When molten material erupts onto the Earth’s surface, it is called lava, and the resulting solid material is a type of rock. The hardest and densest forms of this solidified lava result from rapid cooling in the open air. The names given to these rocks relate directly to their cooling speed and underlying chemical makeup. Understanding these names requires looking at the fundamental scientific categories used to classify all rocks formed from superheated molten material.
The Scientific Classification of Solidified Lava
The overarching group for all rocks that form from the cooling and crystallization of molten material is known as igneous rock. Scientists divide this category based on where the solidification takes place, which directly impacts the rock’s final appearance and density. Lava that solidifies on or very near the Earth’s surface is called extrusive igneous rock, also known as volcanic rock. This classification distinguishes it from intrusive igneous rock, which forms when magma cools slowly underground. Extrusive rocks are defined by the rapid heat loss experienced by surface lava, resulting in fine-grained and dense structures.
How Cooling Speed Determines Hardness and Texture
The rapid cooling of lava is the mechanism that produces a hard, dense rock structure. When the molten material is exposed to the atmosphere or water, it loses heat quickly, limiting the time available for mineral crystal growth. This quick-setting process results in a rock texture where individual crystals are microscopic, known as aphanitic.
A dense rock with an aphanitic texture is inherently harder than one where crystals had time to grow large. In contrast, magma that cools slowly deep underground forms visible, coarse-grained crystals, a texture known as phaneritic. In some cases, the lava cools so extremely fast that no atoms have time to organize into a crystalline structure, creating dense, non-crystalline volcanic glass. This fine-grained or glassy texture is what gives these extrusive rocks their density and strength.
Dense and Fine-Grained Volcanic Rock Types
The most common and widespread type of dense, fine-grained solidified lava is Basalt. This dark-colored rock forms from lava that is low in silica and relatively fluid, allowing it to flow great distances before hardening. Basaltic lava is the primary component of the ocean floor and makes up massive shield volcanoes, like those found in Hawaii. Its low silica content and high concentration of iron and magnesium contribute to its high density and dark appearance, classifying it as a mafic rock.
Another exceptionally hard form of solidified lava is Obsidian, which is a classic example of volcanic glass. Obsidian forms when high-silica lava cools almost instantaneously, preventing any crystal formation. This lack of crystal structure makes it extremely dense and results in a characteristic conchoidal fracture, meaning it breaks with smooth, curved surfaces and very sharp edges. Historically, this property made it highly sought after for creating tools and weaponry.
Other fine-grained volcanic rocks also exist, formed from lavas with a higher silica content than basalt. Andesite is an intermediate-composition, fine-grained rock, often gray in color, typically formed at subduction zones. Rhyolite represents the high-silica end of the spectrum and is chemically identical to granite, but its rapid cooling on the surface results in a fine-grained texture instead of the large crystals found in its intrusive counterpart.