Earth structure and seismic waves

Earth's Subsystems

• Geodynamo system is the Earth’s magnetic field.
• Plate tectonic system circulates the Earth’s rocks.
• Climate system circulates the Earth’s air, water, and ice.

Earth's Chemical Layers

• Crust is the outermost layer.
• Mantle lies beneath the crust.
• Core is the Earth's center.

Earthquake Wave Types

• Body waves move through the interior.
• Surface waves move along the surface.

Types of Body Waves

• Primary waves (P-waves) are fast, compressional waves vibrating in the same direction they travel and can pass through solids and liquids.
• Secondary waves (S-waves) vibrate perpendicularly to their path and are slower,traveling only through solids.

Liquid Outer Core

• S-waves stop, and P-waves slow down when reaching the outer core.
• The outer core is liquid because S-waves cannot travel through it.

S-Wave Shadow Zone

• It's the area where no S-waves are detected after an earthquake.

P-Wave Shadow Zone

• It's the area where P-waves are detected, but with a delay.

Earth's Interior Classifications

• Chemical composition: crust, mantle, core.
• Mechanical properties: lithosphere, asthenosphere, mesosphere, outer core, inner core.

Crust vs. Lithosphere

• Both are solid, outermost layers.
• Lithosphere includes the crust and upper mantle.
• Crust is only the topmost layer.

Core Composition

• Inner core: Primarily iron and nickel.
• Outer core: Primarily iron and nickel.

Density of Earth Layers

• From least to most dense: crust, mantle, outer core, inner core.

Inner Core Temperature

• Roughly equals the surface temperature of the sun.
• The sun’s core is much hotter.

Earth's Magnetic Field

• Generated by the geodynamo.
• The geodynamo involves molten iron moving to create electric currents.
• Electric currents produce magnetic fields.

Lack of Martian Liquid Water

• Mars’ atmosphere is thin.
• Thin atmosphere leads to low atmospheric pressure.
• Low pressure causes any liquid water to evaporate quickly.

Most Voluminous Layer

• Mantle comprises about 84% of Earth’s volume.

Mantle Rock Type

• The mantle is made of peridotite rock.

Rocks vs. Minerals

• Minerals are naturally occurring with specific properties, composition, and structure.
• Rocks consist of two or more minerals mixed through geological processes.

Convection

• Convection occurs when liquid or gas heats up, expands, and decreases in density.

Convection in Earth Layers

• Mantle convects.
• Crust and solid inner core do not convect.

Types of Crust

• Continental crust is mostly granite, while oceanic crust is mostly basalt.
• Continental crust is thicker and less dense than oceanic crust.

Internal Layer Knowledge

• Seismic wave analysis provides information.
• Information is also obtained through volcanic studies and laboratory experiments.

Mantle Xenolith

• Rock fragment from the Earth's mantle carried to the surface by volcanic eruptions.

Meteorite Types

• Iron meteorites contain mostly iron metal.
• Stony meteorites contain mostly silicate minerals.
• Stony-iron meteorites are a mix of iron metal and silicate minerals.

Seismogram Identification

• Seismogram records earthquake details.
• P-waves arrive first, S-waves next, and surface waves last.

Earth's Magnetic Field "Bar Magnet"

• Resembles a bar magnet with poles and field lines.
• It influences compass needles and magnetic interactions.

Earth's Magnetic Field Differences

• The poles wander over time.
• Fluctuates in strength.
• Occasionally reverses.
• Generated by electricity flow in the outer core, driven by liquid metal convection.

Reconstructing Magnetic Field History

• Seafloor spreading: records magnetic field direction and strength when new crust forms.
• Ancient lava flows show the magnetic field direction when lava solidified.

Earth's Formation

• Formed from gas and dust clumping together via gravity.
• Cooled, forming layers, with the moon created from a massive collision.
• Surface solidified, allowing life to emerge.

Earth's Layered Structure

• Assembling debris created planet's layered structure.
• A volatile early atmosphere was created.
• Lastly, the moon was formed.

Age of Earth

• 4.54 billion years old

Determining Earth's Age

• Radiometric dating dates the materials.
• They rely on the decay of radioactive isotopes.
• Radiometric dating provides accurate age estimates for events including Earth's formation.

Dating the Solar System

• Stony meteorites (chondrites) are dated.
• Chondrites are the oldest, most unchanged meteorites, best estimating the Solar System's age.

Law of Superposition

• The oldest rock layer is at the bottom, with the youngest layer at the top of the layers.

Original Horizontality

• Sediments settle in flat, horizontal layers because of gravity.

Lateral Continuity

• Layers spread in all directions, with later separations caused by erosion or faulting.

