Magma is the molten rock material found deep beneath the Earth’s surface. When this material erupts onto the surface, it is known as lava. The cooling and solidification of both magma and lava form igneous rocks. This process is the initial step in the continuous system known as the rock cycle.
The Chemistry of Cooling and Crystallization
The process of solidification begins when the magma or lava loses heat to its surroundings. This drop in temperature causes the randomly moving atoms within the liquid melt to slow down significantly. As their movement decreases, the atoms begin to chemically bond and arrange themselves into highly organized, repeating geometric structures. This transformation from a liquid state to an orderly solid is called crystallization.
The newly formed solid structures are mineral crystals, which combine to make up the final rock. Not all minerals crystallize at the same time, creating a predictable sequence of formation. For instance, minerals rich in iron and magnesium generally form first. Those with higher silica and potassium content solidify later at lower temperatures.
Cooling Speed Determines Rock Location
The rate at which molten rock cools is the primary factor determining the final characteristics of the rock. This cooling rate is directly controlled by where the solidification takes place. Molten rock that remains trapped beneath the Earth’s surface cools very slowly because it is insulated by surrounding rock.
This slow cooling process creates intrusive, or plutonic, igneous rocks. Because the magma is deep underground, it can take thousands or even millions of years for the heat to dissipate. A contrasting process occurs when lava erupts onto the surface.
When lava is exposed to the atmosphere or water, it loses heat very quickly, often solidifying in a matter of hours or days. This rapid cooling forms extrusive, or volcanic, igneous rocks. The difference in cooling time separates the two primary types of igneous rock.
Resulting Textures and Grain Sizes
The difference in cooling speed directly dictates the observable texture and crystal size of the resulting rock. In intrusive rocks, the prolonged cooling time allows mineral crystals to grow large enough to be seen with the naked eye. This results in a coarse-grained texture, also known as phaneritic. Individual crystals, such as quartz and feldspar, interlock, which is characteristic of granite.
Conversely, the rapid cooling of extrusive rocks limits the time available for atoms to form large crystals. These rocks develop a fine-grained, or aphanitic, texture. The crystals are microscopic and too small to identify without a magnifying lens. Basalt, which forms much of the ocean floor, is a common example of a fine-grained extrusive rock.
When cooling happens instantaneously, such as when lava meets cold water, atoms are frozen in place before they can organize. This results in a glassy texture with no crystals, such as in obsidian, which is volcanic glass. Solidification completes the molten stage of the rock cycle, forming a stable igneous rock ready for erosion and weathering.