Earth Science Society

Slipping Away

On the morning of January 10, 2005, the residents of La Conchita, California, awoke to find the highway out of town closed in both directions, due to landslides. Around 12:30 p.m. many residents heard an ominous roar as the bluff above the town unleashed 600,000 metric tons of dirt and mud, covering four blocks in 10 m of debris. Scientists went to the scene to discover exactly what had caused this enormous landslide and whether one could happen again.

The setting La Conchita is built on a narrow swatch of land between the highway and a huge bluff. The bluff is held together weakly, so it is susceptible to being loosened by heavy water content, such as a prolonged, heavy rain. The slope is further weakened from the effects of regular landslides, as well as being on a fault line.

In the two weeks prior to the landslide, the area had received a record amount of rain—about 35 cm—the amount it normally receives in a year! The excess water caused the earth to literally slide off the face of the mountain.

A history of landslides This event was not, however, the first landslide to hit the area. In fact, the mountain bluff is scarred with the evidence of many landslides. Ten years earlier, in March of 1995, two devastating landslides hit the area in the span of a week. These landslides were also caused by a large amount of rain, but the movement of the earth was relatively slow, so residents were able to get away. The 2005 landslide was a continuation of the 1995 slide—the soil that was deposited by the earlier slide was loosened by the rainwater and slipped down the slope. After the 1995 slide, the state government erected a retaining wall to keep the landslides at bay. However, soil, mud, and debris from the 2005 disaster passed right over parts of the wall.

Conchita Mass Movement Event 2005
The mass movement at La Conchita, California, in 2005 killed ten people.

The debate Could the 2005 landslide have been detected and the people warned in time to prevent loss of life? Most likely, yes. In fact, some of the residents of the town are suing the government for failure to protect their citizens, as well as failure to adequately notify them of the impending danger.

Are governments responsible for providing warnings and protection to citizens who move into areas that are prone to natural disasters? Or, does the responsibility lie with the citizens that might not have understood the dangers of living in the area? These questions and more are sure to be considered by the residents and government of La Conchita, as well as cities and local governments of disaster- prone areas throughout the United States for years to come.

CmZ^>Earth Science

Debate Research information about a natural disaster that has occurred near your location. Hold a classroom debate on the topic of why people should, or should not, live in an area where natural disasters have occurred. To learn more about natural disasters, visit glencoe.com.

Earth Science & Society 213

David McNew/Getty Images

Tully Mudflow
This image shows the Tully Valley landslide three days after it occurred. The Tully Farms Road is covered up to 5 m deep with clay.

Background: Around midday on April 27, 1993, in a normally quiet, rural area of New York, the landscape dramatically changed. Unexpectedly, almost 1 million m3 of earth debris slid 300 m down the lower slope of Bare Mountain and into Tully Valley. The debris flowed over the road and buried nearby homes. The people who lived there had no knowledge of any prior landslides occurring in the area, yet this landslide was the largest to occur in New York in more than 75 years.

Question: How can you use a drawing based on a topographic map to infer how the Tully Valley Landslide occurred?

Materials metric ruler

Procedure

Imagine that you work for the United States Geological Survey (USGS) specializing in mass movements. You have just been asked to evaluate the Tully Valley Landslide.

1. Read and complete the lab safety form.

2. Check the map's scale.

3. Measure the length and width of the Tully Valley in kilometers. Double-check your results.

Analyze and Conclude

1. Interpret Data What does the shape of the valley tell you about how it formed?

2. Determine In what direction did the landslide flow?

3. Determine In what direction does the Onondaga Creek flow?

4. Infer from the map which side of Tully Valley has the steepest valley walls.

5. Deduce What conditions must have been present for the landslide to occur?

6. Infer At the time of the Tully Valley Landslide, the trees were bare. How could this have affected the conditions that caused the landslide?

mmitUUMm Farth Science

Explain why the mass movement event you examined in this GeoLab is classified as a landslide. Differentiate a landslide from a creep, slump, flow, avalanche, and rockfall.

Onondaga Creek 1993 Landslide

Brine field

W-E Line of landslide section

Valley floor Valley walls Edge of valley floor

Stream channel (arrow shows direction of stream flow)

Brine field

Download quizzes, key terms, and flash cards from glencoe.com.

Download quizzes, key terms, and flash cards from glencoe.com.

g Study Guide

BIG (Idea

Movements due to gravity, winds, and glaciers shape and change Earth's surface.

Vocabulary

Mass Movements avalanche (p. 198) creep (p. 195) landslide (p. 197) mass movement (p. 194) mudflow (p. 196) slump (p. 198)

Section 8.2

Section 8.3

Glaciers cirque (p. 209) continental glacier (p. 208) drumlin (p. 210) esker (p. 210) glacier (p. 207) kame (p. 210) kettle (p. 212) moraine (p. 210) outwash plain (p. 210) valley glacier (p. 208)

Key Concepts

MAUWJdSa Mass movements alter Earth's surface over time due to gravity moving sediment and rocks downslope.

• Mass movements are classified in part by how rapidly they occur.

• Factors involved in the mass movement of Earth materials include the material's weight, its resistance to sliding, the trigger, and the presence of water.

• Mass movements are natural processes that can affect human life and activities.

• Human activities can increase the potential for the occurrence of mass movements.

I man 4BBS Wind modifies landscapes in all areas of the world by transporting sediment.

• Wind is a powerful agent of erosion.

• Wind can transport sediment in several ways, including suspension and saltation.

• Dunes form when wind velocity slows down and windblown sand is deposited.

• Dunes migrate as long as winds continue to blow.

MAREES Glaciers modify landscapes by eroding and depositing rocks.

• Glaciers are large moving masses of ice that form near Earth's poles and in mountain areas.

• Glaciers can be classified as valley glaciers or continental glaciers.

• Glaciers modify the landscape by erosion and deposition.

• Features formed by glaciers include U-shaped valleys, hanging valleys, moraines, drumlins, and kettles.

216 Chapter 8 • Study Guide

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Assessment

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Responses

  • chantelle
    When the tree were bare in tully valley how could this have affected the cause of the lanslide?
    8 years ago
  • Fikru Brhane
    What direction did the tully valley landslide flow?
    8 years ago
  • YORDA
    What conditions must have been present for the landslide to occur?
    8 years ago
  • matilda
    What direction does the onondaga creek flow?
    6 years ago
  • ulisse
    Which side of tully valley has the steepest valley walls?
    5 years ago
  • gloria
    How could bare trees affect conditions that caused landslide?
    5 years ago

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