Page 481 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 14 Weathering, Karst Landscapes, and Mass Movement 445
F cus Study 14.1 Natural Hazards
Frank Slide: Coal Mining and an Early Morning Disaster
On April 29, 1903, at 4:10 a.m., an esti- mated 82 million metric tonnes of rock and sediment slid down the east side of Turtle Mountain in southwestern Alberta at speeds nearing 140 km·h−1. The slide buried the wetland at the base of the mountain, roared through the southern edge of the town of Frank, continued across the valley, and came to a stop upslope on the opposite side of the val- ley. It took just 90 seconds for the mass of material to travel 1.5 km and to cover an area up to 3 km2 at an average depth of 14 m. One estimate counted 70 lives lost in Canada’s deadliest landslide.
More than a century has passed since the Frank Slide, and scientists still do not agree about what caused the slide. The mountain is made of interbedded Paleo- zoic limestone and shale topping Me- sozoic sandstone, with shale and coal at its base. A coal-mining shaft was sunk at the base of the mountain below the Tur- tle Mountain thrust fault (Figure 14.1.1). Mining at the base of the mountain was likely one of the final triggers that sent the rock downslope. The rockslide took place along the easterly dipping beds
of the main geological structure—the Turtle Mountain anticline.
The actual mechanism of failure is complex and includes limestone creep over shales, siltstones, sandstones, and coal; adverse jointing and faulting of
Turtle Mountain
the rock mass; underground coal mining at the base of
the mountain; ice wedging in cracks and discontinuities of the rock mass; excessive rain- fall in the 4 years preceding the slide; and seismic loading. All these factors contributed to the failure, but the greatest influence came from the geo- logical instability.
The mechanism of move- ment of the material in the slide has also been the sub- ject of considerable academic debate. One theory states that the debris in the slide remained in contact with the surface through most of its travel down the mountainside, across the valley bottom,
and up the facing slope. A
second theory proposes that
the material had to be lubri-
cated at the base by either
compressed air or steam and
that this compressed layer
allowed the slide material to move freely downslope, across the valley, and up the other side.
The slide affected an entire face of the mountain (Figure 14.1.2), buried part of the town of Frank, the Canadian Pacific Railway, and the highway that passed
▲Figure 14.1.2 Turtle Mountain and the scar of the Frank Slide. Rock debris from the slide is shown in the foreground. [IanChrisGraham/iStockphoto/Getty Images.]
Profile before the slide
Rupture surface Slide debris
Splay
through town, and dammed the Crows- nest River, forming a temporary lake. The continued threat from the geological instability of the mountain led to the re- location of much of the town—out of the potential path of future landslides. The road was rebuilt and the rail line recon- structed. The mine reopened, but closed again soon after.
In Canada, thousands of slides change the terrain every year in all parts of the country. From 1850 to the
present, landslides have caused about 600 fatalities in Canada. From an eco- nomic perspective, slides have cost be- tween CAD $100 million and $200 million in property damage, blocked rail lines and roads, and caused pipeline explo- sions. Environmental damages, the costs of which are difficult to estimate, include damage to spawning grounds and local- ized deforestation and habitat destruc- tion. Natural factors cause most slides, owing to geology, water, ice, wind, and temperature changes. However, human activities such as urbanization, deforesta- tion, and mining may also play a role as triggers to mass movement. The results remain a part of the landscape, such as Turtle Mountain—a reminder of times when mountains moved.
Paleozoic limestone and shales
0
Turtle Mountain thrust fault
Frank Mine
100 METRE
Mesozoic sandstones, shales, coals
S
▲Figure 14.1.1 Cross section through the central part of the Frank Slide. Rock struc- ture creates a natural instability in the rock of Turtle Mountain. Splay refers to a minor fault that branches off a major fault. [Illustration reprinted by permission of John Krahn, P. Eng.]