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Introduction In this essay I will be identifying the various types of factors that influence mass movement over a variety of different environmental conditions.  It will help to clarify the key aspects that effect the frequency and magnitude of mass movement events.Mass movement like mudslides and debris flows are characterised by a rapid movement of material due to gravity. Other factors over time like climate tectonic activity and can have varying influence on frequency and magnitude of surface material movement. There are many types of mass movement which are defined by the material they are made of. Avalanches are examples of ice and snow falls whereas lahars are the flow of volcanic rock and water from the slopes of a volcano. Exploration around factors affecting mass movement in lahars A lahar is a flow of volcanic rock or volcanic sediment mixed with water forming a slurry. One key factor that influences this type of mass movement is volcanic eruptions  (Volcanic hazards program, 2017). The eruptions of molten volcanic rock can melt snow and ice found on the slopes of the volcano or can directly erupt through and mix with crater lakes forming the slurry that flows rapidly down the slopes of the mountain. Pyroclastic flows are also a factor that greatly increases the chance of a lahar occurring. The pyroclastic flow itself is a huge eruption of volcanic ash and molten volcanic rock which flow rapidly down the sides of the volcano. It can cause some of the most destructive lahars as the hot cloud of ash and volcanic rock rush down the slopes of the volcano melting snow and ice as it does so forming the mixture of water and hot volcanic debris. This mixture is thick and dense like cement flowing down the sides of the volcano cable of causing massive damage. Lahars can also occur after volcanic eruptions due to heavy and continuous rainfall. The water from the constant precipitation mixes with volcanic ash and sediment leading to the formation of the flowing slurry like that of the lahar formed during an eruption. This is what led to the sudden flank collapse on the Casita volcano in Nicaragua in 1998 (Kerle, 2003).  Figure 1, Casita volcano in Nicaragua in 1998 The flank collapsed due to increased and sustained precipitation caused by the passing hurricane mitch as well as seismic activity occurring near the local fault line. Rainwater was able to deeply infiltrate the volcanic soil and debris built up on the flank due to its fractured nature. Over time seasonal heavy rains and hurricanes are more likely to occur in the Caribbean from the June to November each year, possibly leading to an increase in Lahars and debris flows of a similar nature due to increased rainfall. In the case of the 1998 casita collapse, the combined factors of seismic activity and increased rainfall lead to the collapse of the flank. Unlike seasonal rainfall seismic activity like earthquakes cannot yet be predicted with any certainty.   Exploration around factors affecting landslidesLandslides, unlike Lahars, do not involve the mixture of water and volcanic material making a slurry that flows rapidly down slopes. Instead, landslides are just the movement of surface material like rock and soil as one consolidated mass down slopes and can occur very rapidly or much slower over a longer period. There are also two distinct types of landslide; planar and rotational (British Geological Survey,2017). Planar landslides are characterized by there straight slip surface due to the movement of material along a bedding plane or a long a straight  Figure 2, diagram of different types of landslideThere are many factors that together greatly increase the chances of a landslide event occurring. The type of geology is a key factor in the occurrence of a landslide. Depending on the type of rock either soft or hard with hard rocks providing greater structural strength. Softer material like clay, alluvium or other sediments offer little strength and stability due to there unconsolidated nature.  The steepness of the slope on which the landslide takes place can also affect the likely hood of landslides with steeper slopes generally showing lesser stability. This is due to the shear strength of the material on the slope which is a combination of its friction on the slope and the cohesion of the particles on the slope. The greater the angle of the slope the greater the shear stress is on the slope material. When the shear stress is greater than the shear strength affecting the slope material it will become unstable and begin to move (Nelson, 2018).figure 3, (Nelson, 2018) diagram showing effect of shear strength (gp) and shear stress (gt)These factors also become more prominent when construction of housing and infrastructure takes place on slopes potentially making them more susceptible to landslides. The sudden urbanisation of hillsides and slopes could increase the likely hood of slope collapse and landslide due to increased and concentrated drainage of rainwater from the impermeable urban surface into the slope. The increase in water content of the slope could decrease its stability acting as a lubricant between layers of softer soils and debris and harder rock surfaces. This type of impacts was particularly seen following the 2nd world war in California where due to a rise in population there was a necessity for a greater quantity of housing (R Hollingsworth, 1981). Because of this reason all available space was required to construct housing leading to construction on slopes and hillsides. This, in turn, led to an increase in the possibility of slumps and flows as housing was built on slopes that were made of less stable materials like mudstone and soils.   Section 2: Time variations Time is a key factor in the variation of these mass movement event. Some debris flows, and lahars can occur seasonally in glaciated areas (USGS,2017). During the summer when the snowpack on the glacier begin to melt the sudden increase in water flow can mix with unconsolidated volcanic materiel leading to the formation and flow of a lahar. It can also occur before the formation of snow-packs form in early winter due to intense rainfall mixing with the same unconsolidated material and leading to the formation of a lahar. Climatic factors like seasonal rainfall are far easier to see how they vary over time. Over the span of a year it is easy to see, like in the case of the flank collapse on the Casita (Kerle, 2003), that because of hurricanes and the strong continuous rainfall during monsoon and storm seasons that precipitation has a far greater increase on the frequency and probability of lahars and debris flows. And with the potential increase in the magnitude of hurricanes and in particular because of rising sea surface temperatures due to global climate change, the number of lahars and debris flows could dramatically rise over time as the frequency and magnitude of these storms increases.Some factors cannot be predicted and are not seasonal or have any discerning reoccurring pattern.  Examples of these types of factors would be seismic and volcanic activity. These both can vary in both frequency and magnitude over time as shown in figure 4 below. Depending on the scale of the eruption or quake can greatly affect the likely hood of a mass movement event from occurring. Figure 4, volcanic eruption and strength history from now to the present. (increase in volcanic activity is correlated with increase in known volcanoes over the years)Where smaller magnitude eruptions and earth quakes are more frequent, larger more devastating earthquakes and eruptions happen far more rarely but can have a far greater impact on the mass movement.  ConclusionThere are many different accumulative factors that influence mass movement. From climate conditions like rain and temperature which can bring about consistent seasonal variation in the occurrence of mass movement events. Moreover, the natural topography of the landscape also has an influence on the occurrence of mass movement with slope steepness and composition of the slope material influencing the likelihood of a mass movement event.As well as Seismic and volcanic activity though the most unpredictable factor to affect mass movement are among the most powerful and the most dangerous as they can cause slope collapse, landslides pyroclastic flows and lahars instantaneously.          References   British Geological Survey, undated landslides, accessed 01/11/2017, http://www.bgs.ac.uk/discoveringGeology/hazards/landslides/home.html?src=topNav Hollingsworth R., 1981 Soil slumps and debris flows: prediction and protection, Assoc Eng GeologistsKerle N., 2003 New insight into the factors leading to the 1998 flank collapse and lahar disaster at Casita volcano, Nicaragua. Bulletin of Volcanology Nelson s, 2018 slope stability, Tulane.edu http://www.tulane.edu/~sanelson/Natural_Disasters/slopestability.htmaccesed 15 Jan.2018. USGS, Volcanic hazards program, accessed 5/11/2017,  https://volcanoes.usgs.gov/vhp/lahars.html    

