GENERAL GEOLOGY (GEOL 1113)

STUDY GUIDE FOR EXAM III

The following study guide is provided as an aid to help you identify the major concepts you should have learned concerning PLATE TECTONICS (Ch. 19). Key vocabulary with which you should be familiar is highlighted in GREEN UPPER CASE throughout this document and on future documents of this type for the course. Additionally, some key words are highlighted in blue and underlined which means they contain hyperlinks to additional information of interest. Clicking on the blue words will transport you to various Internet locations or on-line images to enhance your studying. You should, at the very least, be able to define the highlighted terms in order to complete the exam. Ideally, however, I hope you will be able to do more than simply respond to definitions. I would like you to learn to be able to apply the definitions and the concepts they represent to a fuller understanding of the Principles of Geology and Earth as a planet by visiting the added hyperlinks.

PLATE TECTONICS: Chapter 19

Over the last 40 years, geologists have developed a new, comprehensive view of Earth as a dynamic planet. This view originated from the THEORY OF PLATE TECTONICS, which revolutionized the scientific discipline of Geology and has served to greatly increase our understanding of virtually all geologic processes.In many respects, PLATE TECTONICS is the most important topic we will discuss this semester. You will come to understand (I hope) how everything we have already discussed (fundamental chemistry and DIFFERENTIATION of Earth, MINERALS, IGNEOUS ROCKS, WEATHERING & SEDIMENTARY ROCKS, METAMORPHIC ROCKS) are the physical manifestations of plate tectonic processes on our planet. In addition, in the weeks to come, you will see that the history of Earth is, in actuality, the history of plate tectonic processes - by understanding plate tectonics, we understand Earth's history. The geologic story of Fayetteville, Arkansas is itself a tale of tectonic plates. Plate tectonics is (perhaps) the greatest story ever told - it is (for certain) the greatest show on Earth.

COMPOSITIONAL LAYERING OF THE EARTH

The innermost layer is the CORE. The CORE is subdivided into two regions, THE INNER CORE and the OUTER CORE.

The next layer of Earth is the MANTLE. The MANTLE is composed of solid rocks whose average density is approximately 4.5 g cm-3.

The outermost rocky layer of the Earth is very thin (averaging about 35km thick) and is called the CRUST. The CRUST is generally divisible into two primary components:

The CONTINENTAL CRUST (also known as SIAL because it is enriched in silicon and aluminum)

The OCEANIC CRUST (also known as SIMA becaue it is enriched in silicon and magnesium)

The process whereby Earth's interior became layered is known as DIFFERENTIATION. Note that differentiation has occurred because the DENSITY of materials in the Earth differs. The most dense materials have settled to the center of the Earth, while the least dense surround the outermost part of Earth.

Layering of Earth according to DENSITY is also layering according to COMPOSITION of materials.

MECHANICAL LAYERING OF THE EARTH

We might also consider defining the internal layering of Earth in terms of the PHYSICAL BEHAVIOR OF MATERIALS (as opposed to composition).

In general, the PHYSICAL BEHAVIOR of materials can be considered to be their response when extreme forces are applied to them. On Earth, materials behave as BRITTLE SOLIDS, PLASTIC SOLIDS or FLUIDS.

BRITTLE SOLIDS are those which will fracture or shatter when extreme forces are applied to them.

PLASTIC SOLIDS are those which will DEFORM when extreme forces are applied to them (an example of a plastic solid is modeling clay).

FLUIDS respond to extreme forces by FLOWING.

Rocks within the Earth behave as BRITTLE SOLIDS from the surface to depths of about 100 km (i.e. All rocks in the crust and those of the uppermost mantle exhibit brittle behavior). This region of Earth is called the LITHOSPHERE. The LITHOSPHERE includes all of the CRUST and a portion of the UPPER MANTLE (remember, crust and mantle are defined according the composition of rocks).

Rocks below 100 km behave as PLASTIC SOLIDS. This region is called the ASTHENOSPHERE. The transformation from brittle to plastic solids occurs because of the high temperatures and high pressures below 100 km. These conditions cause rocks to behave differently than at the surface.

The OUTER CORE behaves as a FLUID because it is MOLTEN.

The INNER CORE of Earth is a BRITTLE SOLID composed of pure IRON and NICKEL.

PLATES AND PLATE TECTONICS

Recall that the GEOTHERMAL GRADIENT below 100 km is very small - only about 1oC per km. Thus, the upper portions of the ASTHENOSPHERE are nearly ISOTHERMAL.

Point to ponder: Why is the Asthenosphere isothermal? How is this condition maintained?

