EESC Lab - Rock Types and Formations

Sandstone (Formation & Characteristics)

Sand transported by wind or water; clastic sedimentary rock; sand-sized particles; layered; forms in deserts, beaches, rivers; cemented by silica, calcite, or iron oxide; porous

Sandstone with Ripple Marks (What It Shows)

Indicates direction of water/wind; shallow moving water; sedimentary structure (not fossil); shows current speed; found on Mount Everest (marine origin)

Conglomerate (Environment Indicators)

Rounded pebbles indicate long transport; large clasts show high energy; forms in fast rivers; pebbles larger than sand; cement binds sediments

Breccia (Key Features)

Angular fragments indicate little/no transport; forms near cliffs, landslides, faults; rapid deposition; variable fragment sizes

Shale (General Characteristics)

Forms in calm water; made of small mud particles; fine-grained; fissile (splits into sheets); parent rock of slate; most common sedimentary rock

Shale with Mud Cracks (What It Indicates)

Wet then dry conditions; shallow or exposed environments; fine-grained clay; polygon cracks; climate indicator

Fossiliferous Sedimentary Rock (Formation Conditions)

Shells and skeletons accumulate; shallow ocean; often limestone; rapid burial needed; helps reconstruct ecosystems

Fine-Grained Schist (Formation)

Metamorphic rock; formed by heat and pressure; foliated; distorted layers; indicates mountain building

Obsidian (Why No Crystals?)

Lava cools rapidly; no crystals form; glassy igneous rock; conchoidal fracture; used for tools

Mica Schist (Characteristics)

Sparkly from mica; foliated; medium-high metamorphism; splits along layers; indicates strong tectonic forces

Volcanic Porphyry (Cooling History)

Two-stage cooling; large crystals (phenocrysts) and small crystals; slow then fast cooling; complex igneous history; near volcanic arcs

Pegmatite Intruding Schist (What It Shows)

Very large crystals; intrusive igneous rock; schist is older; cross-cutting relationship; may contain rare minerals

Scoria (Formation & Characteristics)

Fine grained or Glassy so no visible minerals; extrusive igneous rock; rapid cooling so it contains air pockets

Basalt (Formation & Characteristics)

Fine grained; minerals are present; extrusive igneous rock with little air pockets

Gabbro (Formation & Characteristics)

Coarse-grained; visible minerals; intrusive igneous rock so no air pockets

Coquina (Formation)

Shells of sea creatures accumulate along the shoreline; later, they are buried and cemented together

Chalk (Formation & Characteristics)

Microscopic shells and plant material accumulate on seafloor forming calcareous mud; burial causes dewatering and compaction; very fine-grained; soft and crumbly sedimentary rock

Limestone (Chemical formation)

Calcium carbonate precipitates from lake water due to changes in temperature, pressure, or chemistry; forms fine mud; burial leads to compaction; very homogenous, smooth, fine-grained rock; low porosity