GEOG 012: AE - Lecture 2 Notes - The Natural Environment

GEOG 012: AE - Small Island Sustainability

University of The Bahamas, Summer 2025

  • Lecture 2: The Natural Environment

Learning Objectives

  • Understand plate tectonics, rocks, earthquakes, and the water cycle
  • Describe weather, climate factors, and wind systems
  • Identify ecosystems, vegetation, and soil processes
  • Analyze the Bahamian climate characteristics
  • Understand erosion and its environmental effects

Plate Tectonics

  • Plate tectonics is the theory that the lithosphere is divided into plates that float independently over the mantle.
  • Boundaries are sites of new crust formation, mountain building, and seismic activity (earthquakes).
  • Seafloor spreading is the mechanism that builds mid-ocean ridges and drives continental movement.
  • Early mapping accuracy revealed the fit of continents like South America and Africa.
  • Alfred Wegener (1912):
    • Found similar rock assemblages on the east coast of South America and the west coast of Africa.
    • Fossil and climatic records supported the idea of connected continents.
    • Proposed Pangaea (meaning "all Earth"), a supercontinent about 225 million years ago.
    • Called this concept continental drift.
    • While his initial model had some inaccuracies, his arrangement and breakup of Pangaea were correct.

Tectonics Definition

  • The word tectonic comes from the Greek word tektonikùs, meaning "building" or "construction".
  • Refers to changes in the configuration of the Earth’s crust due to internal forces.

Earth's Structural Layers

  • Crust: 0-100 km thick
  • Mantle
  • Asthenosphere: part of the mantle
  • Outer Core: liquid
  • Inner Core: solid
  • Lithosphere: crust and uppermost solid mantle

Plate Movements

  • Converging plates:
    • Can form ocean trenches and subduction zones.
    • Example: Andes Mountains.
  • Diverging plates:
    • Create mid-ocean ridges and seafloor spreading.
    • Example: Mid-Atlantic Ridge.
    • Upwelling of basaltic magma occurs along mid-ocean ridges.

Crustal Deformation

  • Rocks are subjected to stress from tectonic forces, gravity, and overlying rocks.
  • Stress is force per unit area on an object.
  • Types of stress:
    • Tension: causes stretching.
    • Compression: causes shortening.
    • Shear: causes twisting or tearing.
  • Strain is the dimensionless measure of deformation in response to stress; it manifests as folding or faulting in rocks.
  • Whether a rock bends or breaks depends on composition and pressure.

Three Kinds of Stress and Strain

  • Tension: stretching, resulting in thinning crust and normal faults.
  • Compression: shortening, resulting in folding and reverse faults.
  • Shear: shearing and twisting laterally, resulting in strike-slip faults.

Folding

  • Folding occurs when rocks deform due to compressional stress and shortening.
  • Anticline: An arch-shaped upward fold where strata slope downward from the center axis.
  • Syncline: A trough-shaped downward fold where strata slope upward from the center axis.
  • Erosion of a syncline can form a synclinal ridge if rock strata have varying resistance to weathering.

Faulting

  • Faulting occurs when rocks fracture and shift relative to each other.
  • Fault zones are areas with fractures showing crustal movement.
  • Types of faults:
    • Normal fault: tensional stress causes one side to move vertically along an inclined fault plane.
    • Reverse fault: compressional stress causes rocks to move upward along the fault plane.
    • Thrust fault (overthrust fault): a low-angle reverse fault where one block shifts far over the other.
    • Strike-slip fault: lateral shear causes horizontal movement along a fault plane.

Earthquakes

  • Tremendous friction exists along plate boundaries.
  • Stress builds strain in rocks until friction is overcome, causing a sudden break.
  • An earthquake is the sharp release of energy at the moment of fracture, producing seismic waves.
  • Energy radiates throughout the planet.
  • Tectonic earthquakes are associated with faulting.
  • Earthquakes can also occur with volcanic activity.

Volcanism

  • A volcano is a structure in the Earth’s crust with an opening connected to a central vent or pipe through which magma rises from the asthenosphere and upper mantle.
  • Magma collects in a magma chamber until conditions are right for an eruption.
  • Lava is magma that has cooled to form rock.

Volcanic Landforms

  • Cinder Cone: A small, cone-shaped hill (typically under 450 m high) formed by cinders accumulating during moderately explosive eruptions.
  • Caldera: A large, basin-shaped depression formed when the summit of a volcanic mountain collapses inward after an eruption or loss of magma.
    • May fill with rainwater to form a lake (e.g., Crater Lake in Oregon).
    • Long Valley Caldera: formed by a massive eruption 760,000 years ago; characterized by hydrothermal activity like hot springs that power the Casa Diablo geothermal plant.
  • Composite Volcano: A mountain formed by multiple layers of lava, ash, rock, and pyroclastics from a series of explosive eruptions.

