Plate Tectonics and Earthquakes

• Notes on Plate Tectonics and Earthquakes

Plate Tectonics

• Students should be able to describe the complex processes that shape the earth, cause earthquakes, volcanoes, and result in changes in the configuration of Earth's landmasses and oceans.

Contracting Earth Hypothesis

• Mountains form as surface contracts and wrinkles
• Ocean basins form from the collapse of the surface
• Continents stay in place

Continental Drift Hypothesis

• Continents have been drifting to their present locations for millions of years
• Proposed by Alfred Wegener
• Supported by matching features such as fossils, rock formations, coastlines, and mountain ranges
• Supported by reconstruction of paleoclimates with glacial deposits and tropical coal beds
• Rejected because Wegener was considered an outsider using deductive reasoning
• Alternative land bridges provide explanations for organisms moving between continents

Pangaea

• Continents were once together as a single supercontinent landmass called Pangaea ("all lands")

Plate Tectonics Theory

• Earth's outermost layer is fragmented into a dozen large and small plates
• The plates are moving relative to one another above hotter "fluid" material

Earth's Internal Layers

• Core
  • Inner core is solid
  • Outer core is liquid
  • Generates magnetic field
• Mantle
  • Consists of low-density rocks made of oxygen, silicon, and magnesium
  • At the top rocks are molten allowing them to flow creating the asthenosphere sublayer
• Crust
  • Made of low-density rocks of oxygen and silicon
  • Lithosphere layer is broken into tectonic plates

Ocean Floor Features

• Continental shelf is a narrow extension of the continent submerged underwater
• Abyssal plain is relatively level seafloor in the deep ocean
• Oceanic Ridges are submarine mountain ranges which contain the world's volcanic activity found toward the center of oceans
• Oceanic Trenches are narrow, deep portions of the ocean floor
  • Most commonly found around the Pacific Ocean
• Ocean floor age varies where the oldest rocks are along the edges of oceans, and the youngest rocks are located at the oceanic ridges
  • Ocean floor rocks are younger than 200 million years ago
  • Rocks on the continent are as old as 4,000 million years
• Highest heat flow is along oceanic ridges

Wadati Benioff Zone

• Earthquake analysis revealed a region of earthquakes along ocean trenches that slope beneath volcanoes
• This region helps visualize the lithosphere position beneath the surface

Seafloor Spreading

• Magma (molten rock) from the mantle rises to create new ocean floor at mid-ocean ridges
• Around the ridge, seafloor moves towards deep-sea trenches, subducts (dragged into the mantle), and is recycled
• Evidence for seafloor spreading includes:
  • Heat flow
  • Paleomagnetism

Magma Formation

• Marine sediments exist
• Three changes lead to partial melting:
  • Increasing temperature
  • Decreasing pressure
  • Addition of water

Hot Spots

• Stationary regions in Earth's mantle continuously convect heat to the surface
• Used to track motion of plates

Plate Motion

• Fastest moving plates: Pacific, Nazca plates
• Slowest moving plates: Antarctic, North American plates

Plate Boundaries

• Divergent
  • Plates move apart e.g. oceanic ridges
  • Features: rift valley and basalt eruptions
• Convergent
  • Plates move toward each other e.g. subduction zones
• Transform
  • Plates slide past each other e.g. San Andreas fault, CA
  • Features: sections of ridge or trench systems
  • Plates move in opposite directions

Ocean-Ocean Plate Boundaries

• Types of features: where the older (colder) oceanic lithosphere is consumed at the subduction zone
  • When 2 oceanic plates collide, the older lithosphere is consumed in the subduction zone
  • Deep earthquakes

Ocean-Continent Plate Boundaries

• Oceanic plate collides with a continental plate and is consumed in the subduction zone
  • Mountain ranges form along active margins
  • Deep Earthquakes

Continent-Continent Plate Boundaries

• Thickening of continental crust forms the tallest mountain ranges
  • Only type of convergent boundary without an oceanic trench
  • No current volcanic activity
  • Earthquakes
  • No lithosphere is created or destroyed

Earthquakes

• Students should be able to describe how earthquakes form, how we measure them, and what hazards are associated with them.

