composition, the physical movement of the lava flow itself dictates the surface texture of the solidified rock. This surface appearance is categorized using two Hawaiian terms: Pahoehoe and A’a. These terms describe the final morphology of the flow surface, which is a result of the lava’s viscosity and flow rate.
Pahoehoe lava flows are characterized by a smooth, billowy, or ropy surface texture. This form develops from fluid, less viscous lava that moves slowly and forms a thin, continuous surface crust. As the molten lava flows beneath this thin skin, it drags and wrinkles the surface into distinctive rope-like folds.
In contrast, an A’a lava flow solidifies into a rough, jagged, and fragmented surface. This texture is created when the lava is more viscous or is flowing quickly, leading to high shear strain on the surface. The cooled outer crust constantly tears apart and crumbles into sharp, angular fragments called clinkers, which are carried along by the still-molten interior. Both Pahoehoe and A’a are surface features that can occur in the same type of rock, such as basalt, and a single flow can transition from the smoother Pahoehoe to the rougher A’a as it cools.