Crosscutting Relations

• A rock or fault cutting through another is younger than the layer it cuts.

Geologic Record Contents

• Contains fossils, minerals, sediment layers, ice cores, chemical signatures, and geological structures.

Uniformitarianism vs. Catastrophism

• Uniformitarianism: gradual change from processes occurring today.
• Catastrophism: sudden events like floods and cave collapses.

Principle of Fossil Succession

• Older layers contain older organism fossils.
• Younger layers contain more recent organism fossils.

Lord Kelvin's Age Calculation

• Calculated Earth’s age by determining how long cooling would take from a molten state.

Isotopes

• Varying forms of an element with different neutron numbers and atomic masses.

Parent vs. Daughter Isotope

• Parent isotope: The original radioactive isotope decaying.
• Daughter isotope: The stable isotope formed from the parent isotope's decay.

Continent Coverage

• Continents cover about 30% of Earth's surface.
• Average elevation is about 840 meters.

Ocean Basin Coverage

• Ocean basins cover about 70% of Earth's surface.
• Average depth is about 3,800 meters.

Bimodal Elevation

• Earth has highland (continental) and lowland (oceanic) regions.
• These have different elevations.

Ocean Basin Ridges

• Caused by seafloor spreading at mid-ocean ridges.
• Tectonic plates move apart, and magma rises to form crust.

Formation of Mountains

• Mountains form due to tectonic plate movement.
• Movement leads to folding, faulting, volcanism, and uplift.
• Movement occurs at collision (convergent), separation (divergent), and sliding (transform) boundaries.

Tectonics Concepts

• Tectonics: study of Earth's surface changes.
• Plate tectonics: how the surface consists of moving plates.
• Continental drift: continents move (now understood as part of plate tectonics).

Vertical Tectonics

• Mountain ranges are rising like the Himalayas.
• Land sinking.
• Land is rising where ice sheets melted.
• Volcanoes create new land, pushing the crust up resulting in Earthquakes.

Glossopteris Distribution

• Pangea broke apart, spreading Glossopteris plants.

Alfred Wegener

• Proposed continental drift based on fossils, geology, climate, and continent shapes.

East Africa, Madagascar, and India

• East Africa was once with Antarctica and India.
• Madagascar was part of this mass.

Alexander Du Toit

• Tested continental connection by studying South American rocks.
• His evidence included similar rock formations, shared fossils, and matching geological structures.

Reginald Daly

• Proposed thermal convection currents.
• Daly's consistency with the knowledge of the time lacked solid understanding.

Arthur Holmes

• Proposed mantle convection drove continental drift in the 1920s.

Holmes vs. Plate Tectonics

• Holmes proposed mantle convection driving continental drift (1920s).
• Plate tectonics (1960s) linked mantle convection to plate movement.

Rejection of Continental Drift

• Oreskes argues that continental drift was not fully accepted due to the lack of evidence.
• There was reluctance to adopt a new theory.

Reasons for Rejection

• Lack of a plausible mechanism for continents to move and belief in a rigid Earth.

Lesson from Isaac Newton

• The scientific method; developing theories, testing them, and revising them based on evidence.

Karl Popper

• Science operates by proposing and testing theories to find falsifying evidence.

Thomas Kuhn

• Science operates within frameworks called paradigms.
• Paradigms shift radically, changing perspectives.

Scientific Study Without Experiment

• Possible through observation, data collection, and modeling.

Continental Drift (1935-1955)

• Unaccepted in North America/Europe due to limited evidence.
• Southern Hemisphere had receptive audiences.

Shift to Plate Tectonics

• Seafloor spreading was demonstrated.
• Subduction zones' role in crust recycling recognized.
• Sonar mapped the ocean floor, and precise measurement tools were developed.
• Fossils, earthquakes, volcanoes, and paleomagnetic data synthesized.

WWII Technological Innovations

• Sonar mapped the ocean floor and Magnetic anomalies suggested new crust creation at mid-ocean ridges.

Discovery Accelerators

• Magnetic striping on the ocean floor.
• Earthquake and volcano distributions mapped plate boundaries.
• Fossil distributions showed connected continents.
• Paleoclimatic evidence indicated continental shifts.

Apparent Polar Wander

• Revealed continental movement
• Records on different continents showed various polar paths. Earth’s crust is dynamic, and continents are part of moving tectonic plates.

Bathymetric Maps

• Revealed mid-ocean ridges and deep ocean trenches.
• They suggested crust creation at ridges and destruction at trenches.

Marie Tharp

• She created detailed maps of the ocean floor showing rift valleys.
• These supported seafloor spreading and plate tectonics.

