Geology: Density, Rocks, Plate Tectonics, and Hazards

Density

Density = mass/volume

Isostatic equilibrium

The ratio of percentage above and below of a 'floating' object

Continental crust

Less dense (granite), higher elevation, thicker crust

Oceanic crust

Medium dense (basalt), lower elevation, thinner crust

Mantle

Most dense (peridiotite)

Luster

How light interacts with mineral surface (metallic, vitreous, dull, pearly, silky)

Color

The visible color of the mineral, mostly unreliable for identification

Hardness

Resistance to scratching

Cleavage

Flat, defined planes and angles

Fracture

Irregular/jagged break

Calcite

Reacts with HCl

Halite

Is salty

Aphanitic

Fine-grained texture in igneous rocks

Porphyritic

Mixed grain sizes in igneous rocks

Phaneritic

Coarse-grained texture in igneous rocks

Foliated

Shows banding/layering in metamorphic rocks

Non-foliated

No banding/layering in metamorphic rocks

Clastic sedimentary rock properties

Change with distance from source

Grain size

Gets smaller farther from source (gravel → sand → silt → clay)

Grain shape

Gets more rounded farther from source

Grain sorting

Gets more well-sorted farther from source

Convergent Plate Boundaries

Denser plate subducts, causing asymmetric earthquakes and volcanoes

Divergent Plate Boundaries

Creates new oceanic crust (magma rises/volcanoes), symmetric earthquakes

Transform Plate Boundaries

No volcanoes, shallow earthquakes

Cross-cutting relationships

An unconformity, fault, or intrusion that 'cuts' through another feature is younger than that feature

Lateral continuity

Sedimentary rock sequences extend laterally in all directions

Superposition

Rock layers increase with age as you go deeper

Original horizontality

Sedimentary rocks are originally deposited in horizontal sheets; any change occurred after deposition

Unconformities

Erosional gaps in time (represented by squiggly lines)

VEI (Volcanic Explosivity Index)

Calculated from volume (magnitude) and height (intensity) of eruption

Shield Volcanoes

Low slope, low silica lavas, flowy/gentle eruptions with gas

Stratovolcanoes

Steep slope, high silica lavas, explosive eruptions, found at convergent boundaries

Recurrence interval

Time between earthquakes

Stream Discharge

Amount of water flowing through a stream at a given point and time (ft³/s)

Discharge formula

Discharge = Velocity × Cross-sectional area

Cross-sectional area

Width × depth