The Dynamic Earth: A Comprehensive Guide to Geologic Processes
Geologic Cycle
Endogenic Processes (Internal)
These processes involve plate tectonics and magma upwellings.
Exogenic Processes (External)
These processes involve wind, water, and erosion.
Igneous Rocks
Formed from the melting of magma (beneath the surface) or lava (magma that has extruded onto the surface). Examples include Granite and Basalt.
Intrusive Igneous Rocks
Cool below the surface. Examples include plutons (large bodies of intrusive igneous rock) and batholiths (very large plutons).
Extrusive Igneous Rocks
Cool above the surface. Make up 90% of the Earth’s crust.
Jointing
Parallel cracks in rocks.
Exfoliation
Peeling off of rock layers (e.g., domes).
Sedimentary Rocks
Formed from the settling of compounds through weathering, compaction, and cementation. Examples include Limestone and Sandstone.
Strata
Layers of sedimentary rock.
Clastic Sediments
Transported by water, wind, ice, or gravity.
Chemical Sediments
Formed from minerals dissolved in solution.
Metamorphic Rocks
Formed by altering existing rocks through heat and pressure. Examples include Marble, Gneiss, and Slate.
Foliated
Texture with distinct wavy lines of minerals.
Plate Tectonics
The theory that the Earth’s lithosphere is divided into plates that are in constant motion, driven by convection currents in the upper mantle. These plates move at a rate of approximately 6 cm/year.
Pangaea
A supercontinent that existed approximately 225 million years ago during the Triassic Period.
Tectonic Activity
The building and deforming of the Earth’s crust.
Sea-Floor Spreading
The process by which new oceanic crust is formed at mid-ocean ridges, where magma rises from the mantle and solidifies. The youngest crust is found at the ridges, while the oldest crust is found furthest away, dating back approximately 208 million years.
Subduction Zones
Areas where oceanic crust is forced beneath continental crust (which is lighter) and back into the mantle.
Mariana Trench
Located near Guam, the deepest part of the ocean, reaching 11,030 meters below sea level.
Plate Boundaries
Divergent Boundaries
Where plates are spreading apart (e.g., the Great Rift Valley in East Africa).
Convergent Boundaries
Where plates are colliding (oceanic-oceanic, continental-continental, or oceanic-continental). The 2011 Japan earthquake was caused by a convergent boundary.
Transform Boundaries
Where plates slide laterally past each other, creating no new crust (e.g., the San Andreas Fault).
Hot Spots
Areas of upwelling magma from the mantle that can create volcanic island chains (e.g., the Hawaiian-Emperor Islands chain, which has been forming for 80 million years).
Seamounts
Underwater mountains formed by volcanic activity.
Ring of Fire
A zone of volcanic and earthquake activity around the Pacific Rim, caused by the subduction of the Pacific Plate.
Volcanoes
Vents or conduits that allow magma to rise from the asthenosphere and upper mantle through the crust and onto the Earth’s surface. They create various landforms, including volcanic cones and mountains.
Magma Chamber
A reservoir of magma beneath the surface that collects before an eruption.
Crater
A round depression near the top of a volcano.
There are approximately 1300-1500 active volcanoes on Earth, with 500-600 considered active (having erupted in the last 10,000 years) and around 50 erupting each year.
Lava
Molten rock that has erupted onto the surface.
Pyroclastics (Tephra)
Chunks of rock and other materials ejected during an eruption.
Aa
Lava with a rough, jagged surface, formed by the cracking of the thick lava skin.
Pahoehoe
Lava with a smooth, coiled, rope-like surface.
Pele’s Hair
Spun volcanic glass, stretched from lava.
Types of Volcanoes
1. Cinder Cone
Small, short, cone-shaped hill formed from previous eruptions, made of pyroclastic material and scoria.
2. Caldera
A basin-shaped depression formed after the top of a volcano collapses (also known as a kettle). Can often become lakes.
3. Flood Basalts (Plateau Basalts)
Long blankets of lava spreading over the surface, with a horizontal nature. Lava comes out of fissures (linear vents).
4. Shield Volcano
Resembles a knight’s shield lying face-down, with a gentle and gradual slope, resulting in a crater at the top. Formed from effusive eruptions.
5. Composite Volcano (Stratovolcano)
Formed by massive explosive eruptions. Cone-shaped with a steep slope.
Types of Eruptions
Effusive Eruptions
Gentler eruptions with fluid, low-viscosity magma, allowing gases to easily escape. Few pyroclastics and small explosions.
Explosive Eruptions
More violent eruptions with high-viscosity magma, trapping gases and building pressure. Large pyroclastics and little lava (e.g., Kilauea in Hawaii, Mount St. Helens).
Volcanic Locations
Volcanoes are typically found in subduction zones (continental-oceanic, oceanic-oceanic), spreading zones (sea-floor spreading, continental rifts), and hot spots.
Earthquakes
Caused by the release of energy along fault lines in the Earth’s crust.
Types of Stress
Tension
Stretching of rock strata, producing thinning of the crust and normal faults.
Compression
Shortening of the crust, resulting in folding of the crust and reverse faults.
