Earth Structure and Plate Tectonics

Seismic Waves

Energy radiates out from the focus. The focus is the place within the Earth where the rock breaks, producing an earthquake. The epicenter of an earthquake is the point on the ground's surface directly above the focus.

Energy moving outward from the focus of an earthquake travels in the form of seismic waves.

Types of seismic waves

1. Body waves
   1. P-waves
      Primary, pressure, push-pull
      Travel fastest of the seismic waves (4-5 km/sec in upper crust; 6-7 km/sec in lower crust)
      Traves through solids and liquids

   2. S-waves
      Secondary, shaking, shear, side-to-side
      Slower (1-2 km/sec)
      Travel through solids only

2. Surface waves
   Often referred to as L-waves or long waves.
   Complex motion. Up-and-down and side-to-side.
   Slowest.
   Causes damage to structures during an earthquake.

   

Internal Structure of the Earth

Earth has a layered structure, as determined from studies of the behavior of seismic waves as they pass through the Earth. P- and S-wave travel times depend on properties of rock materials through which they pass.
Differences in travel times correspond to differences in rock properties.

Wave velocity depends on density and elasticity of rock.
Seismic waves travel faster in denser rock.
Speed of seismic waves increases with depth (pressure and density increase downward).

Examine behavior of P- and S-waves as they travel through the Earth in the diagram.

Note shadow zones and refraction (bending of waves) at boundaries between layers. S-waves cannot pass through the molten (liquid) outer core.

Major layers of the Earth were detected before 1950.
Fine details were delineated in 1960's during nuclear testing.

Crust

• Continental (about 35 km thick; 60 km in mountain ranges)
  Granitic composition
• Oceanic (5 - 12 km thick)
  Basaltic composition
  Thin layer of unconsolidated sediment covers layered igneous rock Layered structure (ophiolite complex):
  1. pillow basalts (basalts that erupted subaqueously)
  2. "sheeted dikes" - interconnected basaltic dikes
  3. gabbro (coarse grained equivalent of basalt; cooled slowly)

Outer core (2270 km thick)

Inner core (1216 km radius)

Crustal Structures

Faults

Folds

During mountain building or compressional stress, rocks may deform plastically to produce folds. Generally, a series of folds is produced, much as a carpet might wrinkle when you push on one end. The up-folds and the down-folds are adjacent to one another, and grade into one another. In geology we give each a separate descriptive name.

Basic types of folds:

1. Anticlines - upfolds.
   When the upper part of the fold is eroded away, the oldest rocks are in the center of the fold, and the youngest rocks are on each side. Also, the rocks dip (or slope) away from the central axis of the fold.

2. Synclines - downfolds.
   When the upper part of the fold is eroded away, the youngest rocks are in the center of the fold, and the oldest rocks are on each side. Also, the rocks dip toward the central axis of the fold.

   Note that in each case, older rocks dip toward younger rocks.

   

   

3. Domes - Similar to anticline, but with circular outcrop pattern.
   Rock layers are up-folded to resemble a stack of baseball caps or inverted soup bowls. When the upper part of the dome is eroded away, the oldest rocks are in the center, and the youngest rocks are around the edges. Also, the rocks dip away from the center of the dome.

4. Basins - Similar to syncline, but with circular outcrop pattern.
   Rock layers are down-folded to resemble a stack of soup bowls. When the upper part of the basin is eroded away, the youngest rocks are in the center, and the oldest rocks are around the edges. Also, the rocks dip toward the center of the basin.

Plate Tectonics

Based on intensive geophysical data collection in Earth's oceans following World War II, including "International Geophysical Year," 1957-1958. Sea floor mapping discovered patterns of midocean ridges and deep sea trenches. Magnetometers to detect submarines also recorded differences in magnetic properties of rocks on sea floor. Global network of seismometers was established to monitor atomic explosions, and also provided information on worldwide earthquake patterns.

Evidence in support of the Theory of Plate Tectonics:

1. Shape of the coastlines (jigsaw puzzle fit)

   

2. Fossil evidence implies once continuous land connections between now-separated areas
   1. Distribution of Glossopteris flora (plant fossils)
      Late Paleozoic seed ferns
      Gondwanaland (India, Africa, Australia, S. America, Antarctica)

   2. Distribution of Mesosaurus (fish-eating freshwater reptile)

      

   3. Distribution of Lystrosaurus (plant-eating freshwater reptile)

   4. Distribution of Cynognathus (small carnivorous reptile transitional to mammals)

   5. Distribution of Paleozoic fishes and amphibians

3. Distribution of present-day organisms indicates that they evolved in genetic isolation on separated continents

4. Geologic similarities between South America, Africa, and India
   
   1. Same stratigraphic sequence (i.e., same sequence of layered rocks of same ages in each place)
      
   2. Mountain belts and geologic structures (trends of folded and faulted rocks line up)
   3. Precambrian basement rocks are similar in Gabon (Africa) and Brazil

5. Geologic similarities between Appalachian Mountains and Caledonian Mountains in British Isles and Scandinavia

6. Paleoclimatic (=ancient climate) evidence
   (ancient climatic zones match up when continents are moved to their past positions)
   1. Layers of glacial deposits (tillites) are found at same place in sequence of rocks
      Note directions of glacial ice movement as indicated by striations or grooves in the rock

      

   2. Coal seams with logs from tropical (low latitude) trees found at high latitudes

   3. Distribution of carbonate deposits (including reefs) and evaporite deposits

7. Rift Valleys of East Africa indicate continent breaking up

8. Youth of ocean basins and sea floor
   Thin layer of sediment on basalt
   Basalt dates to less than 200 million years (most less than 150 million years)

