This earthcache highlights a geological feature known as a landslide or landslip. This geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the slope stability.
The movement of landslide material can vary from abrupt collapses to slow gradual slides and at rates which range from almost undetectable to extremely rapid. Sudden and rapid events are the most dangerous because of a lack of warning and the speed at which material can travel down the slope as well as the force of its resulting impact. Extremely slow landslides might move only millimetres or centimetres a year and can be active for many years.
Types of landslides:
- Fall - This is generally a rapid to sudden rate of movement with the descent of material characterised by a free-fall period. Falls are commonly triggered by earthquakes or erosion processes.
- Topple - This is a tilting of rock without collapse, or by the forward rotation of rocks about a pivot point. Topples have a rapid rate of movement and failure is generally influenced by the fracture pattern in rock. Material descends by abrupt falling, sliding, bouncing and rolling.
- Flow - This is the most destructive and turbulent form of landslide. Flows have a high water content which causes the slope material to lose cohesion, turning it into slurry. They are channeled by the landscape and move rapidly.
- Slide - This is a very common form of failure and can be subdivided into translational and rotational slides. Rotational slides are sometimes called slumps because they move with rotation. Translational slides have a planar, or two dimensional surface of rupture. They have a moderate rate of movement and the coherence of material is retained, moving largely intact or in broken pieces.
- Creep - This is characterised by the gradual lateral displacement of large volumes of distributed material over very gentle or flat terrain. Failure is caused by liquefaction which is the process when saturated loose sediment with little or no cohesion such as sands or silts are transformed into a liquid-like state. This process is triggered by rapid ground motion most commonly during earthquakes.
Causes of landslides:
Landslides can be triggered by natural causes or by human activity. It’s usually related to instabilities within the slope. It is possible to identify one or more landslide causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time. A list of natural and human causes is identified below:
Natural causes include:
- elevation of pore water pressure by saturation of slope material from either intense or prolonged rainfall and seepage
- vibrations caused by earthquakes
- undercutting of cliffs and banks by waves or river erosion
- volcanic eruptions.
Human causes include:
- removal of vegetation
- interference with, or changes to, natural drainage
- leaking pipes such as water and sewer reticulation
- modification of slopes by construction of roads, railways, buildings, etc
- overloading slopes
- mining and quarrying activities
- vibrations from heavy traffic, blasting, etc
- excavation or displacement of rocks.
Potential landslide sites reveal themselves in several ways including:
- saturated ground or seeps in areas which are not typically wet
- new cracks and scarps or unusual bulges in the ground, roads or pavements
- movement of ancillary structures such as decks and patios in relation to a house
- sticking doors and windows
- tilting or cracking of concrete floors and foundations
- broken water lines and other underground utilities
- leaning telephone poles, trees, retaining walls or fences
- offset fence lines
- sunken or displaced road surfaces or kerbs
- A rapid increase in creek water levels, possibly accompanied by greater turbidity.
Landslides in New Zealand:
This country has not been spared its share of landslides. The country’s biggest landslide occurred at Green Lake some 13000 years ago when a 9 kilometre section fell 700 metres into the valley floor. In 1846 a mudslide at Lake Taupo buried 55 people while 14 people died in 1929 at Murchison after an earthquake and resulting landslide. In 1991 a landslide at Mt Cook lowered the peak by some 10 metres, the Tāhunanui slump in Nelson is a good example of an active landslide.
Dunedin has its own history of landslips, in 1979 at Abbotsford some 18 hectares of land slid down the hill resulting in 69 homes being destroyed, thankfully no lives were lost. The Broad Bay landslip is a much smaller example of this geological feature. This slip began in 1986 when an area of land 70m wide, 250m long started to slide downwards towards the bay, today after extensive development the site is inactive and safe.
To claim a find for this earthcache:
You will need to visit the posted coordinates where you will obtain the answers to the following questions.
- Name the 3 rock types that attributed to this landslide.
- Complete the following sentence. These materials form? ______________
- What has been done to stabilise the landslip?
- At GZ what is the altitude shown on your GPS?
- The two photos on the noticeboard at GZ were taken in what two years?
- (Optional) Take a picture of yourself with the bay in the background.
Email your answers to me via my profile and log your find. If there are any issues with your answers I will contact you. Logs without emailed answers will be deleted.
References:
http://en.wikipedia.org/wiki/1979_Abbotsford_landslip
http://www.ga.gov.au/hazards/landslide.html
http://en.wikipedia.org/wiki/Landslip