5.2 Earthquakes, Volcanoes & Surface Processes (Erosion/Weathering)

Earthquakes, Volcanoes & Surface Processes (Erosion/Weathering)

Earth's surface is constantly reshaped by dual geological forces: internal endogenic forces (tectonic stress, seismic energy, volcanic eruptions) that build relief, and external exogenic forces (weathering, erosion, mass wasting, deposition) that wear down landforms.

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1. Earthquakes & Seismic Wave Dynamics

An earthquake occurs when elastic energy stored in rocks under tectonic stress exceeds the rock's shear strength, causing sudden slippage along a fault line.

• Focus (Hypocenter): The exact point beneath Earth's surface where fault rupture originates.
• Epicenter: The point on Earth's surface directly vertically above the focus.

Types of Seismic Waves

Seismic energy radiates outward from the focus in all directions as seismic waves, divided into body waves (traveling through Earth's deep interior) and surface waves (traveling along the surface).

Seismic Waves
├── Body Waves
│   ├── P-Waves (Primary / Compressional): Fastest, moves through Solids, Liquids, Gases
│   └── S-Waves (Secondary / Shear): Slower, moves through SOLIDS ONLY
└── Surface Waves (Love & Rayleigh Waves): Slowest, highest amplitude, causes ground destruction

1. Primary Waves (P-Waves):
   • Motion: Compressional / longitudinal (particles move parallel to wave direction, expanding and contracting like an accordion).
   • Velocity: Fastest seismic waves ($5\text{--}8\text{ km/s}$ in crust).
   • Medium: Travels through solids, liquids, and gases.
2. Secondary Waves (S-Waves):
   • Motion: Shear / transverse (particles move perpendicular to wave direction, in a side-to-side motion).
   • Velocity: Intermediate speed ($3\text{--}4.5\text{ km/s}$).
   • Medium: Travels ONLY through solids. Liquids have no shear strength. The inability of S-waves to pass through Earth's outer core created an S-wave shadow zone beyond $103^\circ$ from an epicenter, proving to geologists that the outer core is liquid.
3. Surface Waves (Love and Rayleigh Waves):
   • Motion: Rolling elliptical and lateral ground oscillations.
   • Velocity: Slowest waves, but carry the highest energy amplitude and cause the greatest structural surface damage.

Triangulation & Epicenter Location

Because P-waves travel faster than S-waves, the interval of time between the arrival of the first P-wave and the first S-wave ($S-P\text{ lag time}$) increases as distance from the epicenter increases.

• Triangulation Method: Seismologists measure the $S-P\text{ time delay}$ at a minimum of three separate seismic recording stations. Each delay is converted to a radial distance. Three overlapping distance circles intersect at a single unique point: the epicenter.

Measuring Earthquake Size

• Richter Scale: Logarithmic scale based on maximum wave amplitude on a seismogram. Each whole number increase (e.g., from Magnitude 5 to Magnitude 6) represents a 10-fold increase in wave amplitude and approximately a 32-fold increase in total released energy.
• Moment Magnitude Scale ($M_w$): Modern scientific standard measuring total energy released based on fault area, slip displacement, and rock rigidity.

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2. Volcanism & Magma Chemistry

Volcanic activity occurs primarily at subduction zones, ocean rifts, and mantle hotspots (localized stationary plumes of hot upwelling mantle independent of plate boundaries, such as the Hawaiian Islands and Yellowstone).

Magma Viscosity & Eruption Style

Viscosity (a fluid's resistance to flow) dictates whether a volcanic eruption is gentle and effusive or violently explosive. Viscosity is controlled primarily by silica content ($SiO_2$) and temperature.

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3. Surface Processes: Weathering, Erosion & Deposition

While endogenic forces construct mountains and crust, surface agents continually alter the topography.

A. Weathering (In-Situ Breakdown)

Weathering is the physical disintegration or chemical alteration of rocks in place without transport.

1. Mechanical (Physical) Weathering: Breaks rock into smaller pieces without changing chemical composition, vastly increasing surface area.
   • Frost Wedging: Water enters rock fractures, expands by ~9% upon freezing, exerting pressure that splits rock.
   • Exfoliation / Unloading: Removal of overlying rock weight releases pressure, causing intrusive granite to expand and peel off in curved sheets.
   • Biological Action: Tree roots wedging open fractures.
2. Chemical Weathering: Transforms rock minerals into new chemical compounds through interactions with water, oxygen, and atmospheric acids.
   • Dissolution / Carbonation: Rainwater absorbs atmospheric $CO_2$ to form weak carbonic acid ($H_2CO_3$). Carbonic acid dissolves calcite in limestone ($CaCO_3$), carving subterranean caves, sinkholes, and karst topography.
   • Oxidation: Atmospheric oxygen reacts with iron-bearing minerals in rocks (e.g., magnetite) to form iron oxides (rust), weakening the rock matrix.
   • Hydrolysis: Hydrogen ions in water react with feldspar minerals to produce soft clay minerals.

Carbonic Acid Reaction: CO2 + H2O <-> H2CO3
Limestone Dissolution: H2CO3 + CaCO3 <-> Ca(HCO3)2 (Soluble Calcium Bicarbonate)

B. Erosion & Deposition (Transport & Settling)

Erosion is the removal and transport of weathered material by dynamic mobile agents: water, wind, ice, or gravity.

1. Fluvial (Running Water): The dominant agent of surface erosion globally.
   • High-velocity mountain streams erode vertically, carving sharp V-shaped valleys.
   • Low-gradient lowland rivers erode laterally, creating meanders, oxbow lakes, and depositing nutrient-rich sediment at river mouths to build triangular deltas.
2. Glacial (Ice):
   • Massive flowing ice sheets and alpine glaciers pluck and abrade valley walls, transforming V-shaped river valleys into broad, steep-sided U-shaped valleys.
   • Leave behind unsorted glacial debris called till, ridge-like deposits called moraines, and massive transported boulders known as glacial erratics.
3. Aeolian (Wind): Dominant in arid regions with sparse vegetation.
   • Deflation strips fine silt/sand, creating desert pavement and sculpting migrating sand dunes.
4. Mass Wasting (Gravity): Downslope movement of rock and soil under direct gravitational force (landslides, mudflows, rockfalls, and slow soil creep).

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4. Worked Example & Scientific Reasoning

GED Practice Scenario: Seismic Distance & Magnitude Calculation

A seismograph records an earthquake. The first P-wave arrives at $08:10:00\text{ AM}$. The first S-wave arrives at $08:14:30\text{ AM}$.

• Seismic calibration charts establish that an $S-P\text{ time delay}$ of $1.0\text{ minute}$ corresponds to an epicenter distance of $800\text{ km}$.

Question 1: Calculate the distance from the recording station to the earthquake epicenter. Question 2: If Station A records a wave amplitude of $5\text{ mm}$ for a Magnitude 4.0 earthquake, what amplitude would be recorded at the same distance for a Magnitude 6.0 earthquake?

Step-by-Step Solution:

1. Calculate $S-P\text{ Time Delay}$: $\text{Delay} = 08:14:30 - 08:10:00 = 4\text{ minutes } 30\text{ seconds} = 4.5\text{ minutes}$
2. Calculate Distance to Epicenter: $\text{Distance} = 4.5\text{ minutes} \times 800\text{ km/minute} = 3,600\text{ km}$
3. Calculate Wave Amplitude Change on Richter Scale:
   • The difference between Magnitude 4.0 and Magnitude 6.0 is $6.0 - 4.0 = 2.0\text{ units}$.
   • Because the Richter scale is logarithmic ($10^n$ factor per unit step), a 2-unit increase represents a $10^2 = 100$-fold increase in wave amplitude. $\text{New Amplitude} = 5\text{ mm} \times 100 = 500\text{ mm}$