9. Evidence for subsidence in oceans -
   
   1. Guyots - flat-topped sea mounts (erosion when at or above sea level)
   2. Chains of volcanic islands that are older away from site of current volcanic activity -
      Hawaiian Islands and Emperor Sea Mounts
      (also subsiding as they go away from site of current volcanic activity)

10. Mid-ocean ridges located near ocean centers are sites of sea floor spreading
    1. High heat flow
    2. Seismic wave velocity decreases due to high temperatures
    3. Valley along center of ridge (graben)
    4. Volcanoes along ridge
    5. Earthquakes along ridge

11. Benioff Zones - inclined zone of earthquake foci (plural of focus) near deep sea trenches

12. Magnetic stripes on the sea floor
    Symmetrical about mid-ocean ridge

Contributions to plate tectonic theory from paleomagnetism

1. Recently magnetized rocks show alignment of magnetic field consistent with Earth's current magnetic field

2. Magnetization in older rocks has different orientations (as determined by magnetometer towed by ship).
   Can determine direction to north magnetic pole and distance to north magnetic pole from inclination and declination of magnetic field in the rock

3. Polar wandering curves
   Different polar wandering paths seen in rocks of different continents.
   Put continents "back together" and the polar wandering curves are superimposed (match up)

4. A test of the hypothesis of sea floor spreading (Vine and Matthews, 1963)
   Magnetic reversal "stripes" are SYMMETRICAL about the ridge.

5. Magnetic reversal time scale -
   Pattern of reversals in sea floor basalts matches known reversal time scale as determined from rocks exposed on land.
   Width of magnetic stripes on sea floor is related to time.
   (Wide stripes = long time; narrow stripes = short time)

Basic Terminology of Plate Tectonics

Asthenosphere is partially molten part of mantle. Below lithosphere.

Rigid lithospheric plates rest (or "float") on flowing asthenosphere.

Two types of crust are present in the upper part of the lithosphere:

• thin, dense, basaltic oceanic crust
• thick, low density, granitic continental crust

Types of plate boundaries:

1. Divergent - where plates move apart from one another (tensional stress).
   Rifting or spreading or "pull-apart" occurs. Rift ing may occur in either oceanic or continental crust.
   Rift zones tend to have normal faults and intrusions of igneous rock coming up from below (commonly basalt - rich in Mg and Fe).
   Examples: mid-ocean ridges, East African Rift Valley

2. Convergent - where plates move toward one another (compressional stress).
   
   1. Continental collisions form mountain belts
      Examples: Himalaya Mountains, Ural Mountains, Appalachian Mountains

   2. Subduction - where one plate is pushed beneath another plate, forming a submarine trench.
      
      1. Ocean-to-ocean subduction - A plate topped by oceanic crust (or "oceanic plate") may be subducted beneath another plate topped by oceanic crust, forming a deep sea trench with an associated basaltic volcanic island arc.
         Examples: Mariana Trench adjacent to Mariana Islands, Aleution Trench adjacent to Aleutian Islands, Java Trench adjacent to Java, Sumatra, and Sunda Islands in the Indonesian region.

      2. Ocean-to-continent subduction - An oceanic plate may also be subducted beneath a continental plate, forming a trench adjacent to a continent, with volcanic mountains along the edge of the continent.
         Example: Peru-Chile Trench adjacent to the western coast of South America with the Andes Mountains. Note that the volcanic rocks in this setting tend to be andesite (named for Andes Mountains).

   

3. Transform - where two plates slide past one another (shear stress).
   Transform faults cut across and offset the mid-ocean ridges.
   Transform faults are a natural consequence of horizontal spreading of the seafloor on a curved globe.
   Many examples can be seen cutting the Mid-Atlantic Ridge or East Pacific Rise.
   The San Andreas Fault is also a transform fault, occurring where the North American Plate has overridden the East Pacific Rise (part of the oceanic ridge-system).

Rates and directions of plate movements vary.

All plates are moving

Benioff Zones. At subduction zones, cold lithosphere descends into the asthenosphere in deep sea trenches
Associated with volcanoes and deep-focus earthquakes (over 190 miles deep). Diagonally-dipping zone of earthquake foci provide evidence for subduction where one plate is sliding beneath another, causing earthquakes.
"Ring of Fire" - volcanoes associated with subduction zones around Pacific Rim.

Hot spots - thermal plumes (heat rising in mantle).
Plates move over hot spots creating a chain of volcanoes.
Hawaiian Islands, Emperor Sea Mounts

Triple Junction - where three plates meet.
Generally associated with divergent plate boundaries and thermal plumes. Rift zones.
Two plate boundaries tend to be most active, and one less active (failed rift).
Example: Afar Triangle of Ethiopia - Red Sea rift, Gulf of Aden rift, and East African Rift Valley.

What forces drive Plate Tectonics?

If the rising part of a convection cell is beneath a continent, it will cause it to RIFT apart.
Also causes seafloor to rift apart at mid-ocean ridge.

Other forces involved are the thermal heating and expansion of the crust over a spreading center (where hot material is rising from below). The crust tends to slide off the thermal bulge, pushing the rest of the oceanic crust ahead of it. This is called ridge-push or slab-push.

Conversely, far from the spreading center, the oceanic crust is cold and dense, and tends to sink into the mantle at subduction zones, pulling the rest of the oceanic crust behind it. This is referred to as slab-pull.

Many seemingly unrelated geologic facts are unified by the plate tectonic theory.

Document created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

February 22, 1999