Unit 2D: Exploring Earth's External Forces

Comprehensive practice vocabulary flashcards covering Earth's surface features, internal and external dynamic forces, weathering, erosion, glacial landforms, fluvial mechanics, and soil properties and profiles.

Internal Forces

Constructive forces driven primarily by plate tectonics that originate from within the Earth and build up the surface, increasing elevation and complexity.

Energy Source of Internal Forces

The Earth's internal heat, generated by radioactive decay in the mantle and core, which powers mantle convection currents to move tectonic plates.

Mantle Convection Currents

Circulation currents within the mantle driven by internal heat that move the Earth's lithospheric tectonic plates.

Builders of the Earth's Surface

A descriptive title for internal forces because they construct massive, large-scale relief features and increase landscape elevation.

External Forces (Exogenic Forces)

Destructive and modifying land-wearing forces that originate in the planet's atmosphere or on its surface to break down, move, and deposit material.

Energy Source of External Forces

Energy powered primarily by the Sun (which drives weather, wind, and the water cycle) and secondarily by the force of gravity.

Sculptors or Levelers of the Earth's Surface

A descriptive term for external dynamic forces because they work to smooth out and level relief features by wearing down high points and filling in low points.

Plate Tectonics

The movement of large lithospheric plates that results in collisions (mountain building), separation (rift valleys, mid-ocean ridges), and subduction (volcanism).

Volcanism and Mountain Building (Orogeny)

Key internal processes responsible for the creation of new crustal material, extensive mountain ranges, and volcanic activity.

Weathering

The breakdown of rocks, soil, and minerals into smaller pieces (sediment) through physical, chemical, and biological means via contact with the atmosphere, biota, and waters.

In-situ Process

A characteristic of weathering indicating that the breakdown of rock material occurs on-site without involving immediate transport.

Erosion and Transportation

The removal and transport of weathered rock material (sediment) across landscapes by natural agents such as water, wind, ice, and gravity.

Deposition

The process by which transported sediment is dropped, settled, or accumulated in new locations when the transporting agent's energy decreases.

Mass Wasting (Mass Movement)

The downslope movement of rock and soil under the direct influence of gravity alone, without the aid of a transporting medium like water or wind.

Triggers and Examples of Mass Wasting

Gravitational movements—which water can trigger—including landslides, rock falls, rock slides, mudflows, and soil creep.

Physical (Mechanical) Weathering

The mechanical disintegration of rocks into smaller fragments without altering their chemical composition, driven by freeze-thaw cycles, temperature changes, and pressure release.

Frost Wedging

A physical weathering process in which water enters rock fractures, freezes, expands, and mechanically pries the rock apart.

Exfoliation

A mechanical weathering process in which curved slabs or concentric outer sheets of rock peel or flake away due to pressure release.

Chemical Weathering

The decomposition of rocks involving alteration of their chemical composition through reactions with water, oxygen, carbon dioxide, or acid rain.

Biological Weathering

The physical or chemical breakdown of rocks caused by living organisms, such as plant roots prying open cracks or burrowing animals displacing material.

Running Water (Rivers and Streams)

The primary agent of external erosion that carves valleys and canyons, transports sediment, and deposits it to form deltas and floodplains.

Wind as an External Dynamic Agent

An agent of erosion and transportation that erodes dry soil and sand, transports fine grains, and deposits them into dunes.

Ice (Glacial Erosion)

Large masses of ice that erode the terrain through grinding and plucking, forming U-shaped valleys and fjords while depositing moraines.

Coastal Waves and Currents

External marine forces that erode coastlines to carve cliffs and deposit transported sediment to build beaches and barrier islands.

Sediment Sorting

The degree of uniformity in particle size within a sedimentary deposit, where water and wind sort grains by size while glaciers deposit unsorted mixtures.

Glacier

A large, perennial accumulation of snow and ice that forms on land and displays evidence of past or present movement driven by its own weight and gravity.

Glacial Formation Process

The accumulation of snow in regions where annual snowfall exceeds annual ablation, compressing over decades into firn and ultimately solid glacial ice.

Glacial Ablation

The total loss of glacial ice and snow through melting, sublimation, or calving.

Firn

A dense, granular transitional form of compacted snow formed over decades that eventually compresses into solid glacial ice.

Alpine (Valley) Glaciers

Glaciers that originate in mountainous regions and flow downhill through pre-existing V-shaped river valleys, widening them into U-shaped valleys.

