Geology Exam Notes: Universe, Earth, Weathering, Igneous Rocks, Minerals, and Plate Tectonics
The Universe and Earth's Formation
The Universe is approximately (billion years) old.
Our Solar System formed around ago.
The Solar System is considered a second-generation system.
Elements heavier than iron require the immense energy of a larger star system supernova to form.
The Solar System's bulk composition is approximately Hydrogen (H) and Helium (He).
Planets in our Solar System are divided into two groups:
Inner rocky planets: Terrestrial planets closer to the sun.
Outer mostly gas planets: Jovian planets further out.
Earth's Composition and Interior Structure
Bulk Earth Composition:
Primarily composed of Iron (Fe), Magnesium (Mg), Silicon (Si), and Oxygen (O).
Earth's Crust:
Significantly more heterogeneous in composition compared to the bulk Earth.
Contains elements such as Iron (Fe), Silicon (Si), Aluminum (Al), Oxygen (O), Calcium (Ca), and Magnesium (Mg).
Earth's Interior:
Is distinctly layered.
At least part of the interior must be liquid, specifically the outer core.
Composition of Earth's Interior Layers:
Crust: Composed of many diverse elements.
Mantle: Rich in Magnesium (Mg), Silicon (Si), Oxygen (O), and Iron (Fe).
Outer Core: Primarily liquid Iron (Fe) and Nickel (Ni).
Its liquid nature is evidenced by the P-wave shadow and S-wave shadow zones observed in seismic wave studies.
Inner Core: Composed of solid Iron (Fe) and Nickel (Ni).
Weathering Processes
There are two main types of weathering:
Physical Weathering:
Involves the mechanical breakdown of rocks without changing their chemical composition.
Examples: Freeze-thaw cycles (water expands when it freezes in cracks), ice wedging, abrasion by waves, and biological activity (e.g., roots growing in cracks).
Dominates in cold, dry climates where ice formation and temperature fluctuations are common.
Chemical Weathering:
Involves reactions that alter the chemical composition of minerals and rocks.
Dominates in hot, wet climates due to increased reaction rates with water and higher temperatures.
Susceptibility: Is inversely related to Bowen's reaction series; minerals formed at higher temperatures (and thus higher on Bowen's series) tend to weather faster chemically.
Generally, most minerals dissolve faster at low pH (acidic conditions).
Types of Chemical Weathering:
Dissolution: The process where a mineral dissolves directly into ions in a solution.
Example: Calcite dissolving in water.
The dissolution of calcite-rich limestone along cracks and fissures leads to the formation of characteristic features like caves and karst topography, including sinkholes.
Hydrolysis: The reaction of a mineral with water (or and ions from water) to form a new mineral and release ions into solution.
Example: The reaction of K-feldspar with water, which forms clay minerals and releases various ions.
Oxidation Reactions: Involve the loss of electrons from a mineral, often occurring when oxygen in water reacts with iron-rich minerals.
Example: The weathering of pyrite (), an iron sulfide mineral, which oxidizes in water to make sulfuric acid () and iron oxides. This process is a major cause of acid mine drainage.
Human Impact on Weathering:
Humans have become the main agent of weathering for approximately the last years, primarily through activities like mining, agriculture, and construction.
Soil-Rock Boundary:
Often appears simple, but is more commonly a complex zone known as 'regolith', which includes loose, unconsolidated material covering solid rock.
Igneous Rocks
Formation: Igneous rocks form from the solidification (cooling and crystallization) of molten rock.
Magma vs. Lava:
Magma: Molten rock found beneath the Earth's surface. It contains dissolved gases called volatiles, such as (water vapor), (carbon dioxide), and (sulfur dioxide).
Lava: Molten rock that has erupted onto the Earth's surface. It typically has lost most of its volatiles to the atmosphere.
Classification of Igneous Rocks:
Field Relationships (where they cool):
Intrusive Igneous Rocks: Cool and solidify below the land surface, allowing for slower cooling and larger crystal formation.
Dikes: Discordant intrusive bodies that cut across existing rock layers.
Sills: Concordant intrusive bodies that emplace themselves parallel to existing rock layers.
Plutons: Large, irregular, km-scale bodies of intrusive igneous rock.
Batholiths: Very large, composite bodies formed from groups of multiple plutons.
Extrusive Igneous Rocks: Cool and solidify on the Earth's surface.
Examples: Lava flows, volcanic ashes, and pyroclastic deposits.
Texture (grain size):
Refers to the size, shape, and arrangement of mineral grains. Controlled by cooling rate.
Composition (silica content):
Classified based on their silica () content, which influences their color and mineralogy.
Mafic: Rocks rich in magnesium (Mg) and iron (Fe), typically dark-colored and low in silica (e.g., basalt).
Felsic: Rocks rich in feldspar and silica, typically light-colored and high in silica (e.g., granite).
Causes of Rock Melting:
Heating: Increase in temperature can melt rocks.
Pressure Drop (Decompression Melting): A decrease in overlying pressure can lower the melting point of rocks, even without an increase in temperature (e.g., at mid-ocean ridges).
Addition of Water (Flux Melting): The introduction of water or other volatiles can lower the melting point of rocks (e.g., in subduction zones).
