Lithosphere and Land-Forming Processes

Introduction to Geomorphology

• Etymology: The word geomorphology is derived from three Greek words:
  • Geo-: the Earth.
  • Morph-: Form.
  • -ology: the science of.
• Definition: Geomorphology is the study of landforms that pattern the Earth's surface and the processes that shape them. Geomorphologists seek to explain the shape, origin, spatial distribution, and development of terrain features, including the processes that modify or destroy them.
• Key Terminology:
  • Landform: An individual feature such as a slope, valley, or mountain. These vary in size and shape and develop over different time-scales.
  • Landscape: The combined effect of numerous landforms within an area, such as a mountainous or desert terrain.
  • Topography: A term used to describe the Earth's surface, specifically referring to the elevation and relief of the surface features (landforms).
  • Relief: The difference in elevation between high and low points on the Earth's surface.

Orders of Relief

Relief is classified into three descriptors based on scale and complexity:

• 1st Order Relief: The broadest landform divisions. This includes continental landmasses and ocean basins.
• 2nd Order Relief: Regional-scale continental and oceanic features:
  • Continental: Mountain ranges, plateaus, plains, and lowlands.
  • Oceanic: Continental shelves, slopes, abyssal plains, mid-ocean ridges, and trenches.
• 3rd Order Relief: Individual landform features that collectively make up the 2nd order relief. Examples include:
  • Volcanoes, glaciers, valleys, rivers, flood plains, lakes, marine terraces, beaches, and dunes.

Classification of Land-Forming Processes

Land-forming (geomorphic) processes are categorized into two primary systems based on their energy source:

• Endogenetic (Internal) Processes: Driven by energy from within the Earth (geothermal gradients and heat flow).
  • Slow/Diastrophic: Epeirogenic (upliftment, subsidence) and Orogenic (compression, tension leading to mountain building).
  • Sudden: Earthquakes and Volcanism.
• Exogenetic (External) Processes: Driven by atmospheric energy (solar radiation), gradients created by tectonic factors, and gravity.
  • Degradation: Weathering, mass wasting, and erosion (by river, glacier, wind).
  • Aggradation: Deposition (by river, glacier, wind).

Endogenic Processes: Volcanism

• Definition: The process where molten rock (magma) moves toward the Earth's surface. Volcanoes are the resulting landforms.
• Explosive Eruptions:
  • Violently blast molten and solid rock into the air.
  • Products: Volcanic ash fall, pyroclastic flow, and gas.
  • Impact: Can send ash clouds $> 25\,km$ into the stratosphere; causes severe environmental/climatic effects.
• Effusive Eruptions: Characterized by the slow outpouring of lava onto the ground.
• Determinants of Eruptive Nature:
  • Magma Viscosity: Resistance to flow. High viscosity (thick) traps gas, leading to explosions. Low viscosity (runny) allows gas to escape, resulting in effusive flows.
  • Silica Content: High silica ($\text{felsic magma}$) is cooler and thicker (e.g., rhyolitic/granitic lava). Low silica ($\text{mafic magma}$) is hotter and more fluid (e.g., basaltic lava).
  • Gas Content: High water vapor/gas leads to higher pressure and bigger explosions.

Volcanic Landforms and Structures

• Shield Volcanoes:
  • Formed by quiet, effusive eruptions of basaltic lava.
  • Broad, gently sloping sides; the largest type of volcano.
  • Example: Mauna Kea and Mauna Loa in Hawaii. Mt. Kilauea (active since 1983).
• Cinder Volcanoes:
  • Formed by explosive eruptions of granitic lava.
  • Smallest type, characterized by steep-sided slopes of loose pyroclastic material called tephra.
  • Example: Par=cutin in Mexico (appeared in a cornfield in 1943).
• Composite Volcanoes (Stratovolcanoes):
  • Mix of quiet and violent eruptions using intermediate andesitic magma.
  • Layered structure of tephra and lava.
  • Example: Krakatau (1883 eruption killed 36,000 people via tsunami and lowered global temperatures for five years).
• Additional Landforms: Lava flows (Pahoehoe and aa), Calderas (large depressions), and Plug domes (steep-sided mounds of viscous lava).
• Anatomy of a Volcano:
  • Crater: Top depression where material is ejected.
  • Vent: Conduit between the crater and magma.
  • Magma Chamber: Underground pool of molten rock.
  • Secondary/Parasitic Cone: Formed by magmatic branches reaching the surface through smaller vents.