Shear
Twisting or tearing motion, causing the crust to bend and strike-slip faults.
Strain
The deformation of rocks under stress.
Folding
When rocks bend under pressure. Ridges are the top peaks, while troughs are low valleys.
Anticline
When rock layers slope downward and away from the top.
Syncline
When rock layers slope downward and in towards the bottom.
Plunges
When the axis of a fold is not parallel to the surface.
Overturned Anticlines
When folds are pushed together so much that they start to topple over.
Faulting
When rocks break under pressure. The sudden release of energy during a fracture causes an earthquake.
Types of Faults
1. Normal Faults
Result from tension stress, where rocks are pulled apart. After fracture, one side will move vertically. The side of the fault that moves downward is the hanging-wall side, and the higher side of the fault is the footwall side. Cliffs formed are called fault scarps or escarpments.
2. Thrust or Reverse Faults
Result from compression stress, where rocks are pushed together. In reverse faults, the hanging wall moves upward, hanging above the footwall side (can cause landslides). If the fault plane is at a low angle, one block will slide on top of the other, creating a thrust fault (e.g., the 1994 Northridge earthquake in Los Angeles).
3. Strike-Slip Faults
Result from shearing stress, where fault movement is horizontal. Can be right-lateral or left-lateral. No cliffs are formed, but rift valleys can be created (e.g., the San Andreas Fault).
Fault-Related Landforms
Horst
Rock blocks that have faulted upward.
Graben
Rock blocks that have faulted downward.
Basin and Range
A type of topography with abrupt relief and angular rock structures.
Bolson
A low area between two ridges, with slopes and a basin.
Playas
Salt crust areas left behind when evaporation occurs in a bolson or valley.
Orogenesis
The process of mountain formation, often caused by the accumulation of materials that thicken the crust, followed by uplift.
Seismic Waves
Waves of energy that travel through the Earth during an earthquake.
Surface Waves
Move along the top of the crust.
P-Waves (Pressure Waves)
Compressional waves that cause the rock or magma they move through to vibrate parallel to the wave motion.
S-Waves (Shear Waves)
Cause rock to move at a 90-degree angle to the wave motion (cannot travel through liquids).
Earthquake Terminology
Focus (Hypocenter)
The point below the surface where an earthquake originates along a fault.
Epicenter
The point on the surface directly above the focus.
Foreshock
Shaking that happens before the main earthquake event.
Aftershock
Shaking that happens after the main earthquake event (usually smaller).
Measuring Earthquakes
Richter Scale
A logarithmic scale that measures the magnitude of seismic waves.
Seismograph
An instrument that records vibrations in the Earth.
Landforms
Individual aspects of the Earth’s topography (e.g., mountains, valleys, hills, caves).
Landscape
A collection of landforms in one area.
Geomorphology
The study of landforms.
Denudation
Any type of wearing away or moving of landforms (e.g., by water, wind, waves, ice), including weathering.
Endogenic Processes
Tectonic uplift creates the initial landscape, the base point for landform development.
Exogenic Processes
Processes such as weathering and erosion that work to wear down the landscape.
Destabilizing Event
A physical event (e.g., fire, volcanic eruption) that creates a new starting point for landform development.
Geomorphic Threshold or Tipping Point
Reached when the system’s energy exceeds the resistance of the rocks.
Stages of Landform Development
- Equilibrium Stability
- Destabilizing Event
- Period of Adjustment
- New Equilibrium Stability
Erosion
The transportation of weathered material away from its source (including large-scale mass movements). Factors that cause erosion include water, glaciers, wind, and waves.
Deposition
The process of material being set down in a new place.
Differential Weathering
When the top layer of rock protects the rock beneath it from weathering.
Slopes
The curved, inclined surfaces of landforms.
Waxing Slope
The convex part of the slope (at the top).
Waning Slope
The concave part of the slope (at the bottom).
Types of Weathering
Mechanical Weathering
Physical breakdown of rocks (e.g., frost action, salt-crystal growth, pressure-release jointing).
Blockfield
An area where rock pieces collect around a central point.
Talus
A steep slope formed by boulders falling and accumulating.
Chemical Weathering
Breakdown of rocks through chemical reactions (e.g., hydration, hydrolysis, oxidation, carbonation).
Biological Weathering
Breakdown of rocks by living organisms (e.g., lichen taking nutrients from rocks).
Angle of Repose
The steepest angle at which a slope can remain stable.
Types of Mass Movement
Creep
Slow, downslope movement of soil and rock.
Solifluction
In cold climates, the slow downslope movement of soil and rock due to alternate freezing and thawing.
Slide
Rapid downslope movement of a mass of earth material.
Slump
A slide that rotates on a curved surface.
Flow
Fluid movement of earth material (e.g., earthflow or debris flow).
Fall
Rapid, free-fall of earth material from a cliff or steep slope.
Groundwater
Water that exists below the ground surface.
Permeable vs. Impermeable
Permeable materials allow water to pass through, while impermeable materials do not.
Infiltration
The downward movement of water into the soil or rock.
Vadose Zone (Zone of Aeration)
The zone of soil that contains both water and air.