The temperature structure of the ASTHENOSPHERE results from CONVECTION. CONVECTION is a process which transfers heat through the circulation of material. Since rocks in the ASTHENOSPHERE are PLASTIC, the will deform over very long intervals of geologic time and transport heat from deep in the Earth to shallower levels. So, in effect, the Earth's mantle is slowly being stirred (not shaken!) by the process of convection. This stirring will, over time, cause the temperature structure to become rather uniform, just as is observed below 100km!

Mantle convection manifests itself on the Earth's surface in a unique manner. As the plastic rocks of the ASTHENOSPHERE circulate, they cause the overlying brittle rocks of the LITHOSPHERE to fracture. These great cracks in the lithosphere then define the boundaries of large blocks of rock called PLATES. As the asthenosphere moves slowly over geologic time, so then do the overlying plates move - and this motion is the essence of the geology we observe at the Earth's surface - indeed, the essence of virtually all geology on Earth.

PLATE MOTIONS, PLATE INTERACTIONS, AND PLATE BOUNDARIES

If we examine the distribution of large-scale geologic phenomena across the Earth, we quickly find that these phenomena are not randomly distributed. For example,

GLOBAL EARTHQUAKE DISTRIBUTION
The distribution of earthquakes is not random across the Earth's surface. Earthquakes are concentrated in narrow belts along the boundaries between the tectonic plates.

 

GLOBAL VOLCANO DISTRIBUTION
The distribution of volcanoes is not random across the Earth's surface. Volcanoes are concentrated in narrow belts along the boundaries between the tectonic plates.

And, finally, the distribution of the great mountain ranges of the world is not random - the Alps, Himalayas, Urals, Appalachians, Andes, etc. all mark the positions of former or present-day plate boundaries! So, since all the geologic "action" appears to occurs near plate boundaries, let's look in detail at the different types of plate boundaries and the geologic phenomena associated with each.

There are 3 types of PLATE BOUNDARIES:

1) DIVERGENT PLATE BOUNDARIES,

2) CONVERGENT PLATE BOUNDARIES,

3) TRANSFORM PLATE BOUNDARIES.

DIVERGENT PLATE BOUNDARIES are those where two or more plates are moving AWAY from each other. This motion is induced by the UPWELLING LIMB of a convection cell in the asthenosphere. Here, hot rocks from deep in the Earth rise toward the surface. As they rise, they spread and cause the overlying LITHOSPHERE to tear (i.e. fracture) - a process geologists call RIFTING. Associated with RIFTING are numerous earthquakes, caused by the movement of the plates away from each other. In addition to rifting, DIVERGENT PLATE BOUNDARIES are characterized by VOLCANIC ACTIVITY. Magma forms in areas of mantle upwelling through the process of ADIABATIC DECOMPRESSION MELTING ("WHAT'S THAT?" you ask).

Rocks deep in the asthenosphere are very hot and under extreme pressure. As they rise toward the surface, they retain much of their heat, but pressure decreases substantially. This great reduction in pressure while maintaining a high temperature causes melting (i.e.magma forms).

The magma which forms from this process has some specific characteristics:

  • 1) it is HIGH TEMPERATURE (because it began as a rock deep in the Earth),
  • 2) it is LOW SILICA (because it is derived from the mantle, where silica content is low),
  • 3) it has a relatively LOW VOLATILE content (because there isn't much water or other gas deep in the mantle - remember, most of it has differentiated and is in the outermost layers of Earth).

At the Earth's surface, above the upwelling portion of a convection cell, a long chain of VOLCANOES forms, giving rise to the longest continuous mountain chain on Earth, the MID-OCEAN RIDGE. The MID-OCEAN RIDGE winds its way through the ocean basins of Earth for approximately 42,000 miles. Though is was discovered in the 1800's, it was not known to be continuous throughout the oceans until 1957.

The island nation of ICELAND is located on the MID-OCEAN RIDGE.

BASALT is the rock type formed by volcanic processes at the MID-OCEAN RIDGE.

The Mid-Ocean Ridge is also the site a many thousands of EARTHQUAKES every year - but most of these earthquakes are relatively weak and since few people live near or on ridges, they do not create any damage.

The AVERAGE RATE OF PLATE MOVEMENT associated with DIVERGENT PLATE BOUNDARIES is about 3 - 5 CENTIMETERS PER YEAR.

On the Mid-Ocean Ridge, rocks are YOUNGEST at the ridge crest and get PROGRESSIVELY OLDER away from the crest.

Along the crest of the Mid-Ocean Ridge, there is often a RIFT VALLEY - a deep canyon formed by the movement of the plates away from each other on either side of the Mid-Ocean Ridge.