Harry Hess

• Inferred ocean basins are young.
• Crust forms at ridges and recycles at trenches.
• This inference was based on magnetic anomalies in seafloor crust.

Guyots

• Submerged, flat-topped mountains were once volcanic islands.
• Guyots provide evidence for plate movement and seafloor spreading.

Hess's "Ocean Basins"

• Ocean basins form at mid-ocean ridges through seafloor spreading.
• Subduction destroys older crust in deep ocean trenches.

Seafloor Magnetic Stripes

• Magnetic minerals in the oceanic crust align with Earth’s magnetic field.
• This occured during crust formation at mid-ocean ridges.

Significance of Magnetic Stripes

• Demonstrated lithosphere dynamism.
• Supported tectonic plate motion.

"Magic" Pacific-Antarctic Ridge

• Symmetrical magnetic stripe arrangement provided seafloor spreading support.

Cold War Impact

• Advanced sonar and magnetic anomaly detectors.

Subduction Zones' Discovery

• Study of earthquake patterns and seismic tomography.

Events at Subduction Zones

• One tectonic over another.
• Crust melts, triggering earthquakes and volcanoes.

Types of Plate Boundaries

• Divergent: Plates move apart.
• Convergent: Plates move toward each other.
• Transform: Plates slide past each other.

Driver of Plate Motions

• Mantle convection
• Slab pull is the most significant force -Ridge push.

Ways to Melt a Rock

• Increase temperature.
• Decrease pressure.
• Add water.

New Oceanic Crust Formation

• Magma rises and solidifies at mid-ocean ridges.
• Plates pull apart.
• Crust pushes away from the ridge.

New Continental Crust

• Subduction ocean plate melts.
• Magma rises through an oceanic plate that sinks beneath a continental plate.
• This creates volcanoes and mountains.

Partial Melting

• Some minerals melt due to temperature, pressure, and sometimes water.
• Generates felsic magma, forming continental crust.

Low Melting Temp. Element

• silicon (Si)

Exotic Terrane

• Crust fragment transported from elsewhere.
• It attaches to another region through tectonics.

New England Assembly

• A series of exotic terrane collisions created New England.

Causes of Earthquakes

• Tectonic plates move.
• Results in stress building up.
• Stress released causes shaking.

Elastic-Rebound Theory

• Stress builds along a fault, rocks deform.
• Rocks break, releasing energy as seismic waves.

Stress Explained

• The force is applied to rocks.
• Normal stress (compression/tension) causes vertical movement.
• Shear stress causes horizontal movement, resulting in earthquakes.

Earthquake Locations

• Occur along tectonic plate edges

Earthquake Prediction

• We cannot.
• Probabilities are assessed based on seismic hazard zones.
• Monitor for foreshocks.
• Early warning systems can give brief warnings.

San Andreas Fault

• Prior to the 1906 earthquake, rocks moved slowly closer to the fault, where strain slowly accumulated.
• During the earthquake, rocks near the fault shifted significantly horizontally.

Stick-Slip Behavior

• Rocks accumulate stress, then suddenly slip.
• Elastic-rebound theory explains stress buildup and release in that behavior.

San Andreas Fault Hypotheses

• The time-predictable model is too simple because it does not take into account the Earth’s increased complexity

Composition of Brownstones Facades

• Facades are made of brown sandstone not brick.
• Brown sandstones were deposited in river and delta environments in foreland basins.

Plate Boundary Types

• Divergent: plates move apart forming new crust
• Convergent: plates move toward one another resulting in mountains or volcanoes
• Transform: plates slide past each other, which often causes earthquakes

Volcano/Earthquake Locations

• Along tectonic plate edges.

Volcano Producing Boundaries

• Convergent boundaries occur when oceanic plate collides with another plate (oceanic or continental) forcing it downwards into the mantle, melting and creating magma that rises to form volcanoes, process is called subduction

Iceland 1973 Volcanic Eruption

• Residents were quick to evacuate the town.

Mechanism Change

• They used cold seawater to cool the lava flow.

Volcanic Melt (Mid-Ocean Ridges)

• Decompression melting occurs where plates pull apart.
• Decompression reduces pressure mantle, allowing it to melt.

Volcanic Melt (Subduction Zones)

• Flux melting occurs in subduction zones with water addition
• Adding water lowers mantle melting point causing magma to form and leading to volcanic activity.

California’s Sierra Nevada (3 Myr Ago)

• Tectonic extension and normal faulting resulted in uplift.
• Valleys and fractured crust were formed creating a modern topographic version of that region of California.

Mid-Plate Volcanoes

• Caused by hotspots from rising mantle plumes forming volcanic chains to track movement of tectonic plates over time.

