Earth and Space Science: Volcanoes and Earthquakes

Earth and Space Science: Volcanoes and Earthquakes

Zones of Volcanism

  • Volcanism Defined: Describes all processes associated with the discharge of magma, hot fluids, and gases.

  • Location of Volcanoes: Most volcanoes form at plate boundaries, mainly at:

    • Convergent Boundaries: Plates collide, leading to significant geological activity.

    • Divergent Boundaries: Plates move apart, which allows magma to rise.

Convergent Volcanism

  • Oceanic-Continental Subduction Zone:

    • The denser oceanic plate slides under the lighter continental plate into the mantle.

    • Melting of the mantle above the subducting plate occurs, leading to magma rise and volcano formation.

  • Characteristics: Volcanoes formed in these zones typically have explosive eruptions. Most land-based volcanoes result from this process.

Major Volcanic Belts

  • Two Major Belts of Volcanoes Associated with Convergent Boundaries:

    • Circum-Pacific Belt: (also known as the Pacific Ring of Fire)

    • Bigger in size.

    • Corresponds to the outline of the Pacific Plate.

    • Mediterranean Belt:

    • Smaller than the Circum-Pacific Belt,

    • Corresponds to boundaries between the Eurasian, African, and Arabian Plates.

Divergent Volcanism

  • Eruption Characteristics: Eruptions at divergent boundaries are typically nonexplosive.

  • Formation of Pillow Lava: At ocean floor divergent boundaries, eruptions produce large formations of lava known as pillow lava due to their shape.

Hot Spots

  • Definition: Areas in the mantle characterized by high-temperature plumes of mantle material rising toward the surface, resulting in volcanic activity away from plate boundaries.

  • Example of Hot Spot Volcanoes:

    • Some volcanoes, like those on Kauai in Hawaii, are inactive because the island has moved off the hotspot.

    • The most active volcano is Kilauea on the Big Island of Hawaii, currently sitting above the hotspot.

  • Volcanic Chains: Volcanoes formed over stationary hot spots offer insights into plate motions; by analyzing their positions, scientists can calculate the rate and direction of plate movements.

  • Example of a Volcanic Chain: The Hawaiian-Emperor volcanic chain, with its oldest volcano, Meiji, estimated to be about 80 million years old.

Flood Basalts

  • Formation Process: Occur when lava flows from long cracks in the Earth's crust, known as fissures.

  • Example: Columbia River basalts in the northwestern USA were formed this way.

  • Deccan Traps:

    • Location: India.

    • Historical Significance: An enormous flood basalt eruption created this plateau around 65 million years ago.

    • Estimated Volume: About 512,000extkm3512,000 ext{ km}^3 of basalt.

Anatomy of a Volcano

  • Structure Overview:

    • Conduit: A tubular structure through which lava travels to the surface.

    • Vent: The opening through which lava emerges.

    • Crater: A bowl-shaped depression at the volcano's top, usually less than 1 km in diameter.

    • Calderas: Larger depressions, which may be up to 100 km in diameter.

Types of Volcanoes

  • Two Key Factors Influencing Volcano Appearance:

    1. Type of material that forms the volcano.

    2. Types of eruptions that occur.

  • Shield Volcanoes:

    • Characteristics: Broad, gently sloping sides, nearly circular base.

    • Formation: Layering of lava during nonexplosive eruptions; they are the largest type of volcano.

  • Lava Domes: A type of shield volcano with lava that piles up around the vent.

  • Cinder Cones:

    • Formation: Created by small lava pieces (tephra) ejected into the air and accumulating around the vent.

    • Characteristics: Steep sides, smallest type of volcano (also called pyroclastic cones).

  • Composite Volcanoes:

    • Composition: Alternating layers of ash, hardened lava chunks from violent eruptions, and lava that flows downslope.

    • Characteristics: Cone-shaped with concave slopes (also known as stratovolcanoes).

Earthquakes: Stress and Strain

  • Mechanism at Plate Boundaries: Rocks in crust resist movement, and stress builds up.

    • Definition of Stress: Total force acting on crustal rocks per unit area.