Partial Melting:
Rocks do not melt uniformly; different minerals melt at different temperatures.
This process changes the chemical composition of both the melt (which moves away) and the residuum (the remaining solid).
Partial melting tends to concentrate silicate-incompatible elements (elements that don't easily fit into common silicate mineral structures) in the molten fraction.
Minerals
Definition: A mineral is a naturally occurring solid with a regular, repeated three-dimensional arrangement of atoms of particular elements. It is usually inorganic.
Formation: Minerals form as the planet's cations (positively charged ions) bond with the planet's anions (negatively charged ions) in a way that minimizes electrostatic charge and maximizes stability.
Cations must possess both the correct charge and ionic size to bond effectively with anions within specific geometric arrangements called coordination polyhedra.
These coordination polyhedra then link up, often incorporating additional cations and/or water molecules, forming complex mineral structures similar to Lego blocks.
Changes in temperature and pressure can cause these cations and anions to rearrange themselves into new, more stable mineral configurations.
Diversity:
Approximately total minerals are known to exist, grouped into about major families based on their chemical composition and structure.
The most common and important mineral families include silicates, carbonates, oxides, and sulfides.
Rapid identification rules (e.g., hardness, cleavage, color, streak) allow for quick identification of common minerals.
Color:
The color of most minerals is derived from trace elements, often transitional metals, present in small amounts, and not necessarily from the bulk composition of the main elements.
Plate Tectonics
Theory Origin: The theory of Plate Tectonics emerged from multiple lines of evidence indicating large-scale motion of Earth's surface over vast geologic time periods (>\text{millions of years}).
Key Observations and Evidence:
Fossil Distribution: Consistent patterns of fossil distribution across different continents, suggesting they were once joined.
Continental Coastlines: The apparent 'matching' shapes of continental coastlines (e.g., South America and Africa).
Mountain Belt Continuity: The continuity of specific mountain belts and rock types across present-day oceans.
Paleomagnetism: Measurements of Earth's magnetic field recorded in rocks over time, showing apparent polar wander and magnetic reversals.
Oceanic Crust Properties: Symmetry of age and magnetic polarity of oceanic crust on either side of mid-ocean ridges, indicating seafloor spreading.
Seismic Activity: The occurrence of most of Earth's earthquakes near mid-ocean ridges (divergent boundaries) and oceanic trenches (convergent boundaries).
Asthenosphere: The discovery of a weak, partially molten layer in the upper mantle at depths of (the 'asthenosphere'), identified through seismic wave analysis, which allows for plate movement.
Theory Description: The theory of Plate Tectonics describes Earth's outermost layer (the lithosphere, which includes the crust and uppermost rigid mantle) as consisting of approximately brittle plates. These plates are constantly moving relative to each other, driven by gravity (e.g., slab pull, ridge push) and sliding over a convecting mantle.
Plate Boundary Processes:
Mid-Ocean Ridges ('Spreading Centers'): Locations where new oceanic crust is continuously produced as magma wells up and solidifies.
Oceanic Trenches ('Subduction Zones'): Locations where oceanic crust is consumed as one plate dives beneath another into the mantle.
Explanatory Power: The plate tectonics paradigm successfully explains the global distribution of earthquakes, oceanic trenches, mid-ocean ridges, volcanoes, and various rock types.
Crustal Age Differences:
Continental Crust: Is significantly older, with rocks dating back up to . It is generally thicker and less dense than oceanic crust.
Oceanic Crust: Is much younger, with a maximum age of approximately (million years), reflecting its continuous production and destruction.
Hot Spots:
Isolated volcanoes located in the middle of oceanic plates (e.g., Hawaii, Iceland).
These form above stationary 'hot plumes' of mantle material that rise from deep within the Earth, creating volcanic activity as plates move over them.
Sediments accumulate predominantly on divergent margins
Convergent margins and transform margins both accumulate sediment, but in lesser amounts
Sediment is transported often by water, but also by wind and ice
Sediment transported by water travels by 3 modes, depending on flow velocity, grain size and chemistry
Bed load, suspended load, and dissolved load
Higher flow velocities are required to transport coarser sediment grains
Sediment sorting indicates relative distance of transport
Four main kinds of sedimentary rocks
Clastics (Conglomerates, sandstones, siltstones, mudstone)
Biochemical (limestones, dolomites, chalk, coal)
Carbonate sediments accumulate on ramps and platforms
Coal is organic matter from swamps
Evaporites gypsum, rock salt)
Other (volcanic ash, rare)
Sediments are lithified by a combination of compaction and cementation
Cements are minerals precipitating out of pore fluids
Quartz, calcite, Fe oxides all common cements, one usually predominates
Sedimentary structures indicate environments of deposition
Sedimentary structures: Bedding, Cross beds, Ripple marks
Sedimentary structures can be used to infer an environment of deposition
Deltas form where moving water meets still water (ocean, lakes)
Tide, River, waves all control sediment distribution and delta shape
Deltas need regular sediment supply to stay above water
Turbidites are gravity-driven 'underwater landslides'
Work like rivers, but in deep oceanic water
Common reservoirs for oil, gas, and CO2 storage