EAPS5 Midterm 1

Natural hazard

A natural process with the potential to threaten human life or property.

Natural disaster

An event where a natural hazard causes significant damage to life or property.

Natural catastrophe

A natural event causing major injury/death or major property damage.

Hazard (probability)

The probability that a natural event will occur, estimated from evidence of past events.

Risk

The probability of an adverse consequence, based on recurrence interval and expected costs

recurrence intervals: probability that a natural event will occur of a particular size

Mitigation

Strategies to minimize risk, e.g. prediction, prevention, education, insurance, zoning, and building regulations.

Why are disaster costs rising over time?

Growing population, higher property values at risk, migration to hazardous areas, and climate change.

Most common & Most changing types of natural disaster

Most common disasters: meteorological events

Most changing disasters: climatological events

Natural hazards most deaths in the USA?

1) heat + drought

2) floods

3) winter weather

All hazards most deaths in the USA?

1) motor vehicle accident

2) smoking 10 cig/day

3) natural causes

4) violence/poisoning

5) flu

Risk perception — commonly overestimated

Nuclear power, hunting, spray cans, and mountain climbing.

Risk perception — commonly underestimated

X-rays, food preservatives

Is there a magma layer below the surface?

No — there is no continuous molten magma layer beneath the crust.

Crust (general)

The thin, brittle outermost layer that insulates and protects the mantle.

Continental crust — thickness, composition, density

~20–70 km thick

granite-like, high-silica (felsic), pale

less dense (~2650 kg/m3)

Oceanic crust — thickness, composition, density

~5 km (±3 km) thick

basaltic, low-silica and high Mg+Fe (mafic), dark

denser (~3200 kg/m3)

Why is oceanic crust generally younger than continental crust?

It continually recycles — new crust forms at ridges (divergent plate boundaries) while old, dense crust subducts.

Mantle — thickness, composition, density

~2700 km thick

peridotite, very low silica (ultramafic), very dark

~3400 kg/m3

Define mafic

Igneous rocks, magmas, or minerals rich in Mg+Fe (low silica)

Core composition

& outer vs inner core — state and thickness

Mostly iron with some nickel

Outer core liquid, ~2100 km

inner core solid, ~1470 km

What does the liquid outer core produce?

Earth's magnetic field, generated by its circulatory motion.

Lithosphere — definition, strength, heat transfer

Rigid outer shell (~100 km) of crust + uppermost mantle

strong because cold

transfers heat by conduction only

How many plates is the lithosphere divided into?

8 large plates plus a number of smaller ones (~15–20 total).

Asthenosphere — definition, strength, heat transfer

Weak, convecting mantle layer below the lithosphere (no defined bottom)

weak because warm (flows but stays solid)

transfers heat by conduction + convection

Mantle convection & what does it drive?

hot, less dense material rises

cooler, denser material sinks

driving plate motion

Tectonic plates

Rigid slabs of lithosphere that move slowly over the solid, semi-fluid asthenosphere.

How old is Earth?

About 4.55 billion years.

How was Earth's age determined?

By radiometrically dating meteorites/asteroid material formed alongside Earth.

What process differentiated early Earth into layers?

Melting allowed dense iron-nickel to sink and separate from silicate melts by density.

Age of the oldest minerals found

About 4.4 billion years (zircon crystals, Jack Hills, Western Australia).

Oldest intact rock

The Acasta Gneiss, about 4.02–4.03 billion years old (NW Canada).

Chronological order of major events in Earth history

Earth formed, 4.55 BYA

Oldest minerals found, 4.4 BYA (Oldest rocks 4.0–4.3 BYA)

First life, 3.1 BYA?

First abundant life (marine invertebrates), 570 MYA

End of the dinosaurs, 65 MYA

First homo sapiens, ~1 MYA

Craton

The ancient, geologically stable core of a continent that holds its oldest rocks

(oldest continental crust)

Geochronology

Determining the ages of rocks, minerals, and fossils and the sequence of geologic events.

Radioactive decay (rate law)

The decay rate is proportional to the number of atoms present, so a fixed fraction decays each equal time interval.

Half-life

The time for half of the radioactive parent atoms to decay to daughter atoms.

Why is Carbon-14 useless for dating rocks?

Its half-life is far too short and carbon is taken up by living things, not rocks.

Precision

The degree to which a number is known — how tightly it can be specified.

Accuracy

The degree to which a number is actually correct.

What do significant figures imply?

Precision — an uncertainty of about ± half of one unit in the rightmost digit.

Rounding rule for calculations

Express the result to as many sig figs as the least precise value used in the calculation.

FINGERNAILS analogy

India collides with Asia at about 4 cm/yr — roughly the speed fingernails grow.

How is present-day plate motion measured?

With GPS.

Seafloor age pattern at divergent boundaries

Youngest crust sits right at mid-ocean ridges and ages with distance away.

Where do most volcanoes occur tectonically & where globally?

