Landslide at Sable Pass EarthCache

Background:
At about mile 40 of the park road and on the south side, a fracture began appearing in the tundra around the second week of June 2009. During the night of 12 June 2009, a landslide (broadly defined as a movement of gravity-driven ground movement) was released and traveled about 30-40 feet down slope (Figure 1). This slump exposed sandstones of the Usibelli Group (20 to 5 million years old) on the right (as viewed from the road), and recent glacial gravels which probably contain silt and/or clay on the left.

Figure 1: Sable Pass landslide in 2009

A landslide at Mile 45 in the Park has been monitored since 1993 because it poses a threat to the road. When it was measured in 1999, the top of the landslide was within 35 feet of the Park road, but in 2007 the headwall of the landslide was only 27 feet away from the road. Accelerated downslope movement on the slump surface is correlated with high precipitation years (1993-94, 1997-98, 1999-2002, and 2004-05). The rates of movement suggest there will be no immediate threat to the Park road for the next 5 to 10 years.

Total dollar losses from landslides in the United States are conservatively estimated to be between $1.6 and 3.2 billion (in year 2000 dollars). Each year between 25 and 50 people in the United States are killed by landslides. Susceptible regions include the coastal and mountainous areas of Alaska, California, Oregon, and Washington, the Rocky Mountain states, and the hilly areas of the east. Geology happens all around us all the time, and places like the one before you show that the land before you may not be as stable as it looks, and what you see today could be different tomorrow. Landscape changes all the time, but maybe not on a human timeline. Denali National Park preserves these natural processes instead of battling them to remain the same.

There are a number of types of landslides:

•Rotational landslides (slumps) (a) displace material along a curved slip surface, and slump blocks tend to form topographic benches (sometimes rotated or tilted). Slumps are most common on soil slopes, but also may occur on rock slopes (especially weak rock like shale).
•Translational landslides (b) displace material along a planar (flat, but tilted) surface with little or no rotation or block tilting. They occur in all rock types, bedding planes, clay partings, and foliation planes in metamorphic rocks.
•Block slides (c) are translational landslides where the material moves as a single unit.
•Rockfalls (d) are very rapid rock failures.
•Topples (e) are defined by a forward rotation of a unit or units about some pivotal point.
•Debris flows (f) are a form of rapid materials movement composed of loose rock, soil, organic matter, air, and water.
•Debris avalanches (g) are a type of very or extremely rapid debris flows.
•Earthflows (h) are the downslope movement of saturated earth materials, slow or rapid, and form an ‘hourglass’ shape.
•Creep (i) is a type of imperceptibly slow movement indicated by curved tree trunks, bent or tilted fences or retaining walls, tilted poles, and small soil ripples and ridges.
•Lateral spread (j) forms on very gentle slopes or flat terrain. Failure is usually caused by liquefaction from rapid ground movement, either natural (earthquake) or artificial.

Landslides occur because of slope instability, dependent on a number of factors:

1. Forces on slopes: The stability of a slope depends on the relationship between driving forces and resisting forces. Driving forces (most commonly the weight of slope material) move earth materials down slope, and resisting forces (most commonly the shear strength of the slope material along slip planes) oppose this.

2. Earth material type: The material affects the type and frequency of down slope movement (see above types).

3. Slope and topography: As the angle of the slip plane increases, the driving force also increases.

4. Climate/water and vegetation: Climate controls the amount of precipitation and moisture content of the slope. Vegetation cushions a slope and slows erosion by having root systems that provide slope cohesion, BUT it also adds weight to the slope.

How to access this EarthCache:

This EarthCache is completely visible from the Denali National Park road on the south side of the road. You can observe it from the bus (seated on the left side inbound and on the right side outbound will help visibility). You may ask shuttle bus drivers to stop for pictures. Other possibilities include biking and hiking the road.

Please remember to practice Leave No Trace ethics to preserve this experience and environment for future visitors:
•Hiking off-road is not required at this EarthCache.
•Collecting samples is not allowed in National Parks.

To log this EarthCache: Email the answers to the following questions to me (click my username up top, send message).

1. What type of landslide was it? See the above figure 2. A combination of landslide types is called a ‘complex landslide’. If you think it is complex, name which types it combines.

2. Describe the difference in the landslide’s appearance between the photo taken in 2009 (above) and how you see it now.

3. Does it seem like this landslide threatens the Park road?

Sources:

Gilbert, Wyatt G. A Geologic Guide to Mount McKinley National Park. Anchorage: Alaska Natural History Association, 1979.

Godt, J.W. Landslide Overview Map of the Conterminous United States. United States Geological Survey. Accessed 7 June 2010. .

Keller, E.A. Environmental Geology. Upper Saddle River: Prentice-Hall, Inc., 2000.

Landslide Types and Processes. Accessed 7 June 2010. United States Geological Survey. .

Radbruch-Hall, D.H., Colton, R.B., Davies, W.E., Lucchitta, I., Skipp, B.A., and Varnes, D.J. 1982. Landslide overview map of the conterminous United States. Geological Survey Professional Paper 1183, U.S. Geological Survey, Washington.

Summary of Current Resource Projects 2009 (Denali National Park and Preserve Center for Resources, Science, and Learning).

Summary of Current Resource Projects 2010 (Denali National Park and Preserve Center for Resources, Science, and Learning).

This EarthCache was created by the Geological Society of America summer 2010 GeoCorps intern at Denali National Park and Preserve.