Surface Hot Spots

• Produced by mantle plumes.
• It consists of columns molten rock rises from Earth’s mantle.

Hot-Spot Track

• To determine the plate speed you calculate/divide - Distance between the youngest and oldest volcanoes/Time it took for the plate to move this distance

Volcanism Stages

• Intense activity, forms chains decreasing in volcanism.
• Followed by volcanic activity's declining.

Yellowstone

• The next volcanism center will be located west moving with North American plate.

Wei et al. (2020)

• Oceanic plateau, are located in the Emperor Seamount Chain and the Hawaiian Ridge, which are to the northwest of the present-day Hawaiian Islands.

Benefits of Tectonics Theory

• It's a good theory because it explains earthquake distribution, mountain formation, and the history of Earth's continents, phenomena that were previously difficult to understand.

Requirements for minerals

• Naturally occurring, solid, inorganic, and having an ordered atomic structure.

Describing Rock Characteristics

• We use mineral composition and texture.

Basic Rock Types

• Igneous: cooling magma/lava.
• Sedimentary: compressed sediments.
• Metamorphic: transformed by pressure, heat, or chemicals.

Rock Cycle

• Rocks change form through cooling, erosion, compaction, metamorphism, and melting.

Rock Types

• Sedimentary rocks cover most of the surface.
• Igneous rocks consist of most of the crust by volume.

"Equation of Life"

• Input-Output= Change in Storage

Stefan-Boltzmann Law

• The Stefan-Boltzmann Law is represented by the equation, F=σT^4, where F is energy flux, σ is 5.67 x 10-8 (W/m2)/K4, and T is temperature in kelvin (K).

Lithosphere and Deformation

• Primarily deforms at plate boundaries where pulling, or sliding.

Dipping Surface Orientation Description

• We require measurements of strike, dip, and dip direction.

Strike

• The horizontal direction.

Dip

• The angle of inclination of the surface from the horizontal.

Rock Responses to Stress

• Rocks deform by fracturing (brittle behavior) or flowing (ductile behavior).
• Temperature dictates the outcome when it comes to pressure, deformation rate, and stress.

Temperature's Effect on Rock

• High favors deformation.
• Low favors deformation.

Confining Pressure

• High favors deformation.
• Low favors deformation.

Deformation Rate: Effect on Rock

• Rapid: Fractures.
• Slow: Ductile flow.

Fault is Defined as..

• A fracture in the Earth’s crust where movement has occurred.

Dip-Slip Faults

• Vertical movement along the fault plane.

Dip-Slip Faults Types

• Normal Fault: Hanging wall moves down.
• Reverse Fault: Hanging wall moves up.
• Oblique-Slip Fault: Combination of vertical and horizontal movement.

Strike-Slip Faults

• Horizontal movement along the fault plane.

Strike-Slip Faults Types

• Right-Lateral (Dextral): Opposite side moves to the right.
• Left-Lateral (Sinistral): Opposite side moves to the left.

Strike-Slip Meaning

• The movement is horizontal, and the name comes from the strike (horizontal direction) and slip (movement along the strike).

Dip-Slip Types

• Normal Fault: Hanging wall moves down.
• Reverse Fault: Hanging wall moves up.
• Oblique-Slip Fault: Combination of vertical and horizontal movement.

Hanging/Foot Wall

• Rock above the fault is the hanging wall.
• Beneath the fault is the footwall.

Movement (Normal/Reverse)

• Normal Fault: Hanging wall moves down.
• Reverse/Thrust Fault: Hanging wall moves up.

Dip-Slip Faults (Typical Dip)

• Steep dip.

Typical Dips

• The primary movement is horizontal, with blocks of rock sliding past each other sideways, the dip of the fault is typically near 0°, meaning the fault plane is almost horizontal

Basin and Range

• Extensional tectonics are happening.
• The region is between the Sierra Nevada mountain range to the west and the Rocky Mountains to the east.

Basin and Range Tectonic History

• Normal faulting, crustal thinning, and volcanism has been in place ever since it has began.

Common Normal Fault Region

• Basin and Range region in the western United States

Thrust Fault Regions

• Himalayas, Andes mountains

Strike-Slip Fault Region

• San Andreas Fault in California.

Accommodates Tectonic Deformation

• Normal Faults: Extension (pulling apart).
• Thrust Faults: Compression (squeezing together).
• Strike-Slip Faults: Shearing (Sliding past).

Fold

• When layered rock bends or has a series of bends.

Anticline

• An upward-folding arch in the rock layers.

Syncline

• A downward-folding trough.

Cross Section Interpretation

• Anticline: In the shape of the letter “A”.
• Syncline: downward trough or "U" shape.