Earth Science Today
Russ Colson
Minnesota State University Moorhead

Physical Geology:

Topic 1: Physical Catastrophes: Earthquakes, volcanoes, and Earth's structure, what's inside the Earth?

We can consider three main sources for energy driving Earth processes.  These are the following:

1)  Remnant energy (such as heat or kinetic energy left over from earlier in the history of the Earth and solar system)

2)  Solar energy (received from the sun in the form of light, other electromagnetic energy, or particles)

3)  Radioactive energy (from the decay of radioactive isotopes in the Earth's interior which releases energy)

Examples of how each of these affects the Earth include the following:

1)  The tides (affecting Earth's oceans and atmosphere, and even the lithosphere) tap energy from the motion of the Moon, Earth, and Sun.  Thus, these would fall under "remnant energy" in the simple classification above.

2)  The primary source of energy for weather and storms is the Sun, which heats the Earth's surface unevenly, resulting in winds and storms, which attempt to even-out the distribution of heat.

3)  The primary source of energy for volcanism is radioactive decay in the Earth's interior, which provides heat that becomes locally concentrated enough to produce partial melting of Earth's rock.

 

Energy moves in the following ways:

1)  Radiation: Every object is "glowing", emitting energy by electromagnetic radiation. Sometimes this radiation is visible (light), sometimes it is not visible (radiation in parts of the spectrum we can't see, such as radio waves, microwaves, infrared radiation, ultraviolet, x-rays, or gamma rays). This energy can go through space until it encounters a material that absorbs it.

Electromagnetic spectrum (est1a1.html)

2)  Conduction: Energy can cause atoms and molecules to move or vibrate differently in a particular material. This energy can move from one material to another when those materials are in contact with each other.

3)  Convection and Advection: Energy present in a particular mass of material can move with that mass if the mass itself moves. Convection refers to up-and-down movement of masses, usually because of differences in density, advection refers to the lateral movement of mass, such as wind or ocean currents.

Examples of how these work on Earth include the following:

1)  The ground cools off at night because energy is radiated into space, mainly as infrared radiation.

2)  You put a metal stirring rod on a hot plate that is turned on. One end of the metal stirring rod is not touching the hot plate. You pick up the end that is not touching the hot plate, but get burned anyway! This happens because the heat was conducted to the metal rod where it touches the plate and then to the end not touching the plate. (Watch out! This really happened one year!)

3)  The surface of your coffee is cooling off, as it gets cooler than the parts deeper in your cup it sinks, and warmer, less dense coffee rises to the surface by convection.

Illustrative activity: Take a hot plate and turn it on. How is heat moving near it? Think about it, investigate it, talk to someone else about it. (answer, est1a2.html)

 

There are many types of conversion. For example, absorption of light can convert electromagnetic energy to heat or chemical energy. A chemical reaction can convert chemical energy to heat energy. Friction can convert mechanical or kinetic energy into heat energy. Chlorophyll can absorb light energy and convert it into chemical energy. Heating of air over a hot plate can convert electrical energy (used to heat the hot plate) into kinetic energy (the moving air). A fundamental property of the universe appears to be that energy is neither created nor destroyed, but simply changes form. Einstein showed that even matter can be viewed as a form of energy which can be converted into other forms.

Thought-problem:  You rub your hands together to warm them in the cold of a Moorhead winter.  Which of the three sources of energy above is the main source of energy heating your hands?  (answer, est1a3.html)
 
 


 

1)  Local areas of heating within the Earth's crust or near the top of the Earth's mantle cause regions where the rock partly melts. The molten rock is less dense than the surrounding solid material, and it rises up through the crust buoyantly. The source of heat is radioactive decay of elements in the mantle, including Potassium (K) and some Uranium (U) as well as other elements with radioactive isotopes. The heat is focussed in areas of melting by concentrations of these isotopes and/or convection of material in the mantle.

(Sketch of Earth's inner structure, est1a4.html)

2)  Lava is expelled onto the Earth's surface primarily by the work of gases that exsolve from the melt as it rises toward the surface. As the magma rises, the pressure on it decreases (because there is less weight of rock above it). The decrease in pressure allows gases to bubble out of the magma (exsolve). This greatly increases the volume of the gas+liquid because the gas takes up so much more volume than the liquid. This increase in volume helps propel the magma onto the surface as lava. Also, gas is compressible, allowing it to store energy like a spring. The compressible gas makes a volcano more explosive.

(activity with a lava simulant and syringe, est1a5.html)

3)  The molten rock may reach the surface where it cools quickly, or it may become lodged within the Earth's crust where it cools more slowly. Both experiments in the laboratory and observations of volcanic eruptions show that the more slowly a magma cools, the larger the crystals that grow in it become. In this way, we can tell the difference between an igneous rock that cooled slowly deep in the crust (big crystals) from one that cooled more quickly at or near the Earth's surface (crystals too small to see without a microscope).

(Historical note, est1a6.html)

  Thought problem: How is a plaster of paris volcano, made by adding vinegar to baking soda in the summit of a home-made plaster of paris cone, like a real volcano? (answer, est1a7.html)
 


 

1)  Energy for Earthquakes comes from radioactive energy in Earth's mantle. Radioactive decay produces heat that causes convection in the mantle. This movement is transferred to Earth's crust where movement stores up energy in rocks, like a spring being stretched. When the rock breaks, the stored energy is released suddenly. This energy is then carried outwards from the break by seismic waves, a form of energy radiation. The prerequisites for an Earthquake are therefore: energy, motion, a material that can store energy like a spring, sudden release of energy, and transport of the energy.

2)  Not all materials can store energy like a spring. A material must be elastic in order to store energy. Elastic means that when it is stretched it will spring back to its original shape and position. Plastic materials on the other hand will stay bent when bent. Most materials can behave both ways depending on the situation. Think of bending a paper clip. If you bend it a little bit, it springs back. If you bend it a lot it will spring back a ways but will stay somewhat bent. Think of bending a stick of sugar candy until it breaks. It stores energy and releases it suddenly when it breaks. But if you heat up the candy stick it will tend to bend more easily and not store energy. Rocks are elastic and store energy when they are cool. As they get hotter and under greater pressure, they behave more plastically. Thus, Earthquakes are restricted to the shallower parts of Earth where rocks are relatively cool.

3)  There are several types of seismic waves that carry the energy of the earthquake away from the focus (where the original break occurred). Note that the wave moves energy, but there is no net movement of matter (think about a fishing bobber on a lake moving up and down as waves pass). A seismic wave is the passing of a vibrational movement in the matter it travels through. There are different types of seismic waves. Some vibrations can pass only through solids. Some vibrations only occur at the boundary between two types of materials.

(Types of seismic waves, est1a8.html) ("Hearing" into the Earth, est1a9.html


Lab on locating time and place of an earthquake (in MSWORD)

Lab on seismic wave velocities and what they tell us (est1a10.html).

Shortened lab on seismic wave velocity and what waves tell us (in MSWORD)

Shortened lab on seismic wave velocity and figuring out length (in MSword)

 

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Photo credit:  Pu'u O'o eruption, Lee Allen Thomas