 Introduction

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In this essay I will be identifying the various types of factors that influence mass movement over a variety of different environmental conditions.  It will help to clarify the key aspects that effect the frequency and magnitude of mass movement events.

Mass movement like mudslides and debris flows are characterised by a rapid movement of material due to gravity. Other factors over time like climate tectonic activity and can have varying influence on frequency and magnitude of surface material movement.

There are many types of mass movement which are defined by the material they are made of. Avalanches are examples of ice and snow falls whereas lahars are the flow of volcanic rock and water from the slopes of a volcano.

Exploration around factors affecting mass movement in lahars

A lahar is a flow of volcanic rock or volcanic sediment mixed with water forming a slurry. One key factor that influences this type of mass movement is volcanic eruptions  (Volcanic hazards program, 2017). The eruptions of molten volcanic rock can melt snow and ice found on the slopes of the volcano or can directly erupt through and mix with crater lakes forming the slurry that flows rapidly down the slopes of the mountain.

Pyroclastic flows are also a factor that greatly increases the chance of a lahar occurring. The pyroclastic flow itself is a huge eruption of volcanic ash and molten volcanic rock which flow rapidly down the sides of the volcano. It can cause some of the most destructive lahars as the hot cloud of ash and volcanic rock rush down the slopes of the volcano melting snow and ice as it does so forming the mixture of water and hot volcanic debris. This mixture is thick and dense like cement flowing down the sides of the volcano cable of causing massive damage.

Lahars can also occur after volcanic eruptions due to heavy and continuous rainfall. The water from the constant precipitation mixes with volcanic ash and sediment leading to the formation of the flowing slurry like that of the lahar formed during an eruption. This is what led to the sudden flank collapse on the Casita volcano in Nicaragua in 1998 (Kerle, 2003).  

Figure 1, Casita volcano in Nicaragua in 1998

 

The flank collapsed due to increased and sustained precipitation caused by the passing hurricane mitch as well as seismic activity occurring near the local fault line. Rainwater was able to deeply infiltrate the volcanic soil and debris built up on the flank due to its fractured nature.

Over time seasonal heavy rains and hurricanes are more likely to occur in the Caribbean from the June to November each year, possibly leading to an increase in Lahars and debris flows of a similar nature due to increased rainfall. In the case of the 1998 casita collapse, the combined factors of seismic activity and increased rainfall lead to the collapse of the flank. Unlike seasonal rainfall seismic activity like earthquakes cannot yet be predicted with any certainty.

