Principles of Geology

Lecture 3 Focus

• Geosystems, geology, geomorphology
• Endogenic and exogenic systems and their driving forces
• The time spans, into which Earth's geologic history is divided, the Anthropocene
• Guiding principles laws in geology
• Principles of uniformitarianism and superposition
• Earth's Structure and Internal Energy
• Earth Materials and the Rock Cycle

Why we need to understand the Natural Environment

• Human dependence on natural environment
• Resolution of environmental issues
• Intrinsic interest
• The Natural Environment consists of four spheres: Atmosphere, Hydrosphere, Lithosphere, and Biosphere
• It is a dynamic system
• It consists of an array of subsystems
• It is a multitude of interconnected processes
• It occurs across diverse spatial and temporal scales

Two Broad Earth Systems

• Earth possesses endogenic and exogenic systems
• The endogenic system encompasses internal processes that produce flows of heat and material from deep below Earth's crust
• The energy source for endogenic systems is heat from pressure (gravity) and radioactive decay
• Geology is the related branch of science
• The exogenic system consists of external processes at Earth's surface that set into motion air, water, and ice
• The energy source for exogenic systems is solar energy and the energy of motion (air, water, ice)
• Geomorphology is the related branch of science
• Diagrammatically:
  • Input to endogenic and exogenic systems
  • Action entails rock and mineral transformation, tectonic processes, weathering, erosion, transport, and deposition
  • Output includes crustal deformation, orogenesis and volcanism, and various landforms

The Geologic Time Scale

• The geologic time scale is based on absolute dating
• The overall time span is greater than 4.567 billion years
• 88.3% of it consists of the Precambrian Eon
• The last 2 million years consist of 0.04% of the time
• Eon > Era > Period > Epoch

Anthropocene

• Human activity affects Earth's systems to a greater degree and at a faster rate than natural forces
• 'Anthropocene' is 'the human epoch', which spans approximately 250 to 6000 years Before Present (B.P.)
• Novel stratigraphic signatures support the formalization of the Anthropocene at the Epoch level

Principles of Geology: Dating

• Absolute Time
  • Actual number of years Before Present (BP)
  • Absolute age is determined using scientific methods like radiometric dating
• Relative Time
  • Events are placed in a sequence of when they happened
  • Relative dating is based on the positions of rock layers above or below each other (stratigraphy)
• Key facts:
  • The oldest rock on Earth is about 4.0 billion years old
  • The oldest fossil is about 3.5 billion years old
• Radiometric Dating:
  • Half-life dictates the decay-rate
  • Time required for one-half of the unstable atoms in a rock sample to decay into "daughter" isotopes
• History:
  • 1669 Steno, N.: Law of superposition
  • Hooke, R.: Fossils as dating instrument
  • 1786 -Cuvier: Extinctions
  • 1799 –Smith, W. : Law of faunal succession, extinction
  • 1841: First global geological record
  • 20th century: Radio-dating sync palaeo and geological records
  • Stratigraphy guides the overall study of rock layers, including the principles, types of stratigraphic units and their interpretation
  • Relative dating is used within stratigraphy to place rock layers and events in chronological sequence

Stratigraphy Principles

• Principle of Original Horizontality
• Principle of Lateral Continuity
• Principle of Superposition
• Principle of Cross-Cutting Relations
• Law of Inclusions
• Law of Faunal Succession
• Superposition and Uniformitarianism
• Superposition: Rock and sediment are always arranged with the youngest bed (stratum) "superposed” toward the top of a rock formation and the oldest at the base, if undisturbed.
• Uniformitarianism: The same physical processes active today have been operating throughout geologic time
• Examples include earthquakes, volcanic eruptions, landslides, and orogenesis

Unconformities

• Unconformity: A pattern that signals erosion, sedimentary rocks, changes in sea level, and/or orogeny uplift
  • Disconformity: An erosional surface where the rocks below the unconformity are much older than the rocks above
    • Nonconformity: Forms when igneous or metamorphic bedrock is eroded, followed by horizontal layers of sedimentary rock deposited on top
    • Angular unconformity: Horizontal layers of sedimentary rock lie on top of tilted and uplifted layers of older sedimentary rock
    • Paraconformity: Similar to a disconformity but lacks evidence of erosion; instead, there was a pause in sediment deposition