Plutonism

Plutonism refers to igneous bodies (plutons) that cool and solidify beneath the Earth's surface. These are classified by size, shape, and relationship to surrounding rock:

• Batholiths: Large-scale plutons that have melted or pushed aside vast quantities of rock.
• Laccolith: Mushroom-shaped structure formed when magma bulges overlying rock layers upward.
• Sill: A horizontal sheet of intrusive rock between existing layers (no bulging).
• Dike: A wall-like structure formed when magma intrudes into non-horizontal fractures.
• Volcanic Neck: The exposed rock spire of a subsurface pipe from a defunct volcano.

Diastrophism and Earthquakes

• Diastrophism: All processes that move, elevate, or build the Earth's crust.
  • Epeirogenic (Faulting): Rocks break and move vertically. Up-thrown blocks are Horsts; down-thrown blocks are Grabens.
  • Orogenic (Folding): Rocks bend under compression. Anticline is a dome-shaped fold; Syncline is a U-shaped fold.
• Earthquakes: Caused by the sudden release of strain energy along faults. Energy radiates from the Focus as seismic waves.
• Measurement:
  • Intensity: Measured by the Modified Mercalli Intensity Scale (based on damage).
  • Magnitude: Measured by the Richter Scale (based on the amplitude of seismic waves).
• Tsunamis: Seismic sea waves resulting from underwater faulting or landslides. In the open ocean, height is $< 1\,meter$; in shallow coastlines, they can exceed $30\,meters$.

Exogenic Processes: Weathering

Weathering is the mechanical disintegration and chemical decomposition of rocks through weather and climate elements.

• Physical (Mechanical) Weathering: Fractures rock into finer particles without changing chemical composition.
  • Unloading: Expansion of rock when overlying pressure is removed.
  • Thermal Expansion: Expansion/contraction due to temperature cycles.
  • Freeze-Thaw (Frost Wedging): Expansion of water into ice within joints.
  • Salt Crystal Growth: Growth of crystals within rock pores.
  • Hydration: Absorption of water into mineral structures causing swelling.
• Chemical Weathering: Alteration of mineral composition, weakening rock structure.
  • Oxidation: Reaction with oxygen (e.g., rusting of iron minerals).
  • Carbonation: Carbon dioxide and water reacting with carbonate rocks like limestone.
  • Hydrolysis: Chemical interaction with water forming new compounds (e.g., silicates turning into clay).
• Biological Weathering: Disintegration caused by plants, animals, and microorganisms.

Erosion, Deposition, and Mass Wasting

• Erosion: Removal and transportation of material by water, wind, ice, or gravity.
• Deposition: Accumulation (aggradation) of eroded materials.
• Mass Wasting: Down-slope movement of rock debris under the direct influence of gravity. It ranges from slow (creep) to rapid (landslides).
  • Types: Rock fall, Rock slide, Mudflow, Solifluction, Earth creep, Slump, and Avalanche.
• Geomorphic Agents and Landforms:
  • Fluvial (Water): V-shaped valleys (erosional); Deltas, Alluvial fans (depositional).
  • Coastal (Waves): Sea cliffs, Arches (erosional); Beaches, Spits (depositional).
  • Glacial (Ice): U-shaped valleys, Cirques (erosional); Moraines, Drumlins (depositional).
  • Aeolian (Wind): Mushroom rocks (erosional); Sand dunes, Loess (depositional).
  • Karst (Groundwater): Sinkholes, Caves (erosional); Stalactites, Stalagmites (depositional).

Summary: Constructive vs. Destructive Processes

• Constructive: Build landforms (Tectonic/Endogenic processes and Deposition).
• Destructive: Break down landforms (Weathering, Erosion, and Mass Wasting).