Expert: Dr Thomas Bell - 6/17/2009

Question
1. What is the principal landform of karst landscapes?
2. Do caves primarily form in limestone?
3. What is the name of the deadly ladslide that took place in North Italy?
4. What is the persistent gradual mass movement of surface material called?

Answer
Nikki,

You are asking questions about two related subjects, mass wasting and karst landscapes.  The