Old Earth Ministries Online Geology Curriculum

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Geology - Chapter 2:  Earth's Basic Systems

    Our planet is perfectly designed by God to function using the physical laws that He set in place.  Through these laws, the earth is constantly being modified.  These laws can be seen operating in several key systems that He designed.  Two systems that we will consider are the hydrologic system and the tectonic system.  It is these two systems that are responsible for both removing crustal material through erosion and subduction, and creating new crustal material through deposition and volcanism.  We will also briefly examine the concept of isostasy.

The Hydrologic System

     The hydrologic system includes all actions performed by both water and air. Most of you are probably already familiar with the water cycle (click on image at right).  As water evaporates, it becomes airborne moisture.  Eventually, this moisture falls to the ground as precipitation.  Some of it runs into lakes, rivers, and ponds, and eventually it evaporates again, starting the process all over again.  Some of the water becomes ground water, where it may be trapped many years before it again reaches the surface where it can evaporate.  The study of the water system is called hydrology.  Hydrologists study the movement, distribution, and quality of water.  To learn more, see Hydrology.

     The main catalyst that causes the water cycle is heat.  The heat from the sun heats water molecules, and it also causes atmospheric circulation (winds), which increases evaporation.   Gravity is another important piece of this puzzle.  It is gravity that causes rain to fall, and it also moves the water down slope once it is on the surface of the earth.  As water moves down slope, it cuts river channels, which can become deep valleys.  Therefore water is the main agent responsible for erosion.  As you will see, many of the chapters in this course deal with topics related to the forces of water.

     There are several sub-systems of the hydrologic cycle.  Probably the most important one is the formation of river systems.  Chapter 10 will introduce you to the different types of river systems.  If you want to read some more about rivers, see River.  Another important sub-system is glaciers.  Glaciers originate from snowfalls.  If the annual snowfall exceeds the amount of moisture lost from melting, a glacier can form.  The picture at right shows a large valley glacier known as the Aletsch Glacier in Switzerland.  Glaciers can also form over a continent, and are called continental glaciers, or ice sheets.  An ice sheet recently covered much of North America during the last ice age.  To learn more, see Glacier. 

     Another sub-system is ground-water systems.  Ground water is water that is mostly underground.  It occupies the pore spaces in between the individual grains of soil and rock.  A large percentage of the world's drinking water comes from this source.  People in rural areas have wells that are drilled through the ground down to the water table.  A large number of jobs in geology have to do with ground water, as people need fresh water for drinking.  In some instances, artesian wells cause water to erupt at the surface.  For an animation on artesian wells, click here.  You will learn more about them in a later chapter.

     As water makes its way back to the ocean, it encounters shoreline systems.  Through the working of water, these systems form beaches, bars, lagoons, and other coastal features. 

     The final sub-system of the hydrologic system is called eolian.  Eolian systems are wind systems.  You may be familiar with the Sahara Desert (see picture).  The sand dunes are blown by the force of the wind.  Specific landforms are associated with eolian processes.  Wind power can move great masses of loose sediment. 

     You may recall that the energy source for the hydrologic system is heat from the sun.  The same is true for its sub-system, eolian systems.  Heat causes the wind currents.  You will learn more about eolian systems in Chapter 16.  To learn more now, see Eolian Processes.

The Tectonic System

     The other major system that affects landforms is the tectonic system.   The word comes from the Greek word for "builder", which is tecton.  The earth's crust is divided into sections, called plates, or tectonic plates.  These plates actually move about the surface of the earth.  Geologists have taken measurements, and can tell how far each plate moves in a year.  For instance, the Indo-Australian plate fused together from two plates about 50 to 55 million years ago, and its current velocity is about 67 millimeters per year (about 2.6 inches).  The collision of the Indo-Australian plate and the Eurasian plate caused the uplift known as the Himalayan Mountains.  To see an animation of this collision between two continental plates, see Fig. 19.28- Convergence of Plates-Continent-Continent (298.0K).

     The plates move because of the convection currents in the asthenosphere, or upper mantle.  These currents are illustrated in the graphic at right.  As the currents move, they pull the plates along, a concept known as slab pull.  When an ocean plate collides with a continental plate, the continental plate overrides the ocean plate, and the point of collision is known as a subduction zone.  The ocean plate dives underneath the margin of the continental plate, and enters the upper mantle (see picture at right).  When two ocean plates or continental plates collide, they normally cause mountain ranges. 

     Tectonic movement is also responsible for the formation of many volcanoes, and they are also the source of many earthquakes.  You will learn more about tectonics in a later chapter.  To learn more now, see Plate Tectonics.

Isostasy

     Another principle that is important to tectonics is isostasy.  Isostasy refers to the state of gravitational equilibrium between the Earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density.  For instance, mountain ranges have roots which are just as deep into the ground as they are tall.  As the mountains weather and get shorter, they lose mass.  They experience isostatic rebound.  The depth of the roots rise as the mountains get smaller.  Click here to see an animation of mountain range isostasy. 

     Isostasy has actually been measured.  When Hoover Dam was constructed in 1935, it added the weight of 24 billion metric tons of water to the area.  Within a few years, it caused the circular area around the lake to subside about 5.5 feet (1.7 meters).  Other items that can cause subsidence is continental ice sheets.  Since we recently had an ice age (11,000 years ago), the weight of the ice caused the land to subside.  Geologists can measure the rate of rebound for this land, which is about 5-10 centimeters per 1,000 years (2-4 inches per 1,000 years).  Also, ancient lakes, such as Lake Bonneville which was located in Utah, show evidences of rebound.

     Gravity is the force which causes isostatic adjustments.  Isostatic subsidence occurs in places where sediment is being deposited, areas of volcanic activity, regions of glaciation, and under bodies of water.  In all these cases, the increase in mass upon the crust causes it to sink until a point of equilibrium is reached.  Conversely, isostatic rebound occurs in mountains and other landforms that are eroding away, where glacial ice is melting, and where lake/sea water is decreasing.  In these cases, the decrease in mass upon the crust lessens the gravitational effect upon that section of crust, and causes it to rise until it reaches gravitational equilibrium.  To learn more, see Isostasy.

End of Chapter

Tuesday - Research

     Research the answers to the following questions about plate tectonics.  Your parents may have you simply answer the questions, or they may have you put it in essay form.  Please follow your parents instructions. 

     To answer these questions, utilize a search engine to locate the best webpages, or consult a textbook/encyclopedia.  You may also use the links at the bottom of this page.

When was the theory of plate tectonics first proposed?

Who proposed it?

Describe the North American Plate.  What plates does it come into contact with, and what impact do they have on each other.

Describe the relationship between the plate tectonics and the San Andreas Fault.

Wednesday - Quiz

     Today you will complete an 11 question practice quiz.  The link to the quiz will open a new window.  You can come back here and check your answers.  Do not click the Back button on your browser during the quiz.  After the quiz, continue your research project, if necessary.

          Geology Chapter 2 Quiz

Thursday - Review

     Please review the terms in bold in the text, and ensure you have completed your research work from Tuesday.

Friday - Test

     Today you will take the end of chapter test.  Please close all other browser windows, and click on the link below.  During the test, do not click on the Back button on your browser.

          Geology Chapter 2 Test

After you have completed the test, you may proceed to Chapter 3 on your next school day.  Please return to the introduction page for the link to the next chapter.

Return to the Old Earth Ministries Online Geology Curriculum homepage.

Helpful Links

Plate Tectonics (Wikipedia)

North American Plate

Research Articles on platetectonics.com

San Andreas Fault