Explain how temperature, pressure, stress and fluids transform a parent rock, classify metamorphic rocks as foliated or non-foliated, sort them by grade and locate metamorphic zones

Learning goal: Explain how temperature, confining pressure, directed stress (compressional forces) and hot fluids change a parent rock in the solid state; distinguish foliated rocks, typical of regional metamorphism, from non-foliated rocks, typical of contact metamorphism; describe the conditions under which common metamorphic rocks formed from their parent rock; sort metamorphic rocks by grade; identify where metamorphic rocks are found; build a dichotomous key; and choose rocks for a construction project by weighing their properties.

What metamorphism is. Metamorphism is the change in the minerals, texture or both of a rock that stays solid while its temperature, pressure or chemical surroundings change. The word matters: if the rock melts, the result is igneous. Metamorphism begins at roughly 200 °C, above the range in which buried sediment simply compacts and cements, and continues until the rock begins to melt, at roughly 650 to 800 °C or more depending on the rock and how much water is present. The original rock is the parent rock (protolith). It can be sedimentary, igneous or already metamorphic, and it controls what the metamorphic rock can become, because metamorphism mostly rearranges the atoms already present.

The agents: temperature, pressure, stress and fluids. Temperature supplies the energy for atoms to break bonds and rebuild into minerals stable at the new conditions; clay minerals, for example, recrystallize into micas. Heat comes from burial along the geothermal gradient or from a nearby body of magma. Confining pressure is the pressure of the overlying rock, equal in all directions, like water pressure on a diver. It rises by about 26 to 30 megapascals for every kilometer of depth and squeezes minerals into denser structures, but because it is equal in all directions it does not align them. Directed stress (differential stress) is unequal squeezing, greater in one direction than in others. It comes from compressional forces where plates converge and mountains are built, and it flattens grains and rotates platy and elongate minerals so that they lie perpendicular to the direction of greatest squeezing. Chemically active fluids, mainly hot water carrying dissolved ions, speed reactions and can carry elements in or out. Time matters too: reactions that take millions of years in nature are not reproduced in a classroom.

Foliated and non-foliated rocks. Foliation is a planar fabric: flat minerals such as micas and chlorite, and elongate ones such as amphibole, lined up in parallel, or minerals segregated into light and dark bands. It forms where directed stress acts on a rock rich in platy minerals, which is why foliated rocks are typical of regional metamorphism, the metamorphism of large areas, often thousands of square kilometers, in the roots of mountain belts at convergent plate boundaries. Foliation need not be parallel to the original sedimentary layers; slate often splits at a steep angle to the bedding, because the fabric records the direction of squeezing, not the direction of deposition. Non-foliated rocks have no planar fabric. That happens for one of two reasons. Either there was little directed stress, as in contact metamorphism, where magma bakes the surrounding rock in a zone (an aureole) from centimeters to a few kilometers wide, producing fine, hard, splintery hornfels from shale; or the parent rock's minerals are equant, the same size in all directions, so they have nothing to align. Relatively pure limestone (calcite) becomes marble and quartz sandstone becomes quartzite in both contact and regional settings, and both are commonly non-foliated, although strong shearing can stretch their grains or impurity layers into a weak fabric. So "non-foliated means contact" is a tendency, not a rule; the condition to check is whether both directed stress and platy minerals were present.

Parent rock and degree of metamorphism. The grade of metamorphism is how intense the conditions were. Low grade corresponds roughly to 200 to 400 °C, intermediate to 400 to 600 °C and high grade to above about 600 °C, with the boundaries shifting with pressure and water. Follow a shale up the grades. At low grade, clay recrystallizes into microscopic mica and chlorite aligned by stress, and the rock becomes slate: dull, dense and splitting into thin, flat sheets. A little higher, the micas grow slightly larger and give phyllite a silky sheen and often a crinkled surface. At intermediate grade, mica flakes are visible to the eye and the rock, schist, glitters and often holds garnet crystals. At high grade, minerals separate into light bands of quartz and feldspar and dark bands of biotite or amphibole: gneiss. If partial melting begins, the rock becomes a mixed igneous-metamorphic migmatite. Grain size therefore generally increases with grade. Basalt follows a different path: greenschist (chlorite, low grade), then amphibolite (hornblende and plagioclase, intermediate to high grade). Bituminous coal becomes anthracite at low grade and graphite at high grade. In the late nineteenth century, geologists mapping the Scottish Highlands noticed that shales of one parent composition carry a sequence of index minerals as grade rises (chlorite, biotite, garnet, staurolite, kyanite, sillimanite), and lines on a map where each first appears divide an area into metamorphic zones.

Here is a model worked through. A geologist walks 20 km across a hypothetical mountain belt, sampling one shale unit. At the start the rock is slate with chlorite; after 6 km, phyllite with biotite; after 12 km, schist with red garnets; at 20 km, gneiss with sillimanite. The rocks came from one parent, so the changes must record conditions: metamorphic grade, and with it the peak temperature, increased toward the far end, and the parallel mica fabric throughout shows directed stress. The traverse crosses metamorphic zones from low to high grade and places the hottest part of the belt at the 20 km end. Grade alone does not measure depth: a high temperature can be reached at modest depth where the geothermal gradient is steep or an intrusion is near, and later faulting can move rocks from where they formed. Depth of burial is inferred only with pressure-sensitive minerals (kyanite, for example, forms at higher pressure than andalusite of the same composition) and the mapped structure of the belt.

