Introduction: (Initial Observation)
Our planet was most probably created about five billion years ago. At first it was just a ball of molten rock and gasses with little order to its structure. The huge amount of heat energy released at this time is still being radiated, even today, as the planet slowly cools down.
As the cooling began, more dense materials such as iron sank into the core of the Earth, while lighter silicates, other oxygen compounds, and water rose towards the surface. In consequence distinct layers began to form. In this project we will study and make a model of the earth cross section. We will also find out how do geologists know about inside the earth.
Information Gathering:
Find out about the earth geology. Read books, magazines or ask professionals who might know in order to learn about the methods used by geologists to learn about inside the earth. Keep track of where you got your information from.
Introduction:
Three centuries ago, the English scientist Isaac Newton calculated, from his studies of planets and the force of gravity, that the average density of the Earth is twice that of surface rocks and therefore that the Earth’s interior must be composed of much denser material. Our knowledge of what’s inside the Earth has improved immensely since Newton’s time, but his estimate of the density remains essentially unchanged. Our current information comes from studies of the paths and characteristics of earthquake waves traveling through the Earth, as well as from laboratory experiments on surface minerals and rocks at high pressure and temperature.
Other important data on the Earth’s interior come from geological observation of surface rocks and studies of the Earth’s motions in the Solar System, its gravity and magnetic fields, and the flow of heat from inside the Earth. The planet Earth is made up of three main shells: the very thin, brittle crust, the mantle, and the core; the mantle and core are each divided into two parts. All parts are drawn to scale on the cover of this publication, and a table at the end lists the thicknesses of the parts. Although the core and mantle are about equal in thickness, the core actually forms only 15 percent of the Earth’s volume, whereas the mantle occupies 84 percent. The crust makes up the remaining 1 percent. Our knowledge of the layering and chemical composition of the Earth is steadily being improved by earth scientists doing laboratory experiments on rocks at high pressure and analyzing earthquake records on computers.
The Crust
Because the crust is accessible to us, its geology has been extensively studied, and therefore much more information is known about its structure and composition than about the structure and composition of the mantle and core. Within the crust, intricate patterns are created when rocks are redistributed and deposited in layers through the geologic processes of eruption and intrusion of lava, erosion, and consolidation of rock particles, and solidification and recrystallization of porous rock.
Figure 1. The oceanic crust at the island of Hawaii is about 5 kilometers thick. The thickness of the continental crust under eastern California ranges from 25 kilometers under the Great Valley to 60 kilometers under the Sierra Nevada.
By the large-scale process of plate tectonics, about twelve plates, which contain combinations of continents and ocean basins, have moved around on the Earth’s surface through much of geologic time. The edges of the plates are marked by concentrations of earthquakes and volcanoes. Collisions of plates can produce mountains like the Himalayas, the tallest range in the world. The plates include the crust and part of the upper mantle, and they move over a hot, yielding upper mantle zone at very slow rates of a few centimeters per year, slower than the rate at which fingernails grow. The crust is much thinner under the oceans than under continents (see figure above).
The boundary between the crust and mantle is called the Mohorovicic discontinuity (or Moho); it is named in honor of the man who discovered it, the Croatian scientist Andrija Mohorovicic. No one has ever seen this boundary, but it can be detected by a sharp increase downward in the speed of earthquake waves there. The explanation for the increase at the Moho is presumed to be a change in rock types. Drill holes to penetrate the Moho have been proposed, and a Soviet hole on the Kola Peninsula has been drilled to a depth of 12 kilometers, but drilling expense increases enormously with depth, and Moho penetration is not likely very soon.
The Mantle
Our knowledge of the upper mantle, including the tectonic plates, is derived from analyses of earthquake waves (see figure for paths); heat flow, magnetic, and gravity studies; and laboratory experiments on rocks and minerals. Between 100 and 200 kilometers below the Earth’s surface, the temperature of the rock is near the melting point; molten rock erupted by some volcanoes originates in this region of the mantle. This zone of extremely yielding rock has a slightly lower velocity of earthquake waves and is presumed to be the layer on which the tectonic plates ride. Below this low-velocity zone is a transition zone in the upper mantle; it contains two discontinuities caused by changes from less dense to more dense minerals. The chemical composition and crystal forms of these minerals have been identified by laboratory experiments at high pressure and temperature. The lower mantle, below the transition zone, is made up of relatively simple iron and magnesium silicate minerals, which change gradually with depth to very dense forms. Going from mantle to core, there is a marked decrease (about 30 percent) in earthquake wave velocity and a marked increase (about 30 percent) in density.
The Core
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The core was the first internal structural element to be identified. It was discovered in 1906 by R.D. Oldham, from his study of earthquake records, and it helped to explain Newton’s calculation of the Earth’s density. The outer core is presumed to be liquid because it does not transmit shear (S) waves and because the velocity of compressional (P) waves that pass through it is sharply reduced. The inner core is considered to be solid because of the behavior of P and S waves passing through it.Cross section of the whole Earth, showing the complexity of paths of earthquake waves. The paths curve because the different rock types found at different depths change the speed at which the waves travel. Solid lines marked P are compressional waves; dashed lines marked S are shear waves. S waves do not travel through the core but may be converted to compressional waves (marked K) on entering the core (PKP, SKS). Waves may be reflected at the surface (PP, PPP, SS).Data from earthquake waves, rotations and inertia of the whole Earth, magnetic-field dynamo theory, and laboratory experiments on melting and alloying of iron all contribute to the identification of the composition of the inner and outer core. The core is presumed to be composed principally of iron, with about 10 percent alloy of oxygen or sulfur or nickel, or perhaps some combination of these three elements. |
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This table of depths, densities, and com-position is derived mostly from information in a textbook by Don L. Anderson (see Suggested Reading). Scientists are continuing to refine the chemical and mineral composition of the Earth’s interior by laboratory experiments, by using pressures 2 million times the pressure of the atmosphere at the surface and temperatures as high as 20000C.