Rocks

  • A rock is an assemblage of minerals bound together, composed of a single mineral, undifferentiated material, or solid organic material.
  • Three main types of rocks based on their formation processes: igneous, sedimentary, and metamorphic.

Igneous Rocks

  • Igneous rock solidifies and crystallizes from a molten state (magma).
  • Magma is molten rock beneath Earth’s surface.
  • When magma emerges at the surface, it becomes lava.
  • Magma can intrude into crustal rocks, cooling and hardening to form intrusive igneous rock, or extrude onto the surface as lava and cool to form extrusive igneous rock.

Sedimentary Rocks

  • Sedimentary rock forms when loose clasts (grains or fragments) are cemented together.
  • Clasts come from:
    • Weathering and erosion of existing rock (e.g., sand forming sandstone).
    • Accumulation of shells on the ocean floor (forming limestone).
    • Accumulation of organic matter from ancient plants (forming coal).

Metamorphic Rocks

  • Metamorphic rock forms when any igneous or sedimentary rock undergoes physical or chemical changes under pressure and increased temperature.
  • Metamorphic rocks are generally more compact, harder, and more resistant to weathering and erosion.
  • Caused by heating, pressure, heating and pressure together, and compression and shear.

Rock Cycle

  • The rock cycle illustrates the relations among igneous, sedimentary, and metamorphic processes.
  • Includes processes like weathering, erosion, transportation, deposition, compaction, cementation, melting, and metamorphism.

Global Water Distribution

  • About 71% of Earth's surface is covered by water, mostly saltwater.
  • Approximately 97% of all water is saline, and 2% is freshwater held in ice caps and glaciers.
  • Less than 1% of total water is available as freshwater for consumption.

Water Cycle (Hydrologic Cycle)

  • Collects, purifies, and distributes Earth’s fixed supply of water.
  • Powered by the sun.
  • Processes:
    • Evaporation: conversion of liquid water to vapor.
    • Condensation: water vapor turns into droplets in the atmosphere.
    • Precipitation: water returns to the surface as rain, snow, sleet, or dew.
    • Surface runoff: precipitation flows into streams, rivers, lakes, and oceans.
    • Seepage: some precipitation sinks into the soil and is used by plants.
    • Aquifers: Some precipitation sinks through soil into underground layers of rock, sand, and gravel, storing groundwater.

Components of the Water Cycle

  • Zone of aeration (unsaturated zone): contains air and little water.
  • Zone of saturation: spaces are filled with freshwater; the top is the water table.
  • Surface water: freshwater from rain and melted snow stored in lakes, reservoirs, wetlands, streams, and rivers.
  • Surface runoff: precipitation that doesn't soak into the ground or return to the atmosphere.
  • Watershed (drainage basin): land area from which surface runoff drains into a particular body of water.

Weather and Climate

  • Weather is the condition of Earth’s atmosphere at a specific time and place.
  • Climate is the aggregate of day-to-day weather conditions over a long period.
  • Weather phenomena mostly occur in the troposphere.
  • Driven by air pressure (temperature and moisture) differences.
  • Pressure and temperature differences vary with the sun’s angle (latitude) and altitude.
  • Temperatures usually range from +40 °C to -40 °C annually.
  • Coldest temperature: -89.2 °C at Vostok Station, Antarctica (1983).
  • Hottest temperature: 57.7 °C at Aziziya, Libya (1922).
  • Solar wind: movement of mass ejected from the Sun.

Elements of Weather and Climate

  • Temperature
  • Pressure
  • Wind
  • Humidity
  • Precipitation

Temperature

  • Temperature indicates how hot or cold the atmosphere is.
  • Influences precipitation, humidity, clouds, and atmospheric pressure.

Atmospheric Pressure

  • Force per unit area of a column of air.
  • Varies with density and decreases with altitude.
  • Wind is generated as air moves from high to low pressure areas.

Wind

  • Movement of air from high to low pressure areas.
  • Affects global ocean currents and precipitation patterns.
  • Caused by uneven heating of Earth’s surface.

Humidity

  • Amount of water vapor in the air (or relative humidity).
  • If humidity exceeds 100%, air is supersaturated, leading to fog, mist, or clouds.
    • Can result in dew (on surfaces) or frost (if below freezing).
    • Can cause rain, snow, or drizzle in the atmosphere.

