Outline of Earth Science by Ellin Beltz
Introduction Part I
Atoms, Minerals, Rocks, Geological Time
Part II
You are here
Plate Tectonics, Earthquakes, Volcanos, Geological Structures
Part III
Fresh Water and its Landforms
Part IV
Part V
2005 by Ellin Beltz

Outline of Earth Science - Unit II

Plate Tectonics

  • Earth's crust is divided into polygonal plates which come together at triple junctions.
  • Volcanoes, earthquakes, mountain building, changing climates, migrating continents and oceanic islands result from plate movements.
  • The plates move by a combination of forces related to convection in the mantle.
  • Isostasy is the measure of the up and down motion of plates due to loading and unloading of earth materials including sediments and ice.
  • Geological structures are produced by plate tectonic forces.

Layers of the Earth are revealed by the study of earthquake wave velocities from network of seismic monitors. They found that the Earth has:
  • A solid inner core from about ~5100 km to ~6600 km
  • A liquid outer core from about ~2900 km to ~5100 km
  • A plastic mantle and rigid crust from the surface to ~2900 km
Another, newer body of work supports the mantle being divided into a
  • Hot, high pressure mesosphere from ~2900 km to ~200 km
  • Covered by a plastic asthenosphere from about ~200 km to ~100 km.
  • Overlain by the rigid lithosphere - formed by two types of crust:
    1. Oceanic crust
      • source of rock is from the mantle
      • about 5 km or 3.1 miles thick
      • mafic base with sediments on top
      • darker in color
      • density around 3.0 g/cc
      • younger than continents, oldest still in sea is only about 180 million years old
    2. Continental crust
      • source of rock is recycled crust
      • about 40 km or 25 miles thick
      • mostly felsic
      • lighter in color
      • density around 2.54 g/cc
      • up to 3.2 billion years old

Convection cells heated by the core and cooled near the surface may be the driving force of plate tectonics.

Earth's crust is divided into polygonal plates by three kinds of boundaries.

Type of BoundaryForceFaults Geological Structure Examples
Divergenttensionalnormal riftsMid-Atlantic Ridge, East Pacific Rise
Convergentcompressionalreverse folded mountains,
island arcs and
Appalachians, Caribbean Islands, Cascade Volcanos
Transformshearstrike-slip shear mountainsCalifornia's Coast Ranges from Eureka south to Los Angeles

Divergent Boundaries occur either:

  • In oceanic crust include the Mid-Atlantic Ridge and the East Pacific Rise
  • In continental crust include the East Africa Rift System, and
  • Can erupt intermediate or mafic magma.

Convergent Boundaries can occur three ways:

  1. Oceanic crust collides with oceanic crust; a trench forms and a line of volcanos forms on the overlying plate. Eventually, the volcanos may become large enough to be exposed as islands. The deepest point in the oceans is the Marianas Trench at a depth of about ~11,000 meters below sea level.
  2. Oceanic crust collides with continental crust; the thinner plate descends, forming a trench, below the thicker plate to a depth of 660 km where the oceanic plate and its sediments melt and rise up to form a line of volcanos about 300 km inland from the trench. The Cascades Volcanos in the northwestern U.S. are an example of this type of collision.
  3. Continental crust collides with continental crust; one plate goes under the other, but cannot descend into the lithosphere, so huge mountains are thrust up as the collision continues. The Himalayas are forming now from a collision between India and China and the North American Appalachians are the eroded roots of a mountain chain built like this about 250 million years ago.

Transform Boundaries permit plates to slide past each other. An example of an active continental strike-slip fault is California's San Andreas fault system while the Cape Mendocino Transform Fault occurs under the Pacific near Petrolia, California.

All three types of plate boundaries are geologically active.

A proof of plate motion comes from hot spots which are believed to be plumes or blobs of hot mantle material which rise more or less continuously in the same place and are not necessarily associated with a plate boundary. The Hawai'ian Islands in the Pacific Ocean were formed as the Pacific Plate moved over the fixed supply of mafic magma.

Plate Tectonics . Earthquakes . Volcanos . Geological Structures


Stress and Strain
  • Stress is force applied to an object.
  • Strain is the result of force.
  • Objects deform both parallel and perpendicular to force.

The Effects of Stress and Strain include:

  • Elastic deformation -- rock springs back to original shape
  • Plastic deformation - rock acquires a new shape
  • Brittle fracture -- rock breaks
    • If no movement takes place, the line of fracture is called a joint.
    • If rocks move, the line of fracture is called a fault.
    • Breaking rocks causes earthquakes. The "epicenter" is the point on the Earth's surface above where the rock actually broke (the "focus").

Seismic waves travel through rock after earthquakes.

  • Body waves are recorded by seismographs
    • P-wave -- primary -- push wave
    • S-wave -- secondary -- shear or shaking wave -- do not go through liquids
  • Surface waves are slower than body waves
    • up and down rolling wave
    • side to side shaking waves
    • Triangulation from three seismic stations shows epicenter.
    • Earthquakes reveal plate tectonic boundaries, their depths and angles as well as Earth's interior.
  • Earthquakes may damage natural and cultural features by means of:
    • fire
    • landslides
    • broken dams and opened waterways
    • changes in river courses
    • changes in well levels or spring flow (fluids follow faults)
    • tsunami
    • liquefaction and subsidence
  • Humans have induced earthquakes by nuclear bomb tests and deep-well fluid injection.
Plate Tectonics . Earthquakes . Volcanos . Geological Structures


Volcanos make magma by:
  • Increasing temperature
  • Decreasing pressure
  • Pressure-release melting explodes volatiles
  • Or by the addition of water or other volatiles: O2, CO2, CO, ClOx, SO4

Magma environments

  • Spreading centers at divergent boundaries
  • Subduction zone
  • Mantle plumes and hot spots

If the magma only partially melts the minerals with the lowest melting points melt first.