Phreatic Zone (Zone of Saturation)
The zone of soil where all pores are filled with water.
Water Table
The top of the phreatic zone.
Aquifers
Permeable layers of soil or rock that can store and transmit groundwater (e.g., sandstone and limestone).
Aquicludes
Impermeable layers that prevent the movement of groundwater.
Fluvial Processes (Streams and Rivers)
Hydrology
The study of water and its movement.
Stream Orders
A system for classifying streams based on their size and tributaries.
First-Order Streams (Primary Streams)
The smallest streams, with no tributaries.
Second-Order Streams
Formed by the confluence of two first-order streams.
Third-Order Streams
Formed by the confluence of two second-order streams, and so on.
Types of Streams
Perennial Streams
Flow year-round.
Ephemeral Streams
Flow only during the rainy season.
Intermittent Streams
Flow for some parts of the year.
Watershed (Drainage Basin)
The area of land that drains into a particular stream or river.
Drainage Divides
Boundaries that separate watersheds.
Drainage Patterns
Dendritic
Resembles the branching pattern of a tree.
Trellis
Resembles a vine growing on a trellis.
Radial
Streams flow outward from a central point.
Deranged
No clear pattern.
Fluvial Processes
Sheetflow
Water moves downslope in a thin sheet.
Rills
Small indentations or cracks that collect water.
Gullies
Larger rills.
Fluvial Discharge
The amount of water that moves through a stream channel during a given time (measured in cubic meters per second or cubic feet per second).
Discharge Equation
Q = wdv (Q = discharge, w = width, d = depth, v = velocity)
Fluvial Landforms
Landforms created by erosion, transportation, and deposition by streams and rivers.
Base Level
The lowest elevation to which a stream can erode its channel.
Hydrograph
A graph that shows the discharge of a stream over time.
Base Flow
The low discharge amount during dry seasons.
Peak Flow
The highest amount of discharge after a precipitation event.
Erosion
Depends on turbulence (how rough the water flow is) and abrasion (how rock particles in the water scrape along the sides of the channel).
Transport of Sediment
Dissolved Load
Material dissolved in the water.
Suspended Load
Small particles that move with the water and eventually settle at the bottom.
Bed Load
Larger pieces of material that are transported along the streambed.
Deposition
Aggradation
The process of a stream channel being filled by deposition.
Alluvium
Material that has been deposited by a stream (often fertile).
Fluvial Depositional Landforms
Alluvial Fans
Fan-shaped landscapes formed when an ephemeral stream empties out from a narrow canyon onto a flatter surface.
Undercut Banks
Areas of high velocity in a stream channel, where erosion occurs. The opposite side of the channel will have point bar deposits, where sediment is deposited.
Oxbow Lake
A cut-off portion of a river meander.
Delta
A triangular shape of alluvium formed at the mouth of a river.
Floodplain
A flat, low-lying area adjacent to a river that is subject to flooding.
Levees
Barriers that run parallel to a river, formed by the deposition of sediment during floods.
Karst Topography (Cave Systems)
Formed by the dissolution of soluble rocks, such as limestone, by acidic water (a type of chemical weathering called carbonation).
Glaciers
Large masses of ice that form on land and move under the force of gravity.
Types of Glaciers
Alpine Glaciers
Form in mountainous areas.
Continental Glaciers (Ice Sheets)
Much larger than alpine glaciers and cover vast areas of land.
Cirque Glaciers
Small glaciers that form in hollows near mountain peaks.
Valley Glaciers
Flow downhill through valleys.
Tidal Glaciers
Reach the sea and break off into icebergs.
Glacier Formation
Glaciers form from the accumulation and compaction of snow over time. Snow transforms into firn (granular snow) and eventually into glacial ice.
Accumulation Zone
The area of a glacier where snow accumulates and ice is added.
Ablation Zone
The area of a glacier where ice is removed through melting, sublimation, or calving.
Glacier Movement
The lower part of a glacier behaves plastically, allowing it to flow downhill.
Glacial Erosion
Plucking (Quarrying)
The process of a glacier snapping off pieces of rock.
Abrasion
The process of a glacier grinding and polishing the underlying rock surface.
Glacial Erosional Landforms
Roche Moutonnée
A rock formation shaped by glacial erosion, resembling a sheep lying down.
Cirque
A bowl-shaped depression carved by a glacier.
Arête
A sharp ridge formed between two cirques.
Horn
A mountain peak carved by multiple glaciers.
Col
A low-lying pass or saddle between two horns or arêtes.
Glacial Deposition
Glacial Drift
All material deposited by a glacier.
Erratics
Large rocks transported by a glacier and deposited in a different location.
Moraines
Landforms created by the deposition of glacial till.
Terminal Moraine
Marks the furthest extent of a glacier’s advance.
Medial Moraine
Formed where two glaciers merge.
Lateral Moraine
Formed along the sides of a glacier.
Esker
A long, winding ridge of sand and gravel deposited by a meltwater stream flowing within or beneath a glacier.
Till
Unsorted glacial sediment.
Outwash
Glacial sediment that has been sorted and deposited by meltwater streams.
Talik
Unfrozen ground beneath a glacier or permafrost.