Continental Glaciers (Ice Sheets)

Vast sheets of ice covering enormous land areas (e.g., Greenland and Antarctica) that flow outward from a central accumulation dome, unconfined by underlying topography.

U-shaped Valleys

Glacially carved valleys characterized by steep, straight sides and a broad, flat bottom, distinguishing them from river-cut valleys.

V-shaped Valleys

Steep-sided valleys cut into the landscape by the rapid downcutting of fast-flowing river currents in their upper courses.

Moraines

Linear ridges or mounds of unsorted rock debris and glacial till left behind when a glacier melts, named according to their depositional position.

Terminal Moraine

The outermost ridge of unsorted glacial debris pushed forward and deposited at the furthest advance or terminus of a glacier.

Lateral Moraine

A linear ridge of unsorted rocky debris deposited along the lateral margins or sides of a valley where a glacier once flowed.

Medial Moraine

A ridge of sediment running down the center of a larger valley glacier, formed when the lateral moraines of two merging tributary glaciers join.

Kettle Lakes

Water-filled depressions created when partially buried or stranded blocks of stagnant glacial ice melt within outwash or till.

Drumlins

Elongated, teardrop-shaped hills of compacted glacial sediment whose blunt, steeper end points in the direction from which the ice advanced.

Glacial Erratics

Massive boulders transported by moving glacial ice and deposited far away from their native bedrock lithology.

Glacial Till

An unsorted, unstratified sediment mixture deposited directly by melting ice, containing all grain sizes from fine clay and rock flour to giant boulders.

Rock Flour

Finely pulverized, silt- and clay-sized mineral grains generated by the mechanical grinding and abrasion of bedrock beneath moving glacial ice.

Glacial Striations

Parallel linear scratches and deep gouges carved into bedrock surfaces by hard rock fragments dragged along the base of an advancing glacier.

Eskers

Long, winding, snake-like ridges of stratified sand, gravel, and boulders deposited by meltwater subglacial streams flowing within or beneath retreating ice.

River Source (Headwaters)

The high-elevation origin of a river where precipitation, snowmelt, or springs collect into a fast-flowing, narrow, and cold stream.

Tributaries

Smaller streams or secondary rivers that flow into a main stem river, contributing to its discharge and forming a drainage network.

Watershed (Drainage Basin)

The entire geographic catchment area where all precipitation and surface runoff funnel down into a single river network.

Drainage Divide

An elevated topographic boundary or ridge of high ground that separates adjacent watersheds from one another.

River Channel

The physical trough or trench in which river water flows, defined by confining banks on both sides and a bed along the bottom.

River Current (Flow)

The velocity and strength of moving stream water driven by gravity, flowing fastest over steep slopes and slowest across gentle lowlands.

River Upper Course Dynamics

The high-gradient section of a river where downward erosion predominates, carving steep-walled landforms like canyons and V-shaped valleys.

River Lower Course Dynamics

The low-gradient section of a river near its mouth where water slows down, causing sediment deposition to dominate over erosion.

Meanders

Broad, sinuous, sweeping loops or curves developed in a river channel across flat terrain through continuous lateral erosion and deposition.

Floodplain

The flat, low-lying land bordering a river that is subject to inundation during floods, covered with nutrient-rich fine sediment deposits.

River Mouth

The downstream endpoint of a river where its discharge empties into a larger water body, such as an ocean, sea, lake, or another river.

Delta

A fan- or triangular-shaped depositional landform formed at a river's mouth where decelerating currents drop their sediment load into standing water.

Estuary

A wide, semi-enclosed coastal transition zone at a river mouth where incoming freshwater mixes with saline ocean water, creating a biodiverse ecosystem.

Alluvial Fan

A fan- or triangle-shaped deposit of sediment formed where a fast-flowing mountain stream abruptly flattens, slows, and spreads at the base of a slope.

Oxbow Lake

A crescent-shaped or horseshoe-shaped standing water body formed when a river cuts through the narrow neck of an exaggerated meander loop, isolating it.

Primary Controls on River Velocity

The three physical parameters controlling stream speed: slope (gradient), channel geometry (including meanders and depth), and discharge (water volume).

Steeper Slope (Gradient) Effect

A steeper incline that accelerates water flow through gravity, giving the current higher kinetic energy to erode stream beds and banks into V-shaped valleys.

