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Time Bombs and Timber

The Mountain Blows Its Top
(These demonstrations were adapted from Volcanoes! Teaching Guide, U.S. Geological Survey.)

Background: On May 18, 1980, Mount St. Helens erupted violently. At 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake occurred about one kilometer beneath the volcano, triggering a catastrophic series of events that transformed Mount St. Helens' picturesque mountain landscape into a gray wasteland. The earthquake shook the walls of the volcano's summit crater and triggered many small rock avalanches. Within seconds a huge slab of the volcano's north flank began to slide, and small dark clouds began to billow out of the base of the slide. Plumes of steam and ash rose from the volcano's crater. As the avalanche of rock and ice raced down the mountain's north flank at more than 250 km per hour, a massive explosion blasted out of the north side of the volcano. This lateral blast became a fearsome torrent of ash and rock that outraced the avalanche. Probably no more than 20 to 30 seconds had elapsed since the triggering of the earthquake!

Mount St. Helens
Mount St. Helens


The eruption of Mount St. Helens was not a surprise to scientists who had been monitoring changes on the mountain for two months prior. For a volcano to erupt, magma must move to the Earth's surface. Increased earthquake activity, eruptions of steam and ash, and changes in the shape of the surface of the volcano all signal that magma is moving toward the surface. Inside the volcano, the solid rock that surrounds the molten rock often cracks from the increased pressure and causes earthquakes. Between March 20 and May 18, 1980, more than 10,000 small earthquakes were recorded beneath Mount St. Helens, and the larger earthquakes were felt by people living near the volcano. In addition to recording the distinct jolts characteristic of earthquakes, seismographs also detected continuous rhythmic vibrations called harmonic tremors—further evidence that magma was moving within the volcano.

As magma forced itself inside, the volcano swelled, or inflated. By early April, Mount St. Helens' north flank began to visibly bulge and crack. The bulge grew 2-3 m a day and moved outward about 150 m in two months. When the 5.1 magnitude earthquake shook Mount St. Helens on May 18, the bulge collapsed. The resulting avalanche was the largest volcanic avalanche recorded in historical times. In turn, the sudden removal of masses of rock and ice by the avalanche triggered an explosive eruption of the steam that was trapped in cracks and voids in the volcano and of the gases dissolved in the magma. Unleashed by the abrupt release of pressure, magma, rock, ash, aerosols, and gases exploded from within the volcano's north flank.

The Mountain Is Transformed
In a few minutes, Mount St. Helens' symmetrical cone was transformed—it was 400 m shorter and a gaping crater was gouged into its north side. An avalanche of rock, ash, ice, water, and fallen trees flowed as far as 9 km down the valley of the North Fork Toutle River. Debris dumped into Spirit Lake, raising the lakebed more than 940 m. The lake's cool, crystal-clear waters became a black stew of rocks, mud, and floating trees. Gone were 70 percent of the glaciers that had crowned the volcano; they were either melted by the heat of the eruption or carried away by the fast-moving avalanche. Trees up to 45 m tall were flattened and strewn like matchsticks in the wake of the lateral blast and debris-laden avalanche.

Eruptions Continue
Between May 18, 1980, and October 1995, Mount St. Helens produced at least 21 eruptions of magma and dozens of smaller gas explosions. All this volcanic activity took place in the bottom of the crater that was created by the eruption on May 18, 1980. Mount St. Helens is rebuilding itself as new lava squeezes up during each eruption to push aside old material from the surface of the dome. The volcanic activity that began in 1980 is not yet over.

By observing the following two demonstrations, students will understand why a bulge developed on the north flank of Mount St. Helens and why the avalanche triggered an explosive eruption.

