Question

When the rail line in the accompanying photo was built in rural Alaska in the \(1930 \mathrm{~s}\), the terrain was relatively level. Not long after the railroad was completed, a great deal of subsidence and shifting of the ground occurred, turning the tracks into the "roller coaster" shown here. As a result the rail line had to be abandoned. Suggest a reason why the ground became unstable and shifted.

Short answer

The ground likely became unstable due to the thawing of permafrost, which caused subsidence and shifting.

Step-by-step solution

Examine Environmental Conditions

In rural Alaska, the climate is characterized by extreme temperatures and variations. This region is known for its frozen soil, or permafrost, which remains at or below freezing for at least two consecutive years.

Understand Permafrost Dynamics

Permafrost is soil that is permanently frozen. When this frozen ground is disturbed or when climate conditions change, the permafrost can begin to thaw.

Consider the Impact of Thawing

Thawing permafrost leads to loss of ground stability. As the ice within the soil melts, the ground can become soggy and lose its structural integrity, resulting in subsidence (sinking) and shifting.

Analyze Human Impact

The construction of rail lines and other structures may disrupt the surface, accelerating the thawing of the permafrost underneath. This happens because the construction process can alter surface insulation and heat balance.

Key concepts

Subsidence

Subsidence occurs when the ground sinks or shifts downward. This can happen for various reasons, but one common cause is the thawing of permafrost. In areas like rural Alaska, permafrost is prevalent. It is soil that remains frozen for long periods, sometimes for many years. When this solid, icy structure melts, the ground can no longer hold itself up and starts to sink.

Imagine the ground as a sponge filled with ice cubes. When the ice cubes melt, the water seeps out, and the sponge collapses. This is very similar to what happens with thawed permafrost. The once-solid structure becomes soft and uneven, leading to subsidence.

Understanding this process is crucial, especially in places where infrastructure like roads or rail lines exists. These structures can be severely affected if the ground supporting them becomes unstable. As the ground shifts or sinks, so too do these man-made structures, which can lead to significant damage or even abandonment.

Climate Impact on Geology

The climate has a powerful influence on geological processes, especially in regions with significant frozen terrain like Alaska. Temperatures in these areas can fluctuate dramatically, from extreme cold to warmer periods. These changes in climate can cause permafrost to thaw.

When the climate warms, even just slightly, the permafrost can begin to melt. This melting leads to a loss of ground stability. The process is slow but can have disastrous effects over time. As the ground loses its icy structure, its ability to support homes, roads, or railways diminishes.

Climate change is accelerating this process in some regions, exacerbating geological problems. The increased temperatures lead to more widespread and faster thawing of permafrost. This is a significant concern for engineers and planners who need to account for these geological shifts when designing infrastructure. Understanding these impacts allows for better preparation and adaptation to maintain structures in place.

Human Activity and Geology

Human activities can significantly affect the geological stability of an area, particularly where permafrost is involved. Activities such as construction can alter the natural balance of the ground, leading to faster permafrost thawing.

When a rail line, road, or building is constructed on permafrost, it can disrupt the insulation that keeps the ground frozen. This makes it more susceptible to warming and melting. The heavy machinery used during construction can also compact the soil, changing its structure and increasing the rate of thaw.

The heat generated by these structures or their ongoing use can further contribute to the warming of permafrost. This can create a feedback loop where human activity inadvertently accelerates geological change, leading to more subsidence and instability. Planning and building on permafrost require careful consideration and innovative solutions to minimize these impacts and keep both the natural environment and human-made structures stable.