Earths+Interior
Page 1: Introduction
Seeing Inside the Earth
Overview of the Earth's inner structure and methods used to study it.
Page 2: Key Concepts Review
Deformation and Mountain Formation
Mountain building is termed orogeny.
Stress vs Strain
Stress: Force per unit area.
Strain: Deformation resulting from stress.
Fault Types
Normal Fault: Occurs due to tension, where one block moves down relative to the other.
Reverse Fault: Resulting from compression, where one block moves up relative to the other.
Strike-Slip Fault: Horizontal movement of blocks across a fault line.
Oblique-Slip Fault: Combination of normal and strike-slip behavior.
Stable Regions: Areas of the continent that are less prone to seismic activity.
Physical Evidence of Faults: Includes offsets in geological features, scarps, and ground cracks.
Seismic Station Location: Importance of determining focus vs. epicenter of earthquakes.
Earthquake Causes & Locations: Related to tectonic plate movements and geological activity.
Depth of Earthquakes: Shallow vs. deep earthquake occurrences.
Seismic Wave Types: Body waves (P-waves and S-waves) and surface waves (R-waves and L-waves).
Page 3: Geophysical Structure of Earth
Crust: 0 - 70 km thick, composed of continental and oceanic crust.
Asthenosphere: 100 km thick, part of the upper mantle.
Mantle: Ranges from 70 km to 2885 km; consists of silicate rocks.
Outer Core: Liquid layer, depth of 2391 - 5155 km.
Inner Core: Solid, depth of 2391 - 6731 km.
Page 4: Basic Structure of Earth
19th-Century Model: Imagery of Earth as having three main layers:
Crust: Composed of lighter materials (continental vs. oceanic).
Mantle: Intermediate density.
Core: Heaviest materials, consisting of iron and nickel (ultramafic rocks).
Page 5: Composition of the Earth's Layers
Mantle: Composed primarily of ultramafic minerals (e.g., peridotite).
Divided into upper, transitional, and lower sections.
Core: Composed of an iron alloy, differentiated into outer (liquid) and inner (solid) parts.
Page 6: Understanding Earth's Interior
Sources of Information:
Volcanic xenoliths, ophiolites, deep mines, drilling projects, meteorites.
Seismic Waves: Analyzed from earthquakes to infer interior structures.
Page 7: Xenoliths: Indicators from the Mantle
Definition: Xenoliths are pieces of the mantle brought to the surface through volcanic activity.
Mainly consist of olivine-rich peridotite.
San Carlos olivine-mantle xenoliths sourced from depths of ~175 km near Peridot, AZ.
Page 8: Kimberlites and Diamonds
Kimberlites: Igneous rocks containing diamonds and xenoliths sourced from deeper crust.
Eruptions often brecciate the rocks, preserving fragments from lower crust and mantle.
Page 9: Ophiolites
Definition: Ophiolites represent slices of oceanic crust upthrusted during subduction processes.
Page 10: Geological Structures in Ophiolites
Features of Ophiolite Structures:
Includes sheeted dikes, plagiogranite, and various types of gabbros.
Observations of layered structures provide insights into crustal processes.
Page 11: Kola Superdeep Borehole
Overview: Reached 12.3 km; significant for understanding continental crust composition.
Aimed to study Baltic Shield, focusing on physical and chemical properties.
Page 12: Discoveries from Kola Borehole
Unexpected findings include:
Failure to encounter anticipated basalt layer at 7 km depth.
Deeper granites and water pooling at 3-6 km.
Presence of hydrogen gas and ancient plankton fossils at 6 km depth.
Page 13: Deepest Active Mine
Mponeng Gold Mine: Located in South Africa, at approximately 4 km depth.
Page 14: Limitations on Sampling Earth's Interior
Earth's Radius: Approximately 6,400 km; access to deep samples is very limited.
Page 15: Planetary Samples: Meteorites
Meteorites: Often remnants from planetary bodies that did not survive.
Important for studying composition and evolution of other celestial bodies.
Page 16: Types of Seismic Waves
P-waves: Compressional waves, fastest traveling through solids and liquids.
S-waves: Shear waves, only travel through solids.