Along plate boundaries (subduction zones, transform boundaries), especially the Ring of Fire around the Pacific.

Depth of most earthquakes (& where do deeper ones occur?)

Relatively shallow (0–70 km), with deeper ones in subduction zones.

Divergent boundary — examples

Mid-ocean ridges (magma→oceanic crust)

East Pacific Rise

Red Sea

How is new oceanic crust formed? (magma type?)

Plates diverge

→ pressure on underlying mantle drops

→ decompression melting produces basaltic magma

→ magma rises and cools at the ridge

→ new lithosphere forms at the trailing edges of both plates

Divergent boundary — magma type & viscosity

Basaltic (mafic): low silica, low viscosity, flows easily

Convergent boundary — examples

Pacific Northwest

Japan

western margin of South America

Three types of convergence

Oceanic-continental, oceanic-oceanic, and continental-continental.

Why are there volcanoes at subduction zones?

Water from the subducting slab lowers the mantle's melting point (flux melting), producing rising magma

Oceanic-continental convergence (forms what?)

Dense oceanic plate subducts under continental plate, forming a trench and a volcanic arc (e.g. Andes).

Oceanic-oceanic convergence (form what?)

The older, denser plate subducts, forming deep trenches and volcanic island arcs (e.g. Mariana).

Continental-continental convergence (form what?)

Neither plate subducts; crust folds upward into mountains with quakes but no volcanism (e.g. Himalayas).

Why does subduction magma become more explosive as it rises?

It melts and mixes with silica-rich crust, raising viscosity so trapped gas erupts violently.

Transform boundary — examples

San Andreas Fault

oceanic fracture zones

Why are volcanoes uncommon at transform boundaries?

Plates slide past each other → no pressure drop, added heat, or fluids to generate magma

Oceanic fracture zone

The inactive scar extending beyond the active transform segment between offset ridge crests

Who proposed continental drift, and when?

Alfred Wegener ~1912

Name of Wegener's proposed supercontinent

Pangaea

Continental Drift — evidence of former super continent plate configurations

contemporaneous shared fossils, rocks/mountain ranges, and glacial deposits in places now separated by ocean

When was plate tectonics generally accepted?

late 1960s

Key evidence for plate tectonics from the seafloor

The symmetric pattern of magnetic reversals striped across the ocean floor

What causes earthquakes?

driving forces > rock strength

→ rocks fracture and grind past one another

Epicenter

The point on Earth's surface directly above where an earthquake originates.

Focus (hypocenter)

The actual underground point where the fault slipped.

Strike

The compass direction of a horizontal line on a planar surface such as a fault.

Dip

The angle of inclination of the fault plane relative to horizontal.

Right-lateral strike-slip fault

The far side moves to the right relative to the near side (e.g. San Andreas).

Left-lateral strike-slip fault

The far side moves to the left relative to the near side.

Reverse fault motion

The hanging wall moves up relative to the footwall (compression).

Normal fault motion

The hanging wall moves down relative to the footwall (extension).

Reverse faults occur in which setting?

Convergent boundaries.

Normal faults occur in which setting?

Divergent boundaries.

Strike-slip faults occur in which setting?

Transform boundaries.

Elastic rebound theory

Stress deforms rock elastically until it suddenly snaps back, releasing stored energy as an earthquake.

Stress

The force imposed on rock.

Strain

The change in a rock's shape in response to stress.

Elastic deformation

Temporary deformation under low stress; rock returns to its original shape when stress is removed.

Plastic deformation

Permanent change of shape or flow under high stress deep in the Earth.

Foreshocks

Smaller slips that occur as a fault begins to fail before the main shock.

Aftershocks

Slips that occur as a fault continues to adjust after the main shock.

Offset (fault)

The distance of movement across a fault.

Surface rupture length

The total length of the break along the fault.

What sets the largest earthquake a fault can produce?

Its total fault length — longer faults allow larger earthquakes.

Liquefaction

Water-saturated soil temporarily behaves like a liquid during strong shaking, then re-solidifies.

Sandblows (sand boils)

Liquefied sand and pressurized water erupting up through the ground to the surface.

Submergence/emergence of coastline

Vertical fault offset that lowers or raises the shoreline during an earthquake.

Wavelength

The distance between successive crests or troughs.

Amplitude

Half the height difference between a crest and a trough.

Velocity (wave)

The speed at which a crest or trough travels.

Frequency

The number of crests passing a fixed point per unit time.

Period

The time interval between successive crests at a fixed point.

Wave equation

v = λf = λ/T.

Body waves + f, λ, v

travel through Earth's interior

high f, short λ, high v

P waves — type and speed

Compressional (longitudinal) waves, fastest to arrive, ~6 km/s at the surface.

P wave particle motion

Parallel to the direction of wave propagation.

Can P waves travel through liquids?

Yes — they pass through the liquid outer core (bending as they do).

S waves — type and speed

Shear (transverse) waves, slower than P, ~4 km/s at the surface.