Earthquake

• Earthquakes occur when the Earth's surface shakes due to the release of energy following the rapid movement of the lithosphere along zones of weakness

Faults

• Faults are where 2 massive sections of rock move past each other

Types of Faults

• Normal
  • Rock block above the fault plane moves down
• Reverse
  • Rock block above the fault moves up
  • In mountains, the angle of fault is low (<30 degrees)
• Strike-Slip
  • Blocks on either side of fault move horizontally, left or right

Earthquake Terminology

• Focus: Location where movement begins on fault
• Epicenter: Location on surface above the focus
• Fault Plane: Boundary between rocks
• Fault scarp: Step in the land surface that forms from the movement along the fault

Friction

• Friction along a fault causes faults to "stick"
• Elastic rocks deform (change shape) due to build up of stress
• After decades or centuries, stress has built up to sufficient levels to cause fault movement

Earthquake Distribution

• Most earthquakes occur along plate boundaries, relatively few in interiors of plates (90% occur along the ring of fire)
• Shallow earthquakes are more common than deep events
• Divergent plate boundaries often have shallow earthquakes
• Convergent plate boundaries see earthquakes along trenches and can be as deep as 800 km and tend to be larger in magnitude

Recurrence Interval

• Time between events
• Longer recurrence intervals (100s of years) are associated with the biggest earthquakes
• Decades of less for smaller events

Earthquake Potential Analysis

• Creep meters: Survey changes in positions between two points on opposite sides of a fault, if the distance changes deformation increases
• Strain meters: Measure the distortion of an originally circular hole
• Satellites: Use Global Positioning Systems to continuously monitor location of receivers on the ground
• Seismic gap: Analysis of a fault leads to the identification of a region in which an earthquake has not recently experienced movement

Seismic Waves

• Body Waves travel through the interior of the Earth
• Surface waves travel on the surface
• P-Waves:
  • Fastest and recorded first
  • Move through earth by compressing the rock through parallel to the direction of travel
  • Can move through liquids, gases, and solids
• S-Waves:
  • Recorded second
  • Move through earth by vibrating the material that the waves pass through perpendicular to the motion of travel
  • Can only move through solids
• Love waves produce surface shearing resulting from the earth moving side to side
• Rayleigh Waves:
  • Cause the surface to rise and fall like how ocean waves move through water

Measuring Earthquakes

• Seismograph records seismic waves. The record is referred to as a seismogram
• The time it takes seismic waves to reach a seismograph station increases with distance from the focus
• Arrival time of the waves and the difference in arrival times of P and S waves increases allowing for the determination of how far the earthquake is from the epicenter
• Data from three seismograph stations can pinpoint the location of the epicenter

Earthquake Strength

• Can be measured in terms of intensity, Richter magnitude, and moment magnitude
• Intensity
  • Earthquakes described in terms of people's experience, as well as damage done to nearby structures
  • Ranked using the Mercalli scale, ranging from 1 to 12 using roman numerals (I-XII)
• Richter Magnitude
  • Determined from the seismograph's data
  • The largest amplitude is used to determine the magnitude
  • Measures the amount of shaking.
  • No upper limit
  • Highest recorded was 9.5 in Chile 1960
• Moment Magnitude
  • Measures the total energy released by an earthquake
  • Calculated from the area of the fault that is ruptured and the distance the ground moved
  • Replaced Richter Scale
  • Each increase results in 32 times more energy released

Earthquake Hazards

• Ground Shaking
  • Seismic waves vibrate through the ground
  • Different types of ground materials respond differently to waves;thick layers with loose materials experience stronger shaking
  • The result is one for a tenfold times more shaking
• Ground Displacement
  • Relative movement of rocks along faults may cause elevation changes
  • Could cause disruption to infrastructure
• Landslides
  • Common where steep slopes and mountainous areas are
  • Direct/indirect effects: Deaths resulting from the burial of buildings, obstruction of roads
• Liquefaction
  • Loose sediments saturated with water experiences shaking causing the sediment to be suspended in the water - ground structures destabilize causing tilting
• Fires occur
  • Typically due to broken pipes and electrical wires