Key teaching points:
1. The bulge that developed on the north flank of Mount St. Helens was evidence of changes occurring inside the volcano. Magma was moving closer to the surface and inflating, or deforming, the side of the volcano.
2. Scientists had been closely monitoring the growth of the bulge for nearly two months to try to forecast an eruption.
3. The 5.1 magnitude earthquake on May 18, 1980 shook the entire volcano.
In turn, the shaking of the bulge area caused a sudden collapse of the volcano's north flank and triggered a large avalanche.
4. The removal of this large mass of rock caused a sudden release of pressure inside the volcano and a violent eruption occurred.


  • 1,500 mL Pyrex beaker
  • damp sand
  • several small balloons
  • rubber bands
  • Bunsen burner or hot plate
  • straight pin
  • bottle of soda water
  • basin or bowl

Before class begins put about 1.25 cm of sand in the bottom of the beaker and level the surface of the sand. Partially inflate a balloon, secure it with a rubber band, and place the balloon on top of the sand in the beaker. Cover the balloon with sand to a depth of about 3.75 cm. Level the surface of the sand.

Begin the lesson by reviewing the series of events that occurred on May 18, 1980. Discuss the following events: the bulge that had been growing on the north side of the volcano for a month, the 5.1 magnitude earthquake that triggered the avalanche, and the avalanche that unleashed an explosive eruption—a lateral blast.

Demonstration 1: Why the bulge grew
1. Partially inflate a balloon. Ask students what would happen to the balloon if you were to heat the air inside the balloon? (The balloon would expand because the air expanded.) Explain that inflation caused a bulge to develop on the north side (flank) of Mount St. Helens (See Figure 1).
2. Tell the students that the inflated balloon represents the magma chamber of Mount St. Helens.

Figure 1: Depiction of magma bulge on the north side of Mount St. Helens
Figure 1

3. Show the prepared beaker to the students and tell them there is a partially inflated balloon inside the beaker. Place the beaker on the Bunsen burner or the hot plate. Heat the beaker until the balloon begins to expand. (The surface of the sand should begin to bulge.) (See Figure 2.)

Figure 2: Bunsen burner, beaker and balloon
Figure 2

4. Students should observe and record the changes in the shape of the surface of the sand. What happens to the "land" as the "magma chamber" expands?

Demonstration 2: Why the avalanche triggered the explosive eruption
1. Ask students what would happen if they were to stick a pin into the balloon.
(It would pop.) Why does it explode? (The balloon bursts because the pressure inside the balloon is suddenly released and the gases can escape rapidly.) Burst the balloon with the straight pin.
2. Ask students what happens when they open a bottle of soda. (It makes a fizzing sound because the carbon dioxide, CO2, in the soda escapes.) Demonstrate this over a bowl or basin by shaking a bottle of soda water and then opening it.
(The soda water "erupts" out of the bottle.)
3. Compare the soda bottle to a volcano's magma chamber. As long as the top is on the bottle there is no eruption. Compare the rock and ice that was unloaded by the avalanche to the soda cap. When this rock and ice "cap" was suddenly removed, the pressure inside the volcano was suddenly released and the volcano erupted.

Salamanders and Slugs and Spruce, Oh My!
(foldout guide)

Welcome to an autumn scene in the Emerald Ecosystem. In the evergreen forests of the Pacific Northwest flora and fauna abound at all levels.

We are in a clearing of the Coastal Range region of this ecosystem. Near the forest floor, purplish Oregon grapes flank a sword fern frond hosting a banana slug. On the ground nearby, a copper-brown ensatina salamander is poised on a piece of tree bark. Alongside the bark lies a Sitka spruce cone under a frond of maidenhair fern. Under a brilliant red mushroom (Amanita muscaria, pretty—but poisonous) a lumpy-backed snail eater beetle is looking for a meal near a patch of yellow Ramaria fungus.

Malone jumping-slug The Malone jumping-slug (Hemphilia malonei) was known to live in only six locations and numbered fewer than 50 individuals prior to 1997. Recent surveys in forested habitat have found the slug to be locally common in southwest Washington and northwest Oregon, especially where there are large and rotting logs on the forest floor.