Magma differentiation results in the heavier crystals settling in magma chamber leading to lighter magma available for eruptions.

Mafic and Felsic eruptions are different.

  • Felsic volcanos have dangerous eruptions.
    • High silica and high volatiles result in high viscosity, very stiff lavas.
    • Nuee ardentes - incandescent ash explosions such as devastated St. Pierre, martinique in 1902 when `40,000 people perished.
    • Thick blankets of dry ash can be deposited quickly such as the burial of Pompei during the AD79 eruption of Mt. Vesuvius.
    • Lahars, volcanic mudslides are often caused by thick felsic lavas arriving in streams or lakes and overflowing like concrete. Herculaneum, Italy was buried by the AD79 eruption of Mt. Vesuvius.
    • Caldera collapse may occur, if the crater had a lake in it, the water becomes steam and adds to the eruption.
  • Mafic volcanos produce less dangerous eruptions.
    • low silica and relatively low volatiles produce low vicosity, more fluid lavas.
    • Pahoehoe is "ropy" basaltic lava and flows at a higher temperature than
    • Aa, which is a "blocky" basaltic lava.

Either type of eruption may produce

  • Columnar jointing is the result of cooling in lava.
  • Pyroclastics, bombs and blocks are hot chunks.
  • Ash and cinders occur when volatilized magma explodes and cools.
Parts of VolcanosWhen cool become
Magma ChamberPlutons, Batholiths, Laccoliths and Stocks
VentsDikes (vertical) or Sills (horizontal)
CraterMay become a crater lake or a plug.
Throat or Magma FeederPitons, Titons and Devil's Tower, WY

Fissure eruptions create flood basalts and lava plateaus. The Kergulen Plateau in the southern Indian Ocean may have been exposed and had life on it during the last glacial period when sea levels were lower. Continental flood basalts can be seen in the Snake River Canyon in North America, the Deccan Flats in India and the Siberian Traps, massive flood basalts which date to about 250 million years ago.

Types of Volcanos

  • Shield Volcanoes -- Hawaiian Volcanos
  • Cinder Cones -- Craters of the Moon N.M.
  • Composite Cones -- Mt. St. Helens N.M.

Other famous volcanos include Stromboli, Etna, Vesuvius, Krakatoa, Montserrat, Long Valley, Yellowstone and Mount Lassen. Several volcanos are active now and several have live cameras pointed at them.

Plate Tectonics . Earthquakes . Volcanos . Geological Structures

Geologic Structures, Mountain Ranges and Continents

Stress and Rock Deformation are caused by:
  • Confining pressure
  • Compressive stress
  • Tensional stress
  • Shear stress

Rocks respond to stress differently due to:

  • Type of rock or geomaterial
  • Temperature or temperature change
  • Pressure or pressure change
  • Fluids, either addition or volatilization
  • Time

Geologic structures

One direction of forceTwo directions of force

Force typeFault typeTectonic environmentExample
TensionNormalDivergentEast Africa Rift
CompressionReverseConvergentHimalaya Mountains
ShearStrike-slipTransformSan Andreas Fault

Slip or throw is the amount of movement along a fault relative to the other side.

Joints form when rocks break but don't move.

Mountains and mountain ranges form byExamples
Subduction of an ocean plate beneath another plate Caribbean and other island arcs
Andes Mountains, Cascade Mountains
Collision of two plates Himalayas Mountains
Appalachian Mountains
Atlas Mountains
Volcanic activityMid-ocean ridges
Composite volcanos
Hot spot volcanos
And/or a combination of more than one of these events over time

Evolution of continents and Wilson's Cycles

Earth cycles and recycles its crust. Microcontinents in the preCambrian joined to form the Old Red Sandstone Continent which rifted in the Ordovician to form the first Atlantic (protoAtlantic). The continental blocks rejoined in the end Paleozoic forming Pangaea from northern Laurentia and southern Gondwana which was over the south pole and glaciated during the Carboniferous. The supercontinent was fused, uplifted and dry during the Permian and part of the Triassic, then heat built up under the continents and rifts formed around 180 million years ago creating the Atlantic Ocean we see today. The latest supercontinent is assembling now in the Northern Hemisphere.

Additional resources

  • Visit plate tectonic links and watch animations of how basalts and gabbros form at midocean ridges while plutons of granitic rocks rise and erode out of the continents.
  • Check out a few volcano links, see pictures of mid-ocean ridges and rifts and find out more information on igneous rocks. Don't miss the live cams on various volcanos but remember to allow for time of day differences.
  • Take an earthquake safety course, find out the most recent earthquake for today, report what you felt during an earthquake - all on earthquake links.
  • View the Earth as a single entity with these links to global data and satellite images.
  • Outline of Historical Geology developed for ESCI 211 and updated in 2005. Includes a more detailed rock cycle review as well as taphonomy, phylogeny, and classification of fossils and living animals as well as discussions of plate tectonics and the formation of North America.
Outline of Earth Science by Ellin Beltz
Part I
Atoms, Minerals, Rocks, Geological Time
Part II
You are here
Plate Tectonics, Earthquakes, Volcanos, Geological Structures
Part III
Fresh Water and its Landforms
Part IV
Part V
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2008 by Ellin Beltz -- January 10, 2008