MASS MOVEMENT
The downhill movement of
regolith due to gravity
• Soil creep (slow)
• Mudflow (fast)
• Bogburst (slow or fast))
• Landslide (fast)
Soil Creep
Soil Creep:
• Soil creep is the slowest form of Mass
Movement
• It may only be noticed when posts, fences
or trees are tilted downhill.
• Another way to notice soil creep on the
landscape is by observing ribbed or
stepped patterns on steeply sloping fields
(called Terracetes)
Terracettes
Mudflows:
• Mudflows are saturated Regolith
• These mudflows flow down mountain sides
• Small mudflows are very common in Ireland on
the Irish hill sides. (Wicklow Glens)
• However they are not all small
• A mudflow or mudslide is the most rapid (up to
80 km/h / 50 mph)
• It is a fluid type of downhill mass movement.
• It is a rapid movement of a large mass of mud
formed from loose earth and water.
Armero Mudflow:
• The Colombian volcano Nevado del Ruiz is an
active volcano with a history of generating
deadly volcanic mudflows (lahars) from
relatively small-volume eruptions.
• In 1595, a lahar swept down the valleys of the
River Guali and the River Lagunillas, killing 636
people.
• In 1845, an immense lahar flooded the upper
valley of the River Lagunillas, killing over 1000
people.
• It continued for 70 kilometers downstream
before spreading across a plain in the lower
valley floor.
Armero Mudflow: Cont
• The young village of Armero was built directly on
top of the 1845 mudflow deposit.
• Over the ensuing years, Armero grew into a
vibrant town with over 27000 residents
• On November 13, 1985, history repeated itself for
the third time in 400 years,
• With another eruption and another deadly lahar
racing down the River Lagunillas.
• This time, over 23,000 people were killed, including
most of the residents of Armero. With proper
planning, this tragedy could have been averted.
Mt. Saint Helens
Bogbursts:
Definition:
• In its simplest form a bog burst occurs when a
mass of bog or peat moves.
• In the case of blanket bogs, this usually
happens when the top layer of the bog literally
starts to slide.
• In raised bogs, movement occurs when the
edges give way, this is usually in areas where
intensive cutting has taken place.
Bogburst
Landslides:
• A landslide is a geological term which includes
a wide range of ground movement.
• These movements can occur in offshore and
coastal environments aswell as in onshore
environments.
• Although the action of gravity is the primary
driving force for a landslide to occur, there are
other contributing factors affecting the original
slope stability.
• Deforestation
• Sudden shock
• Diggers
Landslides Cont:
• Deforestation- this involves cutting down trees
of a steep slope (how would this effect the slope?)
• Sudden shock- a sudden shock of the ground
as a result of an earthquake or volcanic eruption
• Diggers- Cutting under a steep bank or slope
(explain how this would effect the slope stability.)
Landslide!
• Read page 43
in text book:
• Landslidesfast
movement
Landslides