  

Exploration around factors affecting landslides

Landslides, unlike Lahars, do not involve the mixture of water and volcanic material making a slurry that flows rapidly down slopes. Instead, landslides are just the movement of surface material like rock and soil as one consolidated mass down slopes and can occur very rapidly or much slower over a longer period. There are also two distinct types of landslide; planar and rotational (British Geological Survey,2017). Planar landslides are characterized by there straight slip surface due to the movement of material along a bedding plane or a long a straight

 

Figure 2, diagram of different types of landslide

There are many factors that together greatly increase the chances of a landslide event occurring. The type of geology is a key factor in the occurrence of a landslide. Depending on the type of rock either soft or hard with hard rocks providing greater structural strength. Softer material like clay, alluvium or other sediments offer little strength and stability due to there unconsolidated nature.  The steepness of the slope on which the landslide takes place can also affect the likely hood of landslides with steeper slopes generally showing lesser stability.

 This is due to the shear strength of the material on the slope which is a combination of its friction on the slope and the cohesion of the particles on the slope. The greater the angle of the slope the greater the shear stress is on the slope material. When the shear stress is greater than the shear strength affecting the slope material it will become unstable and begin to move (Nelson, 2018).

figure 3, (Nelson, 2018) diagram showing effect of shear strength (gp) and shear stress (gt)

These factors also become more prominent when construction of housing and infrastructure takes place on slopes potentially making them more susceptible to landslides. The sudden urbanisation of hillsides and slopes could increase the likely hood of slope collapse and landslide due to increased and concentrated drainage of rainwater from the impermeable urban surface into the slope. The increase in water content of the slope could decrease its stability acting as a lubricant between layers of softer soils and debris and harder rock surfaces. This type of impacts was particularly seen following the 2nd world war in California where due to a rise in population there was a necessity for a greater quantity of housing (R Hollingsworth, 1981). Because of this reason all available space was required to construct housing leading to construction on slopes and hillsides. This, in turn, led to an increase in the possibility of slumps and flows as housing was built on slopes that were made of less stable materials like mudstone and soils. 

 

Section 2: Time variations

Time is a key factor in the variation of these mass movement event. Some debris flows, and lahars can occur seasonally in glaciated areas (USGS,2017). During the summer when the snowpack on the glacier begin to melt the sudden increase in water flow can mix with unconsolidated volcanic materiel leading to the formation and flow of a lahar. It can also occur before the formation of snow-packs form in early winter due to intense rainfall mixing with the same unconsolidated material and leading to the formation of a lahar. Climatic factors like seasonal rainfall are far easier to see how they vary over time. Over the span of a year it is easy to see, like in the case of the flank collapse on the Casita (Kerle, 2003), that because of hurricanes and the strong continuous rainfall during monsoon and storm seasons that precipitation has a far greater increase on the frequency and probability of lahars and debris flows. And with the potential increase in the magnitude of hurricanes and in particular because of rising sea surface temperatures due to global climate change, the number of lahars and debris flows could dramatically rise over time as the frequency and magnitude of these storms increases.

Some factors cannot be predicted and are not seasonal or have any discerning reoccurring pattern.  Examples of these types of factors would be seismic and volcanic activity. These both can vary in both frequency and magnitude over time as shown in figure 4 below. Depending on the scale of the eruption or quake can greatly affect the likely hood of a mass movement event from occurring.

Figure 4, volcanic eruption and strength history from now to the present. (increase in volcanic activity is correlated with increase in known volcanoes over the years)

Where smaller magnitude eruptions and earth quakes are more frequent, larger more devastating earthquakes and eruptions happen far more rarely but can have a far greater impact on the mass movement.  

Conclusion

There are many different accumulative factors that influence mass movement. From climate conditions like rain and temperature which can bring about consistent seasonal variation in the occurrence of mass movement events.

Moreover, the natural topography of the landscape also has an influence on the occurrence of mass movement with slope steepness and composition of the slope material influencing the likelihood of a mass movement event.

As well as Seismic and volcanic activity though the most unpredictable factor to affect mass movement are among the most powerful and the most dangerous as they can cause slope collapse, landslides pyroclastic flows and lahars instantaneously.

 

 

 

 

 

 

 

 

 

References  

British Geological Survey, undated landslides, accessed 01/11/2017, http://www.bgs.ac.uk/discoveringGeology/hazards/landslides/home.html?src=topNav

Hollingsworth R., 1981 Soil slumps and debris flows: prediction and protection, Assoc Eng Geologists

Kerle N., 2003 New insight into the factors leading to the 1998 flank collapse and lahar disaster at Casita volcano, Nicaragua. Bulletin of Volcanology

Nelson s, 2018 slope stability, Tulane.edu

http://www.tulane.edu/~sanelson/Natural_Disasters/slopestability.htmaccesed 15 Jan.2018.

USGS, Volcanic hazards program, accessed 5/11/2017,  https://volcanoes.usgs.gov/vhp/lahars.html

 

 

 

 

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