Tsunamis

• A set of long-wavelength oceanic waves triggered by disturbance like earthquakes, asteroid impacts, and volcanic eruptions
• Often go unnoticed in the deep ocean
• As the waves approach shore they're compressed allowing it to grow to tens of meters

Volcanoes

• Students should be able to describe the processes that lead to volcanic eruptions
• They will understand the difference between types of magma and their influence to the types of mountains and eruptions that can take place
• They will also know the different types of hazards and volcanic features

Volcano Statistics

• More than 1500 active volcanoes exist on Earth
• 500 million people live near them
• Fewer than 200 have instruments to assess their potential for eruption
• VDAP (Volcano Assistance program) was established after the Nevado del Ruiz eruption
  • takes measures to protect people by deploying monitoring instruments

Molten Rock

• Magma vs. lava: Magma still lies beneath the surface and lava has erupted
• Viscosity: Resistance to flow
  • Depends on temperature and composition
  • Hotter magma is less viscous.
  • Silica influences the viscosity in that higher silica content results in a higher viscosity

Volcano Eruptions Driven by Escaping Gases

• Gases dissolved in magma: Water vapor, carbon dioxide, sulfur dioxide
• Gases are released from magma as pressure decreases
• The ease in which those gases escape influence the type of eruption
  • Higher viscosity results in intense explosions

Mt St Helens

• The most active volcano in the conterminous US
• Forms along mountain range from the subduction of the plate beneath N. America
• Eruption in 1980
  • Increasing frequency of earthquakes
  • Deformation of the cone
  • Increase in release of volcanic gases
  • On May 18, magnitude 5.1 triggered a landslide along N side
  • Pressure released which resulted in a lateral blast

Measuring Eruptions

• VEI (Volcano Explosivity Index) measures the volume of erupted material on a log scale with 8 divisions

Volcano Types

• Basalt - Low silica content -Low viscosity
• Andesite - Int. silica content - Int viscosity from mixing melting rocks
• Rhyolite - High silica content - High viscosity which melts with continental rock

Volcano Hazards

• Tephra - Particles ejected from volcanoes can be ash or boulder size Can have measurable deposits hundreds of km away
• Volcano Gases
• Varying gases that can effect climate temperature in global warming or for blocking sunlight

Airborne Hazards

• Tephra represents particles blasted into air by eruption that vary in size
• Volcanic Gases made of water vapor, sulfur dioxide, carbon dioxide, reduces global temperature - Widespread release of carbon dioxide has warming effects

Flowing Hazards

• Lava Flows that flow far
• Pyroclastic Flows which is a cloud mixture
• Lahars are a mudflow mix

Shield Volcano

• Structures that are broad and gentle
• Made low viscosity lava flows and hot spots

Strato Volcano

• Steeper structures make alternating lava
• Over plate subductions

Cinder Volcanoes

• Small and built with magma composition

Common Volcano Features

• A Calera is formed in collapsed volcano
• Geysers are underground eruptions

Earths Rocks Composition

• Rock cycle links make conditions

Rocks and Minerals

• Minerals define
• Minerals have 5 qualifications
  • Natural and non-organic
  • Solid
  • Specific form
  • Specific structure
• Elements combine and found in crust.
• Atoms are the smallest form with protons to bond structure.
• Can have weaker or stronger atom structures

Minerals and Rock Bonds

• Minerals depend on composition
• 3 Groups depend on silicate atoms with oxygen
• Structure affects hardness on resisting a scratch
• Crystal form is how external is shaped

Rocks Main Definitions

• Rocks are specific that have igneous, sedimentary metamorphism:

Igneous

• Made with cooled magma and classified structure is either a volcano plutonic

Sedimentary

• Made from mineral form and must be combined
• Has 3 types with clastic and chemical

Metamorphic rocks

• Combined with rock texture with rock change.
• 2 changes with contacted and regional.