Definition of a Mineral: A mineral is a naturally occurring, inorganic, solid substance that has a definite chemical composition and an ordered crystalline structure.
Rocks are made of minerals, just as words are made of letters. A rock is an aggregate of one or more minerals.
Basic Rock Forming Minerals
The vast majority of the Earth's crust is made of silicate minerals, which are based on the silica tetrahedron (SiO₄). Common examples include:
Quartz: (SiO₂) Very hard, common, and durable.
Feldspar: (e.g., Potassium Feldspar, Plagioclase Feldspar) The most abundant mineral group in the crust.
Mica: (e.g., Muscovite, Biotite) Flaky minerals that peel in sheets.
Olivine: A green mineral, common in the mantle and oceanic crust.
Non-Silicates: e.g., Calcite (CaCO₃, makes up limestone) and Halite (NaCl, rock salt).
2. Rock Cycle
The rock cycle is a fundamental concept in geology that describes the dynamic processes that create, change, and destroy rocks. It shows how the three main rock types—igneous, sedimentary, and metamorphic—are interrelated.
Diagram: A circular flowchart showing the rock cycle.
1. Magma cools (crystallization) -> Igneous Rock.
2. Igneous Rock weathers/erodes -> Sediment.
3. Sediment compacts/cements (lithification) -> Sedimentary Rock.
4. Sedimentary Rock heats/pressures (metamorphism) -> Metamorphic Rock.
5. Metamorphic Rock melts -> Magma.
Include shortcuts: Igneous -> Metamorphic, Metamorphic -> Sediment, etc.
3. Igneous Rocks: Formation
Igneous rocks (from ignis, Latin for "fire") are formed from the cooling and solidification of molten rock (magma or lava).
Magma: Molten rock *below* the Earth's surface.
Lava: Molten rock that has erupted *onto* the Earth's surface.
The texture of an igneous rock (especially crystal size) is determined by its cooling rate:
Intrusive (or Plutonic) Rocks: Magma cools *slowly* deep underground. Slow cooling allows large, coarse crystals to grow.
Example: Granite
Extrusive (or Volcanic) Rocks: Lava cools *quickly* on the surface. Rapid cooling allows only very small, fine crystals (or no crystals, like obsidian glass) to form.
Example: Basalt
4. Metamorphism
Metamorphism is the process of changing a pre-existing rock (the "parent rock" or "protolith") into a new metamorphic rock. This change occurs in the solid state (the rock does *not* melt) due to intense heat and/or pressure.
This "baking" and "squeezing" causes minerals to recrystallize and reorient, forming new textures.
Foliation: A key feature of many metamorphic rocks. Pressure causes minerals to align in parallel bands or layers.
5. The Three Rock Laws (Principles of Stratigraphy)
These are fundamental principles used to determine the *relative* age of rock layers (i.e., which rock is older or younger than another). They were developed by Nicolaus Steno.
The Law of Superposition: In an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom, and the youngest layers are at the top.
The Law of Original Horizontality: Sedimentary layers are deposited (settle out of water or air) in horizontal or nearly horizontal layers. If we see tilted or folded layers, we know they were deformed *after* they were deposited.
The Law of Lateral Continuity: Sedimentary layers extend in all directions until they gradually thin out or are cut off by a barrier (like the edge of a basin). This allows us to correlate rock layers across a valley.
Exam Tip: A fourth principle, the Law of Cross-Cutting Relationships, is also critical. It states that any feature (like a fault or an igneous intrusion) that cuts across another rock layer must be *younger* than the layer it cuts.
6. Rock Structures
These are features (like folds and faults) that are "imprinted" on rocks, usually by tectonic forces *after* the rocks have formed.
Folds: Bends in rock layers caused by compressive stress (squeezing).
Anticline: An arch-like, upward fold (sad face ∩). Oldest rocks are in the center.
Syncline: A trough-like, downward fold (smiley face ∪). Youngest rocks are in the center.
Faults: A fracture or crack in the rock where movement *has* occurred.
Normal Fault: Caused by tensional stress (pulling apart). The hanging wall moves *down*.
Reverse Fault: Caused by compressive stress (squeezing). The hanging wall moves *up*.
Strike-Slip Fault: Caused by shear stress (sliding past). Blocks move horizontally past each other (e.g., San Andreas Fault).
Diagram: Simple block diagrams showing an anticline, a syncline, and the relative motion of normal and reverse faults.
7. Igneous, Sedimentary, and Metamorphic Rocks (Summary)
Weathering is the in-situ (in-place) breakdown of rocks, soil, and minerals at the Earth's surface. It is the first step in erosion. It does *not* involve movement.
Physical (Mechanical) Weathering
This is the process of breaking large rocks into smaller pieces (called clasts or sediment) without changing their chemical composition. This increases the surface area, which speeds up chemical weathering.
Frost Wedging: Water seeps into cracks, freezes (expands by ~9%), and wedges the rock apart. Common in cold, wet climates.
Salt Wedging: Saltwater seeps into cracks, the water evaporates, and salt crystals grow, prying the rock apart. Common in coastal or arid regions.
Exfoliation (Pressure Release): Deeply buried rock (like granite) is under high pressure. When erosion removes the rock above it, the pressure is released, causing the outer layers to "peel" off in sheets, like an onion.
Thermal Expansion/Contraction: Daily (diurnal) temperature changes (hot days, cold nights) cause the rock to expand and contract, leading to stress fractures. Common in deserts.
Chemical Weathering
This is the process that breaks down rock by altering its chemical composition, often turning hard minerals into softer, weaker ones. Water is the key agent.
Oxidation: The "rusting" of iron-rich minerals. Oxygen dissolved in water reacts with iron (Fe) to form iron oxides (like hematite or rust), weakening the rock.
Hydrolysis: Water (H₂O) chemically reacts with minerals, breaking them down. This is the primary way that feldspar (a hard mineral) weathers into clay (a soft mineral).
Carbonation (Dissolution): Carbon dioxide (CO₂) in the atmosphere dissolves in rainwater to form weak carbonic acid. This acid is very effective at dissolving limestone (calcite), leading to the formation of caves and sinkholes (Karst topography).
9. Erosion, Transportation, and Fluvial Sediment Transport
Erosion and Transportation
Erosion is the process that *removes* and *transports* weathered material (sediment) from one place to another. The main agents of erosion are:
Water (Fluvial): The most significant agent of erosion (rivers, streams).
Wind (Aeolian): Important in arid regions (deserts).
Ice (Glacial): Glaciers are powerful, "bulldozing" agents of erosion.
Gravity: Mass wasting, such as landslides and rockfalls.
Fluvial Sediment Transport (Transport by Rivers)
Rivers transport their sediment (their "load") in three main ways:
Dissolved Load: Dissolved minerals and ions (the "chemical" load) carried in the water. This is invisible.
Suspended Load: Fine, light particles (like silt and clay) that are held up ("suspended") and carried along by the flow of the water. This is what makes river water look muddy.
Bed Load: Coarse, heavy particles (like sand and gravel) that are too heavy to be suspended. They are moved along the river bottom by: