Natural Hazards: Volcanism, Mass Wasting, Flooding, and Subsidence

Comprehensive vocabulary flashcards covering the key concepts of volcanic systems, slope stability and mass wasting events, fluvial dynamics and flooding hazards, and land subsidence mechanisms.

Decompression Melting: Mechanism

Melting that happens when hot mantle rock rises and confining pressure drops without any added heat.

Decompression Melting: Tectonic Settings

Occurs at divergent plate boundaries, mid-ocean ridges, continental rifts, and mantle hotspots.

Flux Melting: Mechanism

Melting caused by the addition of water and other volatiles, which lowers the melting temperature of mantle rock.

Flux Melting: Tectonic Setting

Occurs at subduction zones where a sinking oceanic plate releases trapped water into the overriding mantle.

Heat-Induced Melting

Melting of crustal rock caused by direct heat transfer from rising bodies of hot magma.

Magma Differentiation: Definition

The progressive change in the chemical makeup of a magma body as it cools and evolves.

Magma Differentiation: Compositional Shift

Magma naturally evolves from iron- and magnesium-rich (mafic) toward silica-rich (felsic).

Basaltic (Mafic) Magma: Composition and Temperature

The hottest magma type, characterized by low silica content and high amounts of iron, magnesium, and calcium.

Basaltic (Mafic) Magma: Viscosity and Gas

Has very low viscosity (runny) and low dissolved gas content, resulting in gentle, effusive eruptions.

Rhyolitic (Felsic) Magma: Composition and Temperature

The coolest magma type, characterized by very high silica content and high amounts of sodium and potassium.

Rhyolitic (Felsic) Magma: Viscosity and Gas

Has extremely high viscosity (thick and sticky) and high dissolved gas content, leading to explosive eruptions.

Andesitic (Intermediate) Magma

A magma composition intermediate between basalt and rhyolite, typical of subduction zone stratovolcanoes.

Magma Viscosity: Role in Explosivity

Sticky, high-viscosity magma traps expanding gas bubbles, building high pressure that bursts violently.

Dissolved Volatiles: Role in Explosivity

Higher amounts of dissolved gases expand rapidly near the surface, acting as the driving fuel for explosive blasts.

Volcanic Explosivity Index (VEI)

A relative scale from 0 to 8 ranking eruption explosiveness based on erupted volume and plume height.

Shield Volcano: Structure and Magma

A broad, gently sloping volcano built by successive layers of low-viscosity basaltic lava flows.

Shield Volcano: Eruptive Style and Hazards

Quiet, effusive Hawaiian-style eruptions ($\text{VEI } 0\text{--}1$) where fluid lava flows are the primary hazard.

Fissure Eruptions: Description

Effusive eruptions where fluid basalt pours out along linear fractures in the crust, forming extensive flood basalts.

Cinder Cone: Structure and Formation

A small, steep conical hill built from loose scoria and basaltic cinders piled around a single vent.

Cinder Cone: Eruption Style

Short-lived, mild Strombolian-style explosive spurts ($\text{VEI } 1\text{--}2$) that shoot cinders into the air.

Stratovolcano: Structure and Composition

A tall, steep-sided volcanic cone composed of alternating layers of andesitic lava flows and explosive ash deposits.

Stratovolcano: Eruptive Style and Hazards

Highly explosive Vulcanian to Plinian eruptions ($\text{VEI } 2\text{--}6+$) producing pyroclastic flows, ash, and lahars.

Lava Dome: Characteristics

A steep, bulbous mound created when thick, pasty felsic lava piles up directly over a volcanic vent.

Lava Dome: Hazards

Plugs the vent, trapping expanding gases until the dome fractures or collapses into a sudden explosive blast.

Caldera: Formation Mechanism

A large, steep-sided circular basin formed when a volcano collapses inward after draining its magma chamber.

Caldera: Eruptive Scale

Produced by rare, catastrophic super-eruptions ($\text{VEI } 6+$) that blast out hundreds of cubic kilometers of material.

Lava Flows: Hazard vs Impact

Moves slowly enough that people can easily evacuate, but completely crushes, burns, and buries immovable property.

Pyroclastic Flow: Description

A lethal, ground-hugging avalanche of superheated gas, ash, and rock traveling downhill at speeds over $100\,\text{km/h}$.

Pyroclastic Flow: Human Impact

Responsible for more direct volcanic fatalities than any other hazard due to extreme heat and uncontrollable speeds.

Ash Fall: Key Impacts

Weighs down and collapses roofs, contaminates drinking water, destroys agricultural crops, and causes jet engines to fail.

Lahar: Trigger and Speed

A rapid volcanic mudflow created when loose ash mixes with rain or melted ice, surging down river drainages.

Erupted Volcanic Gases: Types and Threat

Releases water vapor, $SO_2$, and $CO_2$ that can cause asphyxiation, acid rain, and volcanic fog (vog).

Paleovolcanology: Concept and Purpose

The study of ancient ash layers and lava flows to determine how often a specific volcano erupts over long time spans.

Harmonic Tremor

A continuous, sustained seismic vibration produced when magma and volcanic gases force their way up a conduit.

Volcano Ground Deformation Monitoring

Using tiltmeters and GPS sensors on volcanic flanks to track surface swelling caused by rising underground magma.

Acoustic Flow Monitor (AFM)

A solar-powered sensor along river canyons that detects ground vibrations of incoming lahars to send early warnings.

Watershed (Drainage Basin)

The entire land area where all incoming precipitation drains down into a single river network.

Stream Headwaters

The high-elevation, steep-gradient starting reaches of a stream where erosion cuts down into bedrock.

Stream Gradient: Definition

The vertical drop in elevation of a river divided by the horizontal travel distance.

Stream Gradient: Downstream Trend

Steepest near the headwaters and progressively flattens out as the stream nears base level.

Stream Discharge ($Q$): Definition and Equation

The volume of water passing through a channel per unit of time, calculated as $Q = V \times A$.

Stream Discharge ($Q$): Downstream Trend

Increases continuously downstream as more surface tributaries contribute incoming runoff.

Stream Flow Velocity: Downstream Trend

Tends to slightly increase downstream because deeper, broader channels minimize frictional drag along the bed.

Stream Competence: Definition

The maximum grain size or diameter of particle that moving stream water can physically roll or push.

Stream Competence: Controlling Factor

Controlled directly by flow velocity; faster-moving water can transport substantially coarser particles.

Stream Capacity: Definition

The total volume or weight of sediment load that a stream is physically capable of moving.

Stream Capacity: Controlling Factor

Controlled directly by discharge; rivers moving larger volumes of water carry much larger total sediment masses.

Stream Bed Load

Coarse particles like gravel and boulders that roll, slide, and bounce along the bottom of the river bed.

Stream Suspended Load

Fine particles like silt and clay kept floating aloft within the moving water column by turbulence.

Stream Dissolved Load

Soluble mineral ions carried invisibly in chemical solution throughout river water.

Bedrock Streams

Steep, high-energy stream reaches in headwaters where running water carves deep, narrow channels into exposed rock.

Meandering Streams: Defining Features

A single curving river channel with sweeping loops that migrates across a wide, flat floodplain.

Meandering Streams: Gradient and Sediment

Develops in low-gradient, flat landscapes where rivers carry mostly fine, suspended sediment.

Braided Streams: Defining Features

A network of converging and diverging shallow channels separated by shifting gravel and sand bars.

Braided Streams: Gradient and Sediment

Develops in high-gradient, arid, or glacial settings where coarse bedload exceeds the stream carrying capacity.

Stream Base Level

The lowest elevation down to which a running stream can erode its bed, below which deposition dominates.

Alluvial Fan

A fan-shaped deposit of gravel and sand formed where a steep mountain stream spreads onto a flat desert basin.

River Delta

A sediment wedge that builds outwards where a river enters standing water (ocean or lake) and rapidly loses velocity.

Natural Levees: Formation

Low ridges along channel banks built when floodwaters spill over and instantly drop their coarsest sediments.

Flooding: Competence and Capacity Response

Both stream competence and capacity surge dramatically due to higher flood velocities and enormous water volumes.

Surface Runoff vs Infiltration

Runoff is water flowing over the ground into channels, whereas infiltration is water soaking into soil.

Stream Hydrograph

A graph that plots river discharge or water stage over time to show how a basin responds to rainstorms.

Flash Flood: Causes and Setting

A sudden, fast-rising flood caused by intense downpours over steep canyons, headwaters, or dry impermeable ground.

Regional (Downstream) Flood: Causes and Duration

A widespread flood caused by days of steady rain that fully saturates broad river basins, lasting for weeks.

Flood Recurrence Interval ($T$): Formula

Calculated using $T = \frac{n+1}{m}$, where $n$ is the years of records and $m$ is the flood magnitude rank.

Annual Exceedance Probability ($P$): Formula

Calculated using $P = \frac{1}{T}$, showing the annual percentage chance of equaling or exceeding a specific flood size.

Urbanization: Impact on Flood Hydrographs

Asphalt and roofs prevent infiltration, making flood crests occur much faster and rise significantly higher.

Deforestation: Impact on Flood Hazards

Removes vegetation that intercepts rain and absorbs soil moisture, increasing total runoff volume and erosion.

Channelization: Benefit vs Downstream Impact

Clears local runoff quickly by straightening channels, but speeds water up to cause worse flooding downstream.

Artificial Levees: Benefit vs Drawback

Prevents river overflow locally, but forces flood crests higher and risks catastrophic disaster if overtopped.

Dams: Flood Benefit vs Ecological Impact

Stores upstream floodwater to release it slowly, but disrupts natural fish migration and blocks downstream sediment.

Floodplain Zoning (Land-Use Planning)

Restricting high-value building in 100-year floodplains to parks, agriculture, and open space to limit monetary loss.

Personal Flood Safety: Vehicle Danger

Just 12 to 24 inches of rushing water can float and carry away most passenger cars, SUVs, and trucks.

Shear Force ($f_s$) on a Slope

The component of gravitational force acting parallel to a slope surface that pulls earth materials downhill.

Normal Force ($f_n$) on a Slope

The component of gravitational force directed perpendicular into a slope that provides friction to resist sliding.

Criterion for Slope Failure

A slope fails and slides when the driving shear force exceeds the resisting normal force and internal friction.

Angle of Repose

The steepest angle relative to the horizontal at which loose, unconsolidated sediment remains stable without sliding.

Moisture Content: Effect on Slope Stability

Slight moisture binds sand grains by surface tension, but complete saturation pushes grains apart, causing slope failure.

Daylighted Bedding: Slope Hazard

Tilted rock layers that dip gentler than the slope and exit into open air, leaving rock slabs unsupported at their base.

Overloading: Slope Failure Trigger

Adding extra mass, such as buildings or heavy fill, high up on a slope, which increases driving shear force.

Oversteepening: Slope Failure Trigger

Cutting away the bottom of a slope through road excavation or riverbank erosion, removing support for upper layers.

Mass Wasting Classification: Three Criteria

Classified by velocity (slow to fast), type of material (rock vs soil/debris), and movement type (slide, flow, fall).

Soil Creep: Definition and Cause

The extremely slow, continuous downhill crawl of soil particles driven by alternating freeze-thaw expansion cycles.

Pistol-Butt Trees

Trees whose lower trunks curve downhill, serving as a classic visible warning sign of active soil creep.

Rockslide: Slip Surface and Movement

A rapid mass wasting event where a detached slab of bedrock slides downslope along a flat, planar weakness.

Rotational Slide (Slump): Slip Surface and Motion

Downslope movement along a concave-upward, curved shear surface that tilts the sliding block backward as it drops.

Translational Slide: Characteristics

Downslope sliding of unconsolidated soil or rock along a flat failure plane that runs parallel to the hillside.

Earthflow: Material and Speed

A sluggish, slow-moving flow of water-saturated clay and fine-grained regolith that forms rounded, bulbous lobes.

Debris Flow: Material and Speed

A rapid, saturated slurry of water, mud, and boulders traveling at high speeds down preexisting channels.

Rockfall: Movement and Speed

The rapid vertical free-fall of detached bedrock dropping from steep cliff faces at speeds up to hundreds of meters per second.

Talus Slope

An apron of loose, angular rock fragments that piles up along the base of a cliff after repeated rockfalls.

Debris Avalanche: Characteristics

A massive, chaotic, and turbulent flow of rock, soil, and ice tumbling down steep mountain slopes at high speeds.

Slope Re-Weighting: Benching and Toe Berms

Excavating steps into a slope to remove weight from the top, while adding heavy rock fill to support the base.

Slope Stabilization: Mechanical Structures

Using rock bolts, retaining walls, heavy steel wire mesh, and sprayed shotcrete to hold loose slopes in place.

Slope Stabilization: Drainage Methods

Installing perforated horizontal drain pipes and planting deep-rooted trees to dry out water-logged slopes.

Karst Topography: Definition and Rock Types

A landscape formed by acidic groundwater dissolving soluble rocks such as limestone, marble, and gypsum.

Solution Sinkhole: Development and Depth

A shallow, slowly forming depression where surface water dissolves carbonate bedrock directly beneath thin topsoil.

Collapse Sinkhole: Development and Hazard

A deep, catastrophic crater that forms abruptly when a subterranean cavern roof beneath thick clay collapses into the void.

Fluid Withdrawal Subsidence

Permanent land sinking caused by overpumping groundwater or oil, which depressurizes and compacts porous aquifer layers.

River Delta Sediment Compaction

Natural land subsidence that happens when thick, water-saturated silt and clay sediments dewater and settle under their own weight.