Precipitation

  • Crucial part of weather.
  • Rain or snow droplets grow as they collect other droplets in a cloud.
  • Fall when they become heavy enough.

Types of Precipitation

  • Convective precipitation: warmed air near the ground rises. Air rises and cools forming clouds and precipitation.
  • Orographic precipitation: air is forced upward by a topographic obstacle (mountain).
  • Frontal precipitation: interaction of different air masses; warm air rises, expands, and cools, leading to precipitation.

Measuring Weather

  • Thermometer: measures temperature in Degrees Celsius (°C) or Fahrenheit (°F).
    • Conversion: (°C × 9/5) + 32 to convert to °F.
  • Barometer: measures air pressure.
  • Hygrometer: measures relative humidity.
    • Analog hygrometer: uses a coil or hair that expands/contracts with humidity.
    • Digital hygrometer: uses a humidity sensor.
  • Anemometer: measures wind speed using rotating cups or vanes.
  • Wind Vane: indicates wind direction; points from where the wind is blowing.
  • Rain Gauge: measures rainfall amount over a period of time (usually in mm).

Climate Classification

  • Köppen climate classification (Wladimir Köppen):
    • Five main types: A (Tropical), B (Dry), C (Temperate), D (Continental), E (Polar and Alpine).
    • Each type (except B) is defined by temperature criteria.
    • Type B designates climates controlled by dryness.

Alisov Climate Classification

  • A system commonly used in Eastern Europe.
  • Distinguishes four main climate zones: equatorial, tropical, temperate, and polar.
  • Transitional zones: sub-equatorial, sub-tropical, and sub-polar.
  • Air masses in transitional zones change seasonally.
  • Some zones contain specific climate regions (continental, maritime, monsoon).

Bahamas Climate Classification (Alisov)

  • Located north of the Tropic of Cancer (~25°N latitude).
  • Tropical maritime air masses dominate.
  • Classified under the Subtropical Climate Zone, specifically the Tropical Trade-Wind (or Maritime Subtropical) Climate.

Factors that Affect Climate

  • Energy from the Sun
  • Movement of Earth in space
  • Latitude
  • Atmosphere
  • Winds
  • Hydrosphere
  • Plate tectonics
  • Volcanic Eruptions
  • Human Activities

Earth and the Sun

  • Energy from the Sun is the most critical factor.
  • Solar magnetic activity leads to variations in total solar irradiance (TSI).

Movement of Earth in Space

  • Revolution Around the Sun: Earth orbits in an elliptical path taking approximately 365.25 days.
  • Rotation on its Axis: Earth rotates approximately every 24 hours.
  • Axial Tilt: The Earth's axis is tilted at approximately 23.5 degrees, causing seasons.
  • Orbital path: Changes from nearly circular to elliptical over 100,000 years affect the length and intensity of seasons.
  • Earth’s angle of tilt: Changes by ~2.40 over 41,000 years, affecting temperature differences between summer and winter.

Effects of Latitude

  • Sunlight intensity varies with latitude.
  • Tropical regions: high temperatures, abundant rainfall.
  • Mid-latitudes: temperate climates, distinct seasons.
  • Polar regions: cold climates, persistent cold temperatures.

Atmosphere

  • The greenhouse effect: natural warming of Earth caused by gases absorbing thermal energy.

Hydrosphere

  • Oceans and lakes act as heat reservoirs.
  • Snow and ice reflect heat (albedo).
  • Distribution of water, ice, snow, and land affects global temperature.

Plate Tectonics

  • Movement of plates results in new land masses, oceans, and mountain ranges, altering heat transfer, wind patterns, precipitation, and ocean currents.

Volcanic Eruptions

  • Spew ash and aerosols, reflecting solar radiation and causing cooling.
  • Add greenhouse gases (CO2), potentially increasing temperatures.

Human Activity

  • Burning of fossil fuels emits greenhouse gases.
  • Deforestation reduces CO2 absorption.
  • Wetlands stores and filters water, remove CO2 from the air, and provide a habitat for a variety of animals. Lower lake levels shrink wetlands

Greenhouse Gases

  • The greenhouse effect is crucial for maintaining a warm enough temperature to support life.

Bahamas Climate

  • Semi-tropical/subtropical marine climate moderated by the Gulf Stream.
  • Warm, humid conditions year-round.
  • Seasonal variations exist with more rainfall in the northwestern islands.
  • Mean daily temperatures fluctuate between 17°C and 32°C.
    • Temperatures: Mean temperatures have risen by 0.5°C (0.11°C/decade) since 1960.
    • Rainfall: Averages range from 865–1,470 mm, with seasonal droughts.
  • Subject to hurricanes and tropical cyclones.