Gentle Slope (Gradient) Effect

A low slope that reduces stream velocity and energy, causing deposition to dominate and produce features such as floodplains and meanders.

Concave Upward River Profile

The longitudinal profile of an equilibrium river system, characterized by steep slopes near the headwaters that progressively flatten toward the mouth.

Overall Effect of Meanders on River Speed

Meanders lengthen the total channel distance, lowering the effective gradient and absorbing kinetic energy, thereby slowing down overall water speed.

Cut Bank (Outside Bend)

The outer edge of a meander curve where water flows fastest, creating turbulence that actively erodes and undercuts the bank to form a deeper channel.

Point Bar (Inside Bend)

The inner margin of a meander curve where water moves slowest, leading to reduced transport capacity and the deposition of sediment in a shallow zone.

Soil

A dynamic natural terrestrial medium composed of minerals, organic matter, water, and air that supports biological activity and plant growth.

Mineral Component of Soil

Weathered inorganic rock particles (sand, silt, and clay) that constitute approximately $45\%$ of a typical healthy soil's volume.

Organic Matter (Humus) in Soil

Decomposed plant and animal material constituting approximately $5\%$ of soil, which improves soil structure, fertility, and moisture retention.

Water Component of Soil

Soil solution occupying approximately $25\%$ of soil volume that dissolves and transports chemical nutrients into plant root systems.

Air Component of Soil

Atmospheric gases occupying roughly $25\%$ of soil volume within pore spaces, vital for root cellular respiration and aerobic microbial metabolism.

Soil Texture

The relative percentage or proportion of sand, silt, and clay mineral particles present in a soil sample.

Sand Particle Size

The largest mineral soil particle size fraction, defined by particle diameters ranging from $0.05\,\text{mm}$ to $2\,\text{mm}$.

Silt Particle Size

The medium-sized mineral soil fraction, defined by particle diameters ranging between $0.002\,\text{mm}$ and $0.05\,\text{mm}$.

Clay Particle Size

The finest mineral soil fraction, consisting of microscopic particles with diameters smaller than $< 0.002\,\text{mm}$.

Soil Porosity

The total volume of void or open pore space between solid grains within a given volume of soil.

Porosity of Sand, Silt, and Clay

Clay typically possesses the highest porosity due to abundant tiny pores; silt exhibits intermediate porosity; sand typically has low porosity with fewer, larger pores.

Soil Permeability

The rate at which water and air flow and percolate through interconnected soil pores.

Permeability of Sand, Silt, and Clay

Sand has the highest permeability and drains rapidly; silt has moderate permeability; clay has the lowest permeability and is prone to waterlogging.

Water Holding Capacity of Soil

The ability of soil to retain moisture; clay holds the most total water due to strong adhesion, while loamy soils offer the best usable balance for plants.

Loam

An ideal agricultural soil texture featuring a balanced mixture of sand, silt, and clay that provides optimal drainage, aeration, and nutrient retention.

Soil Fertility

The capacity of a soil to supply essential plant nutrients (such as nitrogen ($\text{N}$), phosphorus ($\text{P}$), and potassium ($\text{K}$)), derived from organic matter and mineral composition.

Soil pH

A chemical property measuring soil acidity or alkalinity that directly governs the availability and solubility of essential nutrients to plant roots.

Soil Profile

A vertical section through the ground revealing the sequential arrangement of distinct horizontal layers known as soil horizons.

O Horizon (Organic Layer)

The uppermost surface layer of a soil profile, dominated by decomposing organic litter, leaf matter, and dark humus.

A Horizon (Topsoil)

A nutrient-rich mineral horizon situated beneath the O horizon, rich in organic matter, vital for plant growth, and hosting the highest biological activity.

E Horizon (Eluviation Layer)

A light-colored zone of intensive leaching situated beneath topsoil where dissolved minerals and nutrients are carried downward by percolating water.

B Horizon (Subsoil)

The zone of accumulation where leached minerals, iron compounds, and fine clay from upper horizons are deposited; it is noticeably denser than topsoil.

C Horizon (Parent Material)

The subterranean soil horizon consisting of partially weathered, broken-up bedrock from which the mineral soil above originates.

R Horizon (Bedrock)

The unweathered, solid consolidated rock layer forming the deep structural foundation beneath all overlying soil horizons.