Farther in the distance a red-backed vole peeks its nose out from behind an Amanita stalk, which is nestled among more sword ferns. Beyond them, Rhododendron bushes flourish. To the left, you'll see a black-tailed deer and a snag with ferns growing within and around it. Close by, and near some lady fern, lurk a northwestern garter snake and a furry, weasel-like pine marten. Travel a bit to the left and you'll see, under a branch of Rhododendron, another snag that is home to ferns, Marasmius bresadolae mushrooms, and a Pacific tree frog.

Farther back in the clearing is a fallen tree acting as a nurse log to a western hemlock sapling. A vine maple, its leaves yellow in fall color, rises up from the forest floor. Swooping down from the canopy, a spotted owl seeks a meal of voles. Off in the distance and also on the lookout for food, a bobcat perches on an outcrop. Soaring up to the sky are western hemlock and Sitka spruce trees, their vast branches forming a canopy over the forest clearing. Look closely and you can see epiphytes—air plants—nestled in some of their branches.

Transportation of Logs Using a Single-Span Skyline Yarding System
Bibi Booth, John Caruso, and Frances Philipek

In the logging profession, yarding refers to the transportation of logs from the mountainside where they were cut to a level location (landing) where they can be stacked (decked) and loaded onto a truck or train for hauling from the forest. The two basic forces that any yarding system must overcome are gravity and friction.

What Is a Single-Span Skyline Yarding System?
On many mountain slopes, loggers use a single-span skyline yarding system to transport logs. In this system, logs are attached by a vertical cable (choker) to a carriage, which slides horizontally on wheels along the mainline, a movable cable attached to the skyline, which is a cable strung between two upright spars (cable anchors). The carriage allows logs to be transported along the cable up the mountainside to the landing (see diagram). A motor provides the power to move cables in this type of system.

Simplified, idealized single-span skyline yarding system shown operating on a single mountain slope
Simplified, idealized single-span skyline yarding system shown operating on a single mountain slope. In practical applications of this logging system, the tail of the skyline cable would typically be fastened on a tree stump or other anchor on an opposite mountian slope.

The objective of this type of logging operation is to lift logs as high as possible above the mountain's surface to minimize damage to ground soil, vegetation, and the logs themselves. The height of the spars, the elevation of spar support points, the distance between the spars, the steepness of the mountainside, and the weight and size of the logs all affect the percentage of loaded deflection (sag in the skyline cable) that can be accommodated before the logs or the skyline will begin to drag on the ground. Because natural ground is uneven, bumps and dips in the mountainside's surface must also be taken into account.

How Do You Determine the Best Arrangement of Components?
Today's loggers use computers to determine the best arrangement for single-span skyline yarding systems; however, before computers became commonplace (1980s), loggers created simple "chain and board" models of a mountainside to make their determinations. By experimenting with variations of the model's components, the loggers could decide which arrangement would allow the heaviest load to be transported the farthest distance up a mountain at the lowest cost with the least damage to the ground. These models also helped them determine whether skyline logging of a particular mountainside was feasible at all.

What Is the Best Arrangement?
The optimal arrangement of the components of a single-span skyline yarding system differs from one mountain to the next because of differences in steepness and ground irregularities. For a given mountainside, the best arrangement for the single-span skyline yarding system is one that can accommodate a lot of loaded deflection in the cable line and still allow the heaviest logs to clear the ground surface. At a minimum, the system must transport the heaviest logs up the slope without allowing the skyline cable or the log's leading edge (the uphill-facing edge) to touch the ground at any point. If these conditions are not met, the single-span skyline system will not work to transport logs from the desired stretch of mountainside. In addition, the best single-span skyline yarding system will function over a span of at least 300 m in horizontal distance and will allow erection of the shortest possible spars, permitting logging of a large area at the lowest possible expense.

The single-span skyline yarding system functions much like an elongated version of a motorized crane, such as those used in building construction. Other mechanical systems that apply the same principles include the winches on tow trucks, and the block-and-tackle systems used in construction and other operations that require lifting heavy objects.