Ecology

  • Ecology is the study of the relationship between living organisms and their physical environment.
  • Ecosystem is a set of organisms within a defined area interacting with each other and with their environment.
  • Ecosystem services are services provided by healthy ecosystems to support life and economies.

Ecosystem Types

  • Natural: Terrestrial (grassland, forest, desert) and Aquatic (marine: oceans, freshwater: lakes)
  • Artificial/Manmade: aquarium, crop field

Bahamian Vegetation

  • Blackland Coppice: Occurs away from the ocean on hilltops with a mix of less hardy trees and dark understory. Contains dogwood, lancewood, gumbo limbo and satin leaf trees.
  • Whiteland Coppice: A mixture of trees near the ocean that can withstand salt spray; a transition zone between the beach and the mangroves with white soil. Common trees are Giant Poisonwood, Sea Grape, Gumbo Limbo and Sabal Palmetto.
  • Rocky Coppice: A transition zone between mangroves and pine forest, ground is covered with limestone outcroppings and is often flooded during high tide. Commom trees through the rocky coppice are Ming, mahogany and red cedar.
  • Pine Forest: Found on northernmost islands dominated by Caribbean pine (Yellow pine) home to wild boars, feral cats, racoons, quail, wood doves, white-crowned pigeons, and migratory ducks.
  • Mangroves: Consist of four key species:
    • Red Mangrove: stilt-like prop roots, dominates deeper waters.
    • Black Mangrove: pneumatophores (snorkel roots).
    • White Mangrove: salt-excreting leaf glands.
    • Buttonwood: found on drier land, used in landscaping and traditional medicine.
  • Coral Reef: The Bahamas has one-third of the Caribbean's coral reefs, provide coastal protection, support fisheries, and contribute to tourism. Affected by Climate change, coral disease, coastal development, overfishing, pollution, invasive species, and careless recreation.

Soils

  • Dynamic natural material with water, air, minerals, and organic matter.
  • Basis for functioning ecosystems.
  • Soil texture refers to the proportion of sand, silt, and clay-sized particles in a soil sample.
  • Retains and filters water, habitat for microbial organisms, slow-release nutrients, and stores carbon dioxide.
  • Non-renewable natural resource that develops over long periods of time Soils develop over long periods of time bear the legacy of climates and geological processes over the last 15,000 years or more
  • Soil science is the interdisciplinary study of soil.
  • Pedology deals with the origin, classification, distribution, and description of soils.
  • Edaphology specifically focuses on the study of soil as a medium for sustaining the growth of higher plants.

Soil-Formation Factors

  • Five primary natural soil-forming factors: parent material, climate, organisms, topography and relief, and time.
  • Human activities also affect soil development.

Parent Material

  • Weathering of rocks provides raw mineral ingredients for soil formation.
  • Bedrock, rock fragments, and sediments. Clay minerals are the principal weathered by-products in soil.

Organisms

  • Biological activity is essential.
  • Vegetation, animals, and bacteria determine the organic content of soil.
  • Soil organisms help maintain soil fertility. Soil organisms range in size from land mammals that burrow into the ground , to earthworms that ingest and secrete soil, to microscopic organisms that break down organic matter. The actions of these living organisms help maintain soil fertility

Relief and Topography

  • Affects soil formation. Steep slopes cannot have full soil development and level lands may have drainage issues.
  • Orientation of slopes controls exposure to sunlight. In the Northern Hemisphere, a south-facing slope is warmer overall through the year because it receives direct sunlight
  • North-facing slopes are colder, causing slower snowmelt and a lower evaporation rate, thus providing more moisture for plants.

Time

  • All soil development factors require time. Rate tied to the nature of the parent material and climate. The rate of soil development is closely tied to the nature of the parent material (soils develop more quickly from sediments than from bedrock) and to climate (soils develop at a faster rate in warm, humid climates).

Soil Horizons

  • Soil profile: a vertical section of soil.
  • Pedon: three-dimensional representation of the soil profile.
  • Polypedon: a group of similar pedons.
  • Soil series: Several pedons or polypedons having similar soil profile properties and horizons are classified as a soil series.
  • Typical Soil Profile Horizons
    • O Horizon - Organic Layer
    • A Horizon - Topsoil
    • E Horizon - Eluviation Zone
    • B Horizon - Subsoil
    • C Horizon - Weathered Parent Material
    • R Horizon - Bedrock

Soil-Forming Processes

  • Weathering
  • Organic Matter Accumulation and Decomposition
  • Translocation
  • Bioturbation
  • Horizonation
  • Accumulation
  • Gleization

Weathering

  • Rocks disintegrate, dissolve, or break down.
    • Physical Weathering: Mechanical breakdown of rocks into smaller particles through freeze-thaw cycles, abrasion, or root growth.
    • Chemical Weathering: Alteration of minerals via reactions
    • Biological Weathering: Breakdown by organisms. Breakdown by organisms(e.g., lichens secreting acids, burrowing animals).