Earth's Structure and Internal Energy

• The Earth formed from a nebula 4.6 billion years ago
• Silica and iron are two dominant materials
• As temperature decreased, Earth solidified and gravity sorted materials by density
• Core mass = 1/3 of Earth's mass
• Core volume = 1/6 of Earth's volume
• Magnetic poles and geographic poles are different
• The fluid outer core generates 90% of Earth's magnetic field because it is made of liquid molten metallic iron
• The north magnetic pole (NMP) is migrating
  • It is now at 85.1°N, 134°W
• Each layer exhibits distinct composition or temperature readings
• Heat energy migrates outward by conduction as well as by convection in plastic or fluid layers
• Seismic tomography helps identify boundaries
• Density changes can induce reflection and refraction of seismic waves
• Earth's Mantle: 80% of Earth's total volume, and density is 4.5g/cm³. The lower boundary is called the Mohorovicic (Moho) discontinuity
• Asthenosphere and Lithosphere
  • The asthenosphere is a plastic-like layer between the upper mantle and the uppermost mantle
    • It enables flow when a force is present and enables a rigid layer to bend or break
  • The Lithosphere consists of the crust and the uppermost mantle
  • Oceanic crust has an an average thickness is 5 km, and 30km for continental crust
• Continental crust is low in density (2700 kg/m3), composed of granite, and called sial (dominanted by silica and aluminum)
• Oceanic crust is denser (3000 kg/m3), composed of basalt and called sima (dominanted by silica and magnesium)
• Buoyancy (physics) states less dense items float in something denser
  • Earth's crust floats on the denser/elastic layers beneath it
  • The crust is in a constant state of compensating adjustment (Isostasy)
  • The crust sinks in regions of greater load, and rides higher isostatically with recovery from a load
  • Isostatic (iso = "equal,” static = “standstill") refers to the state of gravitational equilibrium between the lithosphere and asthenosphere
  • Tectonic plates “float” at an elevation dependent on thickness and density
• Is the balance between the buoyancy and gravitational forces, explain the vertical movements of Earth's crust
• Isostatic adjustment also occurs in Canada

Earth Materials and the Rock Cycle

• A mineral is an inorganic (nonliving) natural compound having a specific chemical composition and possessing a crystalline structure
  • A rock is an assemblage of minerals bound together, or a mass of a single mineral, or undifferentiated material, or even solid organic material (such as coal)
  • Three basic rock types: Igneous, Sedimentary, and Metamorphic
• rocks types are classified according to the processes that formed them:
  • Igneous (Most abundant class and formed from molten material)
    • Rapid cooling results in fine texture
    • Slow cooling results in coarse texture
    • Intrusive igneous rocks cool magma below the surface, such as granite
    • Extrusive igneous rocks cool lava on the surface, such as basalt
  • Sedimentary rocks are formed by lithification via cementation, compaction, dehydration, or heating
    • Sediment can be clastic, chemically precipitated, or organic, such as lime stones or coal, or precipitation of minerals
    • Metamorphic rocks are created when rocks are altered by heat, pressure, compression and shear
• More silica: Granite & Rhyolite
• Less silica: Gabbro & Basalt
• Pluton are any intrusive igneous body, including batholiths, laccoliths, sills, and dikes
• Sedimentary Rocks:
  • Clastic Sedimentary Rocks:
    • Formed from fragments broken off a pre-existing parent rock
    • Produced by Lithification (compaction and cementation by minerals, drying and heating)
    • Classified in accordance with by size of the sediment (shale, sandstone, conglomerate)
  • Chemical Sedimentary Rocks:
    • Produced from shells or by chemical precipitation i.e. inorganic mineral compounds derived from a saline solution
    • Forming karst topography, hydrothermal deposits and salt deposits; Ex.: halite and potash.
  • Biogeochemical sedimentary rocks:
    • Formed from organic deposits, i.e. material produced by plants and animals.
    • Ex.: chalk (skeletons and shell fragments) and coal (peat + compaction)
• Foliated rocks show a banding
• Nonfoliated rocks do not show a banding
• Contact metamorphism occurs in the upper crust when rising magma ""cooks"" adjacent rock.
• Regional metamorphism occurs deep in the lithosphere. The rock cycle has two cyclic systems that drive the same rock cycle:
  • The hydrologic cycle: occurs at and above Earth's surface and powered by sun
  • The tectonic cycle: occurs below Earth's surface and is powered by internal heat