  • Types of Stress:

    1. Compression: Shortens material.

    2. Tension: Lengthens material.

    3. Shear: Distorts or twists material.

  • Strain: Deformation of materials in response to stress.

Earthquakes: Stress and Strain (Continued)

  • Failure of Rocks: Rocks can twist, squeeze, or stretch but will fracture when stress and strain hit a critical point, releasing energy.

  • Result: This energy release is what causes earthquakes.

Types of Deformation

  • Elastic Deformation: Occurs under low stress; materials return to original shape when stress is removed.

  • Plastic Deformation: Happens when stress exceeds the elastic limit, causing permanent deformation. At high temperatures, solid rocks may deform fluidly, reducing stress.

Earthquakes: Faults

  • Failure Mechanism: Crustal rocks fail when stress exceeds their strength, resulting in movement along a weakness called a fault—a fracture or system of fractures along which movement occurs.

Types of Faults

  • Reverse Faults: Created by horizontal and vertical compression leading to shortening; rock on one side is pushed up relative to the other.

  • Normal Faults: Involves partly horizontal and part vertical movement, pulling rocks apart and stretching the crust; vertical movement results as one side moves downward.

  • Strike-Slip Faults: Caused by horizontal shear, with movement occurring horizontally in opposite directions, akin to vehicles moving alongside each other on a freeway.

Earthquake Waves

  • Seismic Waves: Result from vibrations produced during an earthquake; irregular rock surfaces can lock and snag, causing elastic deformation and, upon exceedance of the elastic limit, movement.

  • Types of Seismic Waves:

    1. Primary Waves (P-waves): Squeeze and push rocks in the direction the wave travels.

    2. Secondary Waves (S-waves): Move slower than P-waves; they have perpendicular motion to the wave direction.

    3. Surface Waves: Slowest waves, traveling along the Earth’s surface, causing sideways and up-and-down movement.

Seismometers and Seismograms

  • Seismometers: Instruments that measure motions during an earthquake. They consist of a frame anchored to the ground; during a quake, the frame moves while a hanging mass records movement.

  • Seismograms: The output from seismometers, showing individual tracking of seismic wave types.

Earthquake Focus and Epicenter

  • Focus: The initial rupture point of a fault, generally several kilometers below the surface.

  • Epicenter: The point on the Earth’s surface directly above the focus.

  • Measurement of Wave Distance: The separation of seismic waves on seismograms from different distances aids in determining the epicenter's location.

Travel-Time Curves

  • Usage: Average timing data for P- and S-waves to reach seismic stations depend on distance from the epicenter, indicated by travel-time curves.

Visualizing Seismic Waves

  • Function: Travel times and seismic wave behavior provide a detailed understanding of Earth's internal structure, revealing hints about its composition.

Clues to Earth’s Interior

  • Seismic Imaging: Speed of seismic waves varies with temperature and composition; mapping seismic wave paths can generate images of internal structures.

  • General Observation: Seismic wave speeds decrease as temperature increases.

Earthquake Magnitude and Intensity

  • Richter Scale: Developed by Charles Richter; provides a numerical rating of earthquake magnitude based on energy released (measured by the height of the largest seismic wave's amplitude).

  • Magnitude's Impact:

    • Magnitudes correlate with potential damage and energy release; specific examples highlight historical earthquakes and their implications.

Selected Earthquake Data

  • Example Earthquake Impacts:

    • San Francisco, CA (1906) - Major devastation, high loss of life.

    • Chile, 2010: M 8.8 earthquake highlighted for destructive capacity and severe socioeconomic consequences.

Modified Mercalli Scale

  • Definition: Measures earthquake intensity on a scale from I to XII, with higher numbers indicating greater damage.

  • Intensity Dependence: Primarily reliant on the amplitude of surface waves; intensity values peak near the epicenter and diminish with distance from it.

Locating an Earthquake

  • Connectivity of Data: Epicenter locations and occurrences are determined using seismograms and travel-time curves.

  • Epicenter Location Process: Seismic stations record wave patterns and distances; scientists use this data to intersect specific geographic regions to identify the epicenter of seismic activity.

Seismic Belts

  • Distribution: Most earthquakes worldwide occur along narrow seismic belts that define tectonic plate boundaries, areas characterized by significant geological activity.