Organic Matter Accumulation and Decomposition

  • Accumulation: build-up of plant and animal-derived materials.
    • Sources include: plant litter (such as dead leaves, stems, and roots), root exudates (such as sugars, acids, and other compounds released by living roots), animal residues (faeces, dead insects, and microorganisms), and manure/compost (added by humans to enrich soil
  • Decomposition: Breakdown of organic material by soil organisms into simpler compounds.
    • Humification: Residual organic matter is transformed into humus. It improves soil structure (aggregation), enhances water and nutrient retention, and acts as a slow-release nutrient reservoir
      Mineralisation: Organic compounds are converted into inorganic nutrients such as Ammonium (NH₄⁺), Nitrate (NO₃⁻), Phosphate (PO₄ ³⁻), which are usable by plants.

Translocation

  • Movement of dissolved or suspended materials (clay, organic matter, salts, or iron/aluminum oxides) within the soil profile.
    • Leaching: downward movement of dissolved substances through the soil.
    • Eluviation: The loss of fine particles (clay, iron, aluminum, organic matter) from the upper soil layer (typically the E horizon) due to downward water movement.
    • Illuviation: The deposition of leached or eluviated materials (clay, iron oxides, humus) in a lower soil layer (typically the B horizon).

Bioturbation

  • Transport and mixing of soil by living organisms.
    Organisms actively rework soils and sediments by creating burrows, feeding, and seeking refuge leading to enhanced soil structure, fertility, and nutrient cycling, and plays a role in carbon sequestration and the distribution of organic matter

Accumulation

  • Build-up of specific minerals or compounds in the soil.
    • Calcification: Accumulation of calcium carbonate (CaCO₃) in subsurface layers
    • Salinisation - accumulation of soluble salts in the upper soil layers.
    • Laterization is the intense accumulation of iron (Fe) and aluminum (Al) oxides in tropical soils, creating a hard, brick-like layer.

Gleization

  • Soil formation process with a glei (or gley horizon) resulting from poor drainage conditions and waterlogged conditions.
  • The ‘gleization’ process creates soils called ‘gleys’

Erosion

  • Act in which earth is worn away, often by water, wind, or ice, is a part of the rock cycle.
  • Weathering, breaks down or dissolves rock, weakening it or turning it into tiny fragments.
  • Agents include: water, wind and glacier/ice.

Water Erosion

  • Moving water carries away bits of soil and slowly washes away rock fragments.
    • Splash erosion - The impact of a falling raindrop.
    • Sheet erosion - the transport of loosened soil particles by overland flow.
    • Rill erosion - development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes.
    • Gully erosion - accumulation of runoff water rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth.
  • Streams erode their banks through hydraulic action, corrosion, and particle abrasion.

Wind Erosion

  • Also known as Aeolian erosion; occurs mostly in deserts, forming sand dunes.
    • In dry areas, windblown sand blasts against rock with tremendous force, slowly wearing away the soft rock. It also polishes rocks and cliffs until they are smooth.

Glacial/Ice Erosion

  • Abrasion/Scouring
  • Plucking
  • Ice Thrusting

Gravitational Erosion

  • Mass movement is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of gravity.

Exfoliation

  • A type of erosion that occurs when a rock is rapidly heated up by the sun, resulting expansion of the rock.

Factors Affecting Erosion Rates

  • Precipitation and wind speed
  • Soil structure and composition
  • Vegetative cover
  • Topography
  • Human Activities
  • Agricultural practices
  • Deforestation
  • Roads and urbanization
  • Climate change

Prevention and Remediation

  • Windbreaks: rows of trees reduce wind speed to prevent erosion
  • Strip Cropping: alternating strips of crops to reduce soil erosion and improve soil fertility
  • Terracing: creating steps on hillsides reduce erosion and water runoff
  • Contour Cropping: planting crops in rows that follow the land’s curves reduce water runoff
  • No-Till Planting: planting seeds directly without plowing reduces soil erosion
  • Cover Crops: plants grown to improve the soil between main crops prevent soil erosion