R-waves: Surface waves causing up-and-down motions.
L-waves: Surface waves causing side-to-side motion.
Page 17: Seismic Wave Concepts
Wave Front: The boundary of a wave's influence.
Seismic Ray: Indicates positions of points on the wave front over time.
Page 18: Factors Affecting Wave Velocity
Density: Greater density slows P-wave velocities.
Rigidity: More rigid materials transmit waves faster.
Compressibility: Highly compressible materials allow for faster wave travel at depth.
Page 19: Wave Velocities in Different Rocks
Depth Dependence: P-waves travel at 8 km/s in peridotite, slower in sandstone.
Page 20: Material States and Wave Travel
Both P and S-waves can travel through solids; only P-waves can traverse liquids.
Wave speed varies significantly in liquid versus solid materials.
Page 21: Propagation Differences between Waves
P-wave Properties: Can propagate through both solid and liquid; induce compressive pulses.
S-wave Properties: Cannot propagate through a liquid; flows without generating shear waves.
Page 22: Repeat of Material States and Travel
Emphasizes differences between wave movement through liquid and solid states.
Page 23: Reflection and Refraction of Waves
Reflection: Wave bouncing off the boundary.
Refraction: Wave bending as it transitions between materials.
Page 24: Crust-Mantle Boundary
Discovery: Moho Line identified by differences in P-wave velocities, indicating a boundary at 25-70 km depth beneath continents.
Page 25: Seismographic Observations
Seismic Wave Detection: Close stations pick up crustal waves before distant stations detect mantle waves.
Page 26: Structure of the Mantle
Low-Velocity Zone: Found at depths 100-200 km due to partial melting of peridotite.
Seismic waves behave differently above and below this zone.
Page 27: Mantle Structures Overview
Upper and Lower Mantle: Defined by changes in wave velocities beneath 660 km.
Page 28: Core-Mantle Boundary Discovery
Early 1900s Work: Revealed a depth of 2900 km where P-waves were absent, indicating a significant density change.
Page 29: Nature of the Core
Core primarily consists of iron alloy; liquid outer core identified by S-wave shadows.
Page 30: Inner Core-Outer Core Boundary
1930s Studies: Detection of P-waves reflecting from core boundaries, confirming solid inner core presence.
Page 31: Structure of the Core
The core features a solid inner part and a liquid outer part due to variable pressure and temperature conditions.
Page 32: Seismic Reflection Techniques
Technique: Use natural or artificial seismic events to analyze layers within the crust for economic purposes.
Page 33: Seismic Reflection Profiling
Illustrative techniques for analyzing subsurface layers, vital for geological surveys.
Page 34: Seismic Reflection Cross-Sections
Detailed views of crust structures showcasing stratigraphy and underlying compositions through seismic methods.
Page 35: Seismic Tomography
Analogy to CAT scans; reveals temperature variations and supports the model of mantle convection currents.
Page 36: Tomographic Results
Results emphasize differences in seismic-wave velocities; colored representations indicate geological variations.
Page 37: Modern View of Earth's Interior
Artistic renditions of Earth's dynamic layers, including mantle convection and tectonic interactions.
Page 38: Understanding the Geoid
Geoid: Represents variations in Earth’s gravitational force and is not a perfect sphere due to density changes.
Page 39: Gravity Anomalies
Difference between observed geoid and reference geoid indicates strength variation in gravitational pull.
Page 40: Isostasy Principles
Archimedes’ Principle: Describes balance forces exerted by different strata, influencing their stability.
Page 41: Earth's Magnetic Field Structure
The magnetic field arises from movements in the outer core and differs from geographic orientations.
Page 42: Self-Exciting Dynamo Concept
The flow of molten metal in the outer core generates electric currents, maintaining magnetic fields throughout history.
Page 43: Lecture 14 Review
Consolidation of learned materials regarding seismic studies and Earth’s interior.
Page 44: Review Questions
Critical thinking prompts on Earth's internal structure and wave behavior are presented for assessment.
Page 45: Key Terminology
Important terms related to Earth's geology and seismic activities summarized for reference.
Page 46: Isostasy Animation
Visual representation of the isostatic adjustment in relation to ice weights on Earth's crust.