Metamorphism Basics & Pressures
How do metamorphic rocks form?
Formed by recrystallization of pre‑existing rocks in the solid state, beneath Earth’s surface.
Rock with alternating dark/light bands
Gneiss (gneissic banding).
Protoliths: quartzite, marble, slate, gneiss
Quartzite = quartz‑rich rock; Marble = limestone; Slate = shale; Gneiss = shale (high grade) or granite (felsic).
What is downcutting?
Downward erosion; forms slot canyons in solid rock and V‑shaped valleys in loose sediment.
What does a hydrograph show?
Flood frequency and strength over time.
What is a protolith?
The pre‑existing rock before metamorphism.
Rock with platy mineral alignment
Schist (schistosity).
Difference between marble & quartzite
Marble = calcite, coarse crystals; Quartzite = quartz, very hard.
Angle of repose
The maximum slope angle a material can maintain while stable.
Reducing flooding hazards
Levees raise banks; dams prevent flooding; floodways divert excess water; zoning maps restrict development.
Mineral alignment vs. pressure direction
Minerals align perpendicular to differential pressure, forming foliation.
Rock with foliation only seen when broken
Slate (slaty cleavage only visible in thin section or when broken).
Difference between slate & hornfels
Slate = foliated, regional; Hornfels = non‑foliated, contact.
Why V‑shaped valley widens
Increased slope → lateral erosion widens valley.
Discharge, frequency, recurrence interval
Larger floods = lower frequency, longer recurrence interval; smaller floods = higher frequency.
Confining vs. differential pressure; which causes foliation?
Confining = equal from all sides. Differential = unequal; causes foliation.
Slate → Phyllite → Schist → Gneiss (grades)
Slate = low; Phyllite = low–intermediate; Schist = intermediate; Gneiss = high.
Contact aureole
Zone around magma where contact metamorphism occurs.
Changes downstream
Gradient decreases, discharge increases, velocity changes, downcutting decreases.
Dam problems & pros/cons
Pros: flood control, electricity, water source. Cons: loss of fertile land, sediment trapped.
What is foliation?
A planar arrangement of minerals caused by differential pressure; minerals become aligned.
Regional vs. contact metamorphism
Regional = differential pressure; produces foliated rocks. Contact = temperature only; produces non‑foliated rocks.
Base level
The lowest elevation a river can erode.
Cutbanks & pointbars
Cutbank = erosion; Point bar = deposition; found in meandering rivers.