Where metamorphic rocks are found. Metamorphic zones lie (1) in the cores of mountain belts at convergent boundaries, such as the Appalachians, the Alps and the Himalaya; (2) in the ancient continental shields, such as the Canadian Shield, where old mountain roots are exposed by erosion; (3) in contact aureoles around intrusions; (4) in subduction zones, where rock is carried deep quickly and stays relatively cool, making high-pressure, low-temperature blueschist; and (5) along fault zones, where grinding produces sheared rocks. Ohio has no metamorphic bedrock at the surface; the gneiss, schist and quartzite boulders in its glacial deposits were carried from the Canadian Shield by ice.

Choosing rock for construction. Properties that follow from the mineralogy decide use. Slate splits into thin, impermeable sheets and has long been used for roofing and flooring. Marble takes a polish and is easy to carve, but its calcite is only hardness 3 and dissolves slowly in acidic rain, so outdoor marble loses detail over decades. Quartzite, made of interlocking quartz grains, is very hard and resistant to weathering but costly to cut. Schist is weak along its mica planes, poor for load-bearing blocks and a slope hazard where those planes dip toward a road cut. Gneiss is strong but may split along its bands.

Worked example, a dichotomous key: Step 1: Does the rock have foliation (aligned minerals or bands)? If yes, go to 2; if no, go to 5. Step 2: Are there light and dark bands? Yes: gneiss. No: go to 3. Step 3: Are mica flakes visible to the eye? Yes: schist. No: go to 4. Step 4: Does it have a silky sheen? Yes: phyllite. No, dull and splits into flat sheets: slate. Step 5: Does it fizz in dilute (5%) vinegar or 1 M hydrochloric acid applied by the teacher? Yes: marble. No: go to 6. Step 6 (for the classroom set only): Is it light-colored, with sugary quartz grains visible under a hand lens that the rock breaks through? Yes: quartzite. No, dark, too fine to see grains, splintery, and labeled as collected beside an intrusion: hornfels. (Both can scratch glass, so hardness does not separate them; a sample that fits neither is reported as not resolved.) Test on a sample that glitters with flakes and has no bands: foliated, no bands, visible mica, so schist.

Worked example, conditions at depth: A shale is buried 15 km in a hypothetical mountain belt. Confining pressure ≈ density × g × depth = 2,700 kg/m³ × 9.8 m/s² × 15,000 m ≈ 397,000,000 Pa ≈ 400 MPa, nearly 4,000 times atmospheric pressure (0.101 MPa). With an assumed gradient of 25 °C per km and a 15 °C surface, the temperature is about 15 + 375 = 390 °C. That is low grade: with directed stress the shale should be slate or phyllite with chlorite and perhaps biotite, not gneiss. A steeper gradient near a hot intrusion would raise the grade at the same depth.

Worked example, sorting by grade: Four specimens from one shale parent: garnet-mica schist, slate, gneiss and phyllite. Order from lowest to highest grade: slate (microscopic micas, dull), phyllite (fine micas, sheen), schist (visible micas and garnet), gneiss (segregated bands). The evidence is the growth of mica grains and then the separation of minerals into bands, both requiring more heat and time.

Worked example, contact or regional: In a hypothetical field area, a dark, splintery, non-foliated rock is collected 50 m from a gabbro intrusion, and the same shale unit 3 km away is unaltered. The change dies out with distance from the intrusion and there is no fabric: contact metamorphism, and the rock is hornfels. A garnet schist collected in a mountain belt, with its mica fabric continuous across many kilometers, records regional metamorphism.

Investigation, modeling foliation: Materials: modeling clay, 40 paper-punch circles or flat sequins, two flat boards, a protractor and a marker. Knead the flakes randomly into a ball of clay, slice it in half and measure the angle of 20 flakes on the cut face relative to the table. Reform the ball and press it firmly between the boards (pressing from above only, which models directed stress). Slice again at right angles to the flattened face and measure the same 20-flake sample. Record both sets in a table and compute the mean deviation from horizontal. Result that supports the claim: after squeezing, flakes lie close to perpendicular to the squeezing direction. For contrast, squeeze a second ball evenly from all sides in your hands (a rough model of confining pressure) and compare. What the model cannot show: in real rock the minerals recrystallize and grow into new minerals, at temperatures of hundreds of degrees over millions of years; the clay only rotates existing flakes.

Key-building task: Construct your own dichotomous key for six metamorphic specimens (slate, phyllite, schist, gneiss, marble and quartzite) without copying the order of the worked key. Every step must offer exactly two choices about one observable property. Swap keys with a partner, test each key on specimens whose labels are hidden, and record every specimen the key misnames or cannot resolve and the step you would change.

Representation task: Make a grade chart with three parent rocks down the side (shale, basalt, limestone) and low, intermediate and high grade across the top. Fill every cell with the rock formed, one diagnostic feature and the approximate temperature range; mark with a star any cell whose rock can also form by contact metamorphism.

Independent performance, building with rock: Your group designs a small public library with an outdoor entrance stair, a roof, an interior lobby floor and a carved dedication panel. Choose a rock for each from marble, slate, quartzite, schist, gneiss and granite. Success criteria: each choice is justified by at least two properties that follow from the rock's minerals and texture; at least one rejected rock is explained (for example, why marble is a poor outdoor stair in acid rain); costs of cutting and durability are weighed; and the plan names one test you would run on a sample before ordering.