Summarizing, DIVERGENT PLATE BOUNDARIES are characterized by:

Point to ponder: Can you name at least two places where plates are diverging?

CONVERGENT PLATE BOUNDARIES are those where two or more plates are moving TOWARD each other. These are the most complex types of plate boundaries with the greatest diversity and complexity of geologic processes. When two plates collide, invariably one of the plates is forced beneath the other. The process whereby one plate is forced beneath the other is known as SUBDUCTION. The region of Earth where SUBDUCTION OCCURS is called a SUBDUCTION ZONE. As with all collisions, SUBDUCTION is a process of great geologic violence. As these great plates smash together, one plate is forced to descend back into the asthenosphere. Along the upper surface of the descending plate, great earthquakes will occur. This zone of intense earthquake activity is called the BENIOFF ZONE. All along the descending plate, rocks will be exposed to tremendous HIGH PRESSURE - but at relatively LOW TEMPERATURE. Exposure to these conditions of pressure and temperature will induce TECTONIC METAMORPHISM in these rocks.

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At convergent plate boundaries, the collision of two tectonic plates generates great volumes of metamorphic rocks over broad regions. So, convergent plate boundaries are the principal areas where TECTONIC METAMORPHISM occurs. In cross section, we can predict the temperature and pressure conditions at various locations surrounding the plate boundary. Generally, however, very HIGH PRESSURE will be generated along the collision front while relatively LOW TEMPERATURES will prevail because rocks are at relatively shallow depths.

In addition, magma will be generated in proximity to subduction zones - but it's a magma with very different characteristics than that which occurs at divergent boundaries.

The magma formed during subduction is:

  • 1) it is LOW TEMPERATURE, (because it forms from rocks at shallow depths),
  • 2) it is HIGH SILICA (because it is derived in part from crustal rocks where silica content is high),
  • 3) it has a relatively HIGH VOLATILE content (because there is much water and other gases near Earth's surface).

The magma at convergent plate boundaries forms by a very different process than magma at a divergent plate boundary. Within a SUBDUCTION ZONE, the DESCENDING PLATE is subjected to increasing pressure as it plunges back into Earth's interior. As this pressure increases, WATER which was absorbed by the plate as it traveled across Earth's surface is squeezed from the plate into the surrounding mantle rocks. The addition of this water to the mantle causes the rocks to melt at around 800oC. Thus, the magma formed at CONVERGENT PLATE BOUNDARIES is at a relatively LOW TEMPERATURE and contains an appreciable quantity of GASES (WATER VAPOR). This is why convergent plate boundary volcanoes are characterized by explosive eruptions.

There are 3 types of CONVERGENT PLATE BOUNDARIES:

1) OCEAN-OCEAN CONVERGENT PLATE BOUNDARIES:

  • At these boundaries, two plates generated at a Mid-Ocean Ridge collide with each other. The resulting plate boundary is characterized by

    1) DEEP OCEAN TRENCH
    2) STRONG EARTHQUAKES
    3) EXPLOSIVE VOLCANOES developed as a VOLCANIC ISLAND CHAIN

2) OCEAN-CONTINENT CONVERGENT PLATE BOUNDARIES:

  • At these boundaries, a plate generated at a Mid-Ocean Ridge collides with a continent. The resulting plate boundary is characterized by

    1) DEEP OCEAN TRENCH
    2) STRONG EARTHQUAKES
    3) EXPLOSIVE VOLCANOES developed as a VOLCANIC MOUNTAIN CHAIN on the continent.

2) CONTINENT-CONTINENT CONVERGENT PLATE BOUNDARIES:

  • At these boundaries, a two continents collide with each other. The resulting plate boundary is characterized by

    1) STRONG EARTHQUAKES
    2) NO EXPLOSIVE VOLCANOES
    3) LARGE-SCALE MOUNTAIN BUILDING     (like the Ouachitas or Appalachians).

Point to ponder: Can you name at least two places where plates are converging?

TRANSFORM PLATE MARGINS are those where two plates move horizontally past each other. These boundaries are characterized by large faults (such as the SAN ANDREAS FAULT of California) which are capable of generating damaging earthquakes. The San Andreas Fault is the most famous of the TRANSFORM PLATE BOUNDARIES, but there are others worldwide.

Point to ponder: Can you locate on a map at least two places where plates are slipping past each other?

This concludes the study guide for Exam III. As you can see, it is quite comprehensive and I hope that it serves as a valuable aid to your studying. I hope you will download this guide as soon as possible and study it thoroughly. If you know everything on this guide, you should do very well on the exam next week. HAPPY STUDYING and GOOD LUCK! - Dr. Boss :-)