Why Are Logs Transported Up Mountainsides?
Please note that, in this type of system, logs are transported up the side of the mountain—against the force of gravity—to the landing. Though it is natural to think that downward transportation (working with gravity) would be easier, the reasons for upward transportation in logging are very practical. In order to minimize damage to the logs and remaining trees, mountainside trees are felled so that their tops either fall toward the top of the mountain or to the side of the path of the skyline. The trees actually fall a lesser vertical distance to the ground when felled in these directions rather than downhill. Typically, felling to the side results in a herringbone arrangement of logs on the mountainside. The choker cable then lifts the trees by their bottoms, so that the greater proportion of log weight is suspended close to the carriage. The uphill direction of transport prevents the trees from rotating in the process of being lifted. Also, uphill transportation of logs allows operators more control of the logs' weight; if the logs were coming downhill, their momentum might pose a risk to human life and logging machinery.

Moving a Load
This activity demonstrates to students how loaded deflection changes when you change the height of a load.

  • 16 m length of rope
  • 7.5-11 kg weight
  • stable chairs or table

1.Attach the weight to the middle of the rope. Set the weight on the ground. Have two students (one on each end of the rope) stretch the rope (from waist level) until the weight has been raised 0.3 m off the ground.
2.Place the weight on the seat of a chair or on a low table and have the same students pull the rope from waist level to raise the weight 0.3 m above the chair seat or table.
1.Was it more, less, or equally difficult to raise the weight 0.3 m (obtain 0.3 m of lift) when the weight was raised from the floor versus from the chair?
2.What changes could be made to this simulation to make it easier to obtain 0.3 m of lift with the weight on the chair? (Try having the students who are doing the lifting stand on chairs or tables, or have them pull the rope from shoulder height rather than waist height.)
3.Is it easier or harder to obtain 0.3 m of lift if the students are positioned closer to the weight? Is it easier or harder when one student pulls while standing on a chair or table and the other student pulls from ground level? How about when both students are on chairs or tables?

"Chain and Board" Activity
The "chain and board" approach allows loggers to determine the suitability of a mountainside for log transport without the use of complicated mathematical formulas or computers. In this activity, students will design a model of a single-span skyline yarding system for a hypothetical mountain, adjusting various components of the system to achieve the most efficient arrangement for log transportation. They may find that some mountains simply cannot be logged with a single-span skyline yarding system without damaging the ground or cannot be logged over a great enough horizontal distance (at least 300 m) to be cost-efficient. Keep in mind that this activity simulates a hypothetical situation in which single-span skyline yarding is performed on one portion of a single mountainside. Normally, single-span skyline yarding involves anchoring the cables across a valley from the mountainside to be logged. This arrangement increases the amount of deflection that can be accommodated by the system. (Note: Students should work in groups of four. Also, to simplify this experiment, only the skyline cable is represented. The mainline cable is not represented, and its functions are combined with those of the skyline cable.)

Students follow a planning/design procedure used by loggers to determine:

  • whether or not a particular mountainside can be logged with a single-span skyline yarding system,
  • how to best construct a single-span skyline yarding system to transport the most logs and minimize environmental damage,
  • the maximum load that the system can transport up a particular mountainside.

Materials for each group of students:
  • one 1 m x 1.25 m or larger sheet of graph paper
  • one 1 m x 1.25 m or larger corkboard, bulletin board, foam board, or other board suitable for inserting pushpins
  • 7-10 pushpins with elongated heads
  • 1 m length of beaded metal chain (such as that used for pull-chains on light sockets)
  • one standard paper clip (approximately 0.9 cm x 4 cm)
  • one narrow, cylindrical eraser (such as the type used as eraser refills for mechanical pencils); using the scale you have selected (see Procedure, Preparing the Board, Step 3 below), cut the eraser to a length representing 12.5 m
  • meter stick or ruler with metric units
  • one 3 cm length of drinking straw
Photo of single-span skyline yarding system
Single-span skyline yarding systems enable loggers to remove trees from steep slopes.

