All rocks are broadly categorized into three families based on their formation process: igneous, sedimentary, and metamorphic. Understanding the differences between igneous and metamorphic rocks requires examining their distinct origins, which dictate their internal structure and external appearance. This comparison reveals how Earth’s internal heat and pressure create two fundamentally different materials from the same basic mineral components.
Differences in Formation and Origin
Igneous rocks originate from the cooling and solidification of molten rock material. This material is called magma beneath the surface and lava when it erupts. Slower cooling deep underground creates intrusive (plutonic) rocks like granite. Conversely, rapid cooling on the surface forms extrusive (volcanic) rocks such as basalt.
Metamorphic rocks arise from the transformation of a pre-existing rock, the parent rock (protolith), which can be igneous, sedimentary, or another metamorphic rock. This transformation, called metamorphism, occurs deep within the Earth when the protolith is subjected to intense heat and pressure, causing physical and chemical changes without completely melting the material.
Two primary settings drive this alteration: contact metamorphism and regional metamorphism. Contact metamorphism involves a rock being “baked” by intense heat from an adjacent magma intrusion, which causes localized changes. Regional metamorphism occurs when rocks are buried deeply or subjected to the immense directed pressure and heat associated with mountain-building events and colliding tectonic plates. The temperature and pressure ranges are sufficient to recrystallize the minerals, but not enough to initiate melting, typically above 150 degrees Celsius.
Structural and Textural Distinctions
The formation environment of igneous rocks directly influences their crystalline structure, or texture. The rate of cooling is the primary factor controlling the resulting crystal size. Intrusive igneous rocks, which cool slowly, develop a phaneritic texture, meaning they have large, interlocking crystals easily visible to the naked eye, as seen in gabbro or granite.
Extrusive igneous rocks, which cool rapidly, form aphanitic textures, where the crystals are too small to be seen without magnification, like those in basalt. If the cooling is extremely fast, such as when lava meets water or air, the resulting rock may form a glassy texture with no crystalline structure, such as obsidian. Igneous rocks typically display a random distribution of crystals because they solidify from a uniform liquid.
Metamorphic rocks exhibit unique textures created by the directed stress and heat of their formation. One distinctive feature is foliation, the parallel alignment of mineral grains into layers or bands. This alignment results from pressure that squeezes the rock, causing platy minerals like mica to reorient perpendicular to the direction of the force. Rocks like slate and gneiss show this layering, with gneiss displaying alternating light and dark mineral bands.
Not all metamorphic rocks are foliated. When a protolith is composed of minerals that are not platy or elongated, such as quartz or calcite, it forms a non-foliated texture. In these cases, the minerals recrystallize into a dense, interlocking mosaic of equal-sized grains, known as a granoblastic texture, which is characteristic of marble and quartzite.
Their Role in the Rock Cycle
Igneous rocks hold the position of the initial material in the rock cycle. They form from the solidification of magma that originated deep within the Earth. Once formed, igneous rocks can follow two main pathways: they can be exposed at the surface and broken down by weathering and erosion into sediment, which eventually forms sedimentary rock. Alternatively, if buried and subjected to deep subsurface heat and pressure, they can be directly transformed into a metamorphic rock.
Metamorphic rocks are formed when any pre-existing rock is changed by environmental conditions. Their role in the cycle is to act as a transitional form before undergoing further change. A metamorphic rock can be uplifted and exposed to the surface, where it is then broken down into sediment to begin the sedimentary rock pathway.
A metamorphic rock can complete the cycle by being subjected to such extreme heat that it melts. When this happens, the material becomes magma, and its subsequent cooling and solidification will create a new igneous rock, effectively resetting that portion of the rock cycle. The interplay of melting, solidification, alteration, and erosion links these two rock types in an ongoing geological process.