Metamorphic rocks, often called “changed rocks,” form when existing igneous, sedimentary, or other metamorphic rocks are transformed. The question of whether these rocks are smooth or rough has a complex answer, as their texture is highly variable. Some types, like slate, can exhibit a remarkably smooth, flat surface. Others, such as schist and gneiss, are distinctly rough, layered, and uneven to the touch, depending entirely on the specific rock type and its formation history.
How Metamorphic Rocks Form
Metamorphism is the process where a parent rock, known as a protolith, is transformed by intense heat and pressure deep within the Earth’s crust without fully melting. This transformation occurs at temperatures ranging from approximately 200°C up to 1,100°C, and pressures that can reach 50,000 bars. The protolith undergoes a physical or chemical change, leading to the formation of new minerals and a new texture.
These extreme conditions are typically found in two main settings: regional and contact metamorphism. Regional metamorphism occurs over vast areas during mountain-building events, where rocks are subjected to high heat and directed stress from continental collisions. Contact metamorphism involves localized heating, where surrounding rock is “baked” by the intrusion of hot magma, resulting in high temperatures but lower, more uniform pressure.
The Key to Texture: Mineral Alignment and Foliation
The texture of a metamorphic rock, and consequently its smoothness or roughness, is determined by two primary outcomes: foliation and non-foliation. Foliation is a pervasive planar fabric caused by the alignment of minerals, such as mica, into parallel layers or bands under differential pressure. This directed stress causes platy or elongated mineral grains to rotate or grow perpendicular to the maximum stress, creating a distinctly layered structure.
This mineral alignment is the reason many metamorphic rocks are rough and easily split, as the parallel planes represent zones of weakness. Non-foliated textures, conversely, lack this parallel alignment of mineral grains. These textures typically form under uniform pressure, such as in contact metamorphism, or when the protolith is composed of minerals that are not platy or elongated, like quartz or calcite.
In non-foliated rocks, minerals recrystallize into interlocking, equant grains. This interlocking network makes the rock dense and massive, often resulting in a smooth or granular surface texture. The absence of layered mineral planes means the rock does not split easily and presents a more uniform feel. The type of pressure and the chemical composition of the original rock determine which of these two contrasting textures develops.
Textural Differences in Common Metamorphic Rocks
Marble and quartzite are prime examples of the smoother, non-foliated rocks. Marble forms from the metamorphism of limestone and is composed of large, interlocking calcite crystals, resulting in a dense, uniform, and often polished surface.
Quartzite, which originates from sandstone, is characterized by quartz grains that have enlarged and fused together into a solid, smooth network. These non-foliated rocks feel smooth and dense because their mineral grains are randomly oriented and tightly bound.
On the rougher side are the foliated rocks, like slate, schist, and gneiss, where the layering dictates the feel. Slate exhibits a fine-grained foliation, which allows it to be split into flat sheets, creating a surface that can be relatively smooth but is noticeably layered.
Schist represents a higher degree of metamorphism where platy minerals like mica have grown large enough to be easily seen, giving the rock a rough, sparkly, and fractured surface. Gneiss forms at the highest temperatures and is characterized by distinct light and dark mineral bands, which create a coarse and unevenly rough texture.