Metamorphic Rocks: Heat, Pressure and Texture
What Is Metamorphism?
Metamorphism is the solid-state transformation of existing rock in response to increased heat and/or pressure. The rock does not melt — if melting occurs, the process becomes igneous. Instead, minerals recrystallise into new, more stable forms, and the overall texture of the rock changes. The starting material (protolith) and the temperature and pressure conditions together determine what metamorphic rock type results. Understanding this process allows geologists to reconstruct the depth and heat conditions rocks were once subjected to, long after they have been uplifted and exposed at the surface.
A common misconception is that any heat or pressure will metamorphose a rock, or that metamorphism is “a bit of melting”. Neither is true. There is a threshold below which ordinary burial and diagenesis occur; above it, true metamorphic recrystallisation begins. The boundary is gradational rather than sharp, but for practical purposes metamorphism produces new minerals not present in the original rock.
Two Types of Metamorphism
Contact metamorphism occurs when rock is baked by heat from an igneous intrusion. The intrusion — a dyke, sill or pluton — raises temperature in the surrounding country rock. The affected zone is called the metamorphic aureole. Within the aureole, pressure is not significantly elevated, so the metamorphism is mainly thermal. The most intensely altered rock is immediately adjacent to the intrusion; alteration diminishes with distance. Clay-rich rocks near a granite pluton may be converted to hornfels — though note that hornfels itself is not on the specification as a named rock type; what you need to recognise is the concept of the aureole and the principle of thermally driven recrystallisation.
Regional metamorphism operates at a much larger scale, affecting huge areas of crust during mountain-building (orogenic) events. Both temperature and pressure increase as rocks are buried to great depth along collision zones. The Caledonian orogeny that built mountains across Scotland, Wales and Ireland subjected vast quantities of sedimentary rock to regional metamorphism — producing the slates of North Wales and the schists of the Scottish Highlands. The pressures involved are far greater than in contact metamorphism, and directed pressure (stress) is the key agent that produces the characteristic aligned textures of regional metamorphic rocks.
Contact metamorphism: localised around an intrusion, driven by heat alone, produces a metamorphic aureole. Regional metamorphism: large-scale, driven by both heat AND directed pressure during mountain-building, affects huge crustal volumes. The presence of directed pressure is the distinguishing factor.
Foliated and Non-Foliated Textures
The most visible effect of directed pressure in regional metamorphism is the alignment of minerals perpendicular to the stress direction. This produces a family of planar textures collectively called foliation. Foliated rocks split along these planes of aligned minerals, producing flat surfaces. The style of foliation depends on the grade (intensity) of metamorphism — how high the temperature and pressure conditions were. Not all metamorphic rocks are foliated: rocks that recrystallise under contact metamorphism, or that are composed of minerals that don’t align easily, may be non-foliated.
Slate — produced from mudstone or shale by low-grade regional metamorphism. The clay minerals recrystallise into microscopic mica grains aligned perpendicular to pressure, producing slaty cleavage. Slate splits into thin flat sheets along this cleavage. The individual mica crystals are too small to see with the naked eye — the rock looks dull and fine-grained. Classic Welsh slate (quarried at Blaenau Ffestiniog) is a textbook example.
Schist — produced from mudstone, shale or slate by medium-to-high-grade regional metamorphism. Higher temperatures cause larger mica grains to grow, visible to the naked eye as glittering flakes. The alignment of these large mica flakes produces a coarser foliation called schistosity. Schist also commonly contains index minerals such as garnet, which grow as large porphyroblasts within the foliated mica matrix. Scottish Highland schists are classic examples.
Marble — produced from limestone or chalk by metamorphism (contact or regional). Calcite grains recrystallise into a coarser interlocking mosaic. Marble is non-foliated because calcite crystals do not align easily under pressure in the way mica does. Pure marble is white; impurities in the original limestone may produce coloured or veined varieties. The rock has a granular, sugary texture when freshly broken.
Metaquartzite — produced from sandstone by metamorphism. Quartz grains recrystallise under heat and pressure into an interlocking mosaic of quartz crystals. Like marble, metaquartzite is non-foliated because quartz does not produce platy aligned crystals. It is extremely hard (hardness 7, the hardness of quartz itself), tough and resistant to weathering. The boundary between individual grains is destroyed by recrystallisation — a fresh break passes through grains, not around them (unlike sandstone where the break goes around grains).
Foliated: Slate (from mudstone, slaty cleavage, low grade) and Schist (from mudstone/slate, schistosity, higher grade). Non-foliated: Marble (from limestone, granular calcite) and Metaquartzite (from sandstone, interlocking quartz). Foliation requires directed pressure and platy minerals.
Marble and limestone can both be white and react with acid; the key differences are texture and crystal size. Limestone has a fine-grained or bioclastic texture (visible shell fragments or micrite); marble has a coarser, granular, interlocking crystal texture and lacks visible fossils (they were destroyed during recrystallisation). Metaquartzite and sandstone can both be made of quartz; in sandstone, breaks go around grains (the cement fails first), whereas in metaquartzite, breaks go through grains because recrystallisation has destroyed original grain boundaries.
Layered rocks are not automatically sedimentary. Schist can show pronounced foliation bands that look like sedimentary bedding. The giveaway is the alignment of mica crystals: run your finger across a schist surface and it feels rough in one direction (against the mica grain) and smooth in the other. Sedimentary layers do not show this directional mineral alignment.
Metamorphic rocks complete the three-rock-type classification of the specification. The next major topic in the course — the rock cycle — ties all three rock families together, showing how igneous, sedimentary and metamorphic rocks are continuously transformed from one into another over geological time. Recognising the metamorphic rocks by their parent rock, metamorphic type and texture is the foundation for understanding how the rock cycle operates in continental collision and subduction settings.