John C. Stewart

I.  Preparing the board

  • Step 1. Place the corkboard, foam board, or bulletin board flat on a table with the 1.25 m side running left to right and the 1 m side running up and down.
  • Step 2. Attach the graph paper at the four corners of the board with pushpins; if necessary, anchor with additional pins at top, bottom, and sides.
  • Step 3. Select a scale to use to create your model. For example, for a 1 m x 1.25 m board, a scale of 1 cm = 3 m works well. Note: the horizontal and vertical scales must be the same. Show your scale in a legend block in one corner of the graph paper.
  • Step 4. Starting near the upper left corner of the graph paper, draw a line down to the lower right hand corner to represent the profile of one side of a mountain. Remember, the profile should not be an even line or curve because natural mountainsides have bumps and dips and often change steepness from section to section.

II.  Creating the model of the single-span skyline yarding system

  • Step 1. Drawing the spars: Draw two vertical lines whose bottom ends touch the surface of the mountainside; one line should be near the top of the slope, the other near the bottom. These represent the headspar and tailspar of the logging transport system. Typically, spars are between 16 m and 27 m tall; make sure you draw your spar lines to scale. For example, if you are using a scale of 1 cm = 3 m, your spar lines should be between 5 cm and 9 cm high. The distance between spars can vary; position the spars at least 300 m apart, according to your scale.

1Graphic showing method of determining the spar connection points for the skyline cable

  • Step 2. Determining the spar connection points for the skyline cable: Measure the width (narrow dimension) of your paper clip. Now, measure that distance down from the top of each spar line and place a dot at each of those points. On the lower spar line, mark a second dot 3 m-4.5 m below the first dot, measured to scale.

2Graphic depicting how to draw the chord and midspan line

  • Step 3. Drawing the chord and midspan line: Draw a straight line between the dot on the upper spar and the lower dot on the lower spar. These two dots represent the support points for the skyline cable. The straight line between them, called the chord, represents the shortest distance between these points. Now, using a yardstick or ruler, mark the midpoint of the chord and draw a straight vertical line between the midpoint and the mountainside. This is the midspan line.

3Graphic illustrating how to erect supports for the skyline—stringing the skyline

  • Step 4. Erecting supports for the skyline—stringing the skyline: Firmly insert pushpins at the two marked skyline support points on your spars. (Remember, on the tailspar this is the lower dot.) Insert a third pin a couple of centimeters downslope from the lower support point; this will be used later to adjust the tension on the skyline. Secure the skyline (chain) at the headspar support by knotting it around the head of the pushpin. Pass the skyline gently over the tailspar support and secure it to the pushpin located downslope from the tailspar support. Now, raise your board to an upright position and lean it against a wall or other support.

4Graphic illustrating how to construct the carriage apparatus

  • Step 5. Constructing the carriage apparatus: Partially unbend one end of your paper clip to create a straight section that forms a right angle with the body of the paper clip. Beginning at the end of the straightened section of the paper clip, gently slide the drinking straw onto the paper clip. Push the point of the straight end of the paper clip partly through the narrow width of the eraser near one end (see diagram). The body of the paper clip represents the carriage; the straightened section of the paper clip represents the choker; and the eraser represents a log.

5Graphic illustrating the carriage apparatus

  • Step 6. Attaching the carriage to the skyline cable: Temporarily remove one end of your chain, threading it through the straw, and reattach the link of the chain to its anchor. The carriage must be able to move freely on the skyline. (Note: If you have trouble maintaining attachment of your chain on the pins, one group member should hold the chain at the headspar support pin and another student should hold the chain at the pin downslope of the tailspar.) Slide the carriage back and forth along the skyline a few times to test its freedom of movement.