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to build a cross section of earth model. During this project we will investigate geological resources to find out what is inside the earth and how do geologists know about that.
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Identifying variables does not apply to this project.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Hypothesis does not apply to this project, however you may hypothesize about the results of the research that you will do.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1:
Trying to “see” what is beneath the surface of the Earth is one of the jobs of a geologist. Rather than digging up vast tracts of land to expose an oil field or to find some coal-bearing strata, core samples can be taken and analyzed to determine the likely composition of the Earth’s interior. In this activity, we simulate core sampling techniques to find out what sort of layers are in a cupcake.
MATERIALS NEEDED:
Cupcake mix
Plastic knife
Drawing paper
Frosting Food coloring
Foil baking cups
Toothpicks
Plastic transparent strawsDIRECTIONS:
Prepare cupcakes according to package directions, but use at least three different colors of batter. Layer batter in colors in the muffin cups. Using foil baking cups and frosting will prevent us from seeing the interior of the cupcakes, in the same way that a geologist can’t see the interior of the Earth. (the frosting layer is equivalent to the soil.) Use a straw, a toothpick, and a piece of paper. Fold a piece of drawing paper into four sections and in one of the sections draw what you think the inside of the cupcake would look like. How can you get more information about the cupcake without peeling the foil or cutting it open with a knife. You may suggest using the straw to take a core sample. Push the straw into the cupcake and pull out a sample. Remember to use the straw like a drill, rotating it through the cupcake (straws can be cut to a length slightly longer than the depth of the cupcake.) You should make a second drawing of the cross section of your cupcake based on the information from three core samples. Each new drawing should be carefully labeled and placed in a different section of the recording paper. Finally, you should cut open the cupcakes with a knife to compare them to the drawings.
Questions:
How did this experiment help you to understand what geologists do? Can you think of other ways or reasons (other than looking for oil or coal) that we might learn something by drilling core samples? (Groundwater resources, for example, or deep-sea drilling in paleoceanography.)
Stewardship Did you make a “mess” when coring your cupcake? Do you think that coring rocks also produces material that has to be cleaned up? What steps can be taken to minimize the environmental impact of drilling?
Earth Science is all around you What objects in your room come from petroleum products or mineral resources? Make a list of these materials. What mineral resources are found underground in your state?
Experiment 2: (Earth Bowel)
Scientists know that the earth is made of four layers: the inner core, outer core, mantle, and crust. The Earth Bowl is a three dimensional, edible representation of the earth in cross section.
Materials:
4 oz. pkg. raspberry gelatin dessert
4 oz. pkg. instant vanilla pudding
8 oz. pkg. black cherry gelatin dessert
4 cups boiling water (can be boiled and kept hot in thermos)
4 cups cold water
3 mixing bowls
12 graham crackers
1/2 cup melted margarine
1/4 cup granulated sugar
10″ diameter clear glass bowl
(Small paper cups and spoons for after discussion)
Procedure:
Make the gelatin desserts in separate bowls and according to the directions on the side of the box. Put in refrigerator to set.
Crush the graham crackers into fine crumbs. This can be done by putting the crackers in a zip-lock bag and pound the bag until the crackers are in very fine crumbs.
Mix the graham cracker crumbs with the melted margarine and granulated sugar. Press the mixture on the bottom and along the sides of the glass bowl to form a crust. Set aside.
After two hours the gelatin will set. Spoon the black cherry into the graham cracker crust. Form it so that there is about a five inch pocket in the middle.
Next, spoon in the lemon gelatin, leaving a two inch hole. Into this center, spoon the raspberry gelatin.
Important Concepts:
(1) In the Earth Bowl the graham cracker crust represents the earth’s crust which is about 20 miles deep; the black cherry is the mantle (4,000 miles deep); the lemon gelatin is the outer core (2,200 miles deep); and the raspberry gelatin is the inner core (800 miles deep).
(2) Although the Earth Bowl is made of cool gelatin, the center of the earth is actually extremely hot.
(3) In the classroom the Earth Bowl took only a few hours to make; in actuality the earth took 5 billion years to form.
Experiment 3:
Use a large Styrofoam ball to make a model of the earth cross section. Use the information and pictures in this page or other resources to make a model of the earth cross section. If you can not find a large Styrofoam ball in your local craft store, simply use Styrofoam sheets and make a cutaway model.
Cutaway views showing the internal structure of the Earth. Below: This view drawn to scale demonstrates that the Earth’s crust literally is only skin deep. Below right: A view not drawn to scale to show the Earth’s three main layers (crust, mantle, and core) in more detail (see text).
Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. The mantle, which contains more iron, magnesium, and calcium than the crust, is hotter and denser because temperature and pressure inside the Earth increase with depth. As a comparison, the mantle might be thought of as the white of a boiled egg. At the center of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy) rather than stony. Unlike the yolk of an egg, however, the Earth’s core is actually made up of two distinct parts: a 2,200 km-thick liquid outer core and a 1,250 km-thick solid inner core. As the Earth rotates, the liquid outer core spins, creating the Earth’s magnetic field.
Materials and Equipment:
List of materials can be extracted from the experiment design section.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Calculations:
Description
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