6Graphic depicting how to adjust the skyline cable for ground clearance

  • Step 7. Adjusting the skyline cable for ground clearance: For the transport system to function successfully, your skyline cable must be situated such that neither the skyline cable nor the leading edge of the log touches the ground at any point along the log's path up the slope. The log does not have to be fully suspended, but its leading edge must clear the ground. Slide the carriage the length of the skyline cable, maintaining the carriage in a horizontal position along the cable at all times. Whenever the skyline cable and/or log dips below the ground profile, adjust the tailspar pin up or down on the spar line so that the skyline cable and log just clear the profile along the entire distance between spars. Check the entire span between the spars in this manner. When you have determined that there is clearance along the entire span, detach the end of the skyline from the lowermost pin and anchor it to the tailspar pin at the support point.

7How to measure the loaded deflection distance and the allowable percentage of loaded deflection that can be accommodated on the slope

III.  Measuring the loaded deflection distance and the allowable percentage of loaded deflection that can be accommodated on the slope:

  • Move the carriage and log to the midspan position on the skyline cable, and measure the vertical distance between the skyline cable (chain) and the chord (the straight line between spars). This midspan vertical distance is the amount of loaded deflection.
  • Measure the chord between your spar supports. To obtain the allowable percentage of loaded deflection, divide the loaded deflection by the chord distance and multiply this figure by 100. For example, if loaded deflection is 15.25 m and chord distance is 300 m, the allowable percentage of loaded deflection is 15.25 m / 300 m x 100, approximately 5 percent.

IV.  Discussion:

  • Were you able to transport logs up your mountainside? If not, what could you have changed to make log transportation possible?
  • Compare your results with those of other groups that used steeper or gentler slopes, more even or bumpier ground surfaces, different weights of chain, higher spars, etc. What differences existed in the ways their systems functioned?
  • Which elements of this model could be altered to change the amount and allowable percentage of loaded deflection and other system characteristics?
  • Try these variations:
    • raise or lower the height of the headspar and/or tailspars
    • increase or decrease the distance between the spars
    • change the ground profile (this is equivalent to changing your spar locations or selecting a different mountainside profile)
    • use a heavier or lighter chain
    • use a lighter load (break the eraser in half lengthwise, use a smaller eraser, and/or use a smaller paper clip, etc.)
    • use a heavier load (attach two erasers)
    • try arranging a single-span skyline system in which the two ends of the skyline are separated by a canyon, river, fragile habitat, etc. (Note: In this case, the tail of the cable should be anchored close to the ground, not on a spar.)

Logging Vocabulary
carriage: a wheeled device to which the choker is attached; the carriage slides along the mainline cable (Note: The carriage and choker are attached directly to the skyline in the "Chain and Board Activity.")
a short cable, running vertically down from the carriage, to which a log can be attached
chord: a straight line between the skyline support points on the spars
sag; even when it is not transporting logs, a skyline cable always experiences some deflection
headspar: the spar that is farther up the slope
the level area at the top of the slope where logs are stacked; from this area, logs are loaded onto vehicles for hauling
leading edge: during uphill transportation, an edge on the uphill-facing end of the log
loaded deflection:
the amount of sag in the skyline cable that is caused by the weight of the log and carriage; the greater the log weight, the greater the loaded deflection of the skyline cable
the cable that moves the carriage and is parallel to the skyline cable (not modeled in the "Chain and Board Activity")
a vertical line drawn between the midpoint of the chord and the surface of the slope
single-span skyline:
a skyline without intermediate supports, i.e., supported only by a headspar and tailspar or other tail anchor
the property of a body that determines the length of time required to stop its motion
a heavy cable, suspended between the headspar and tailspar, used as a track along which the mainline, carriage, choker, and log move up the slope
one of two upright poles to which the skyline cable is anchored
a large cylinder around which cable is wrapped to control slack
the spar that is farther down the slope
yarding: the process of conveying logs to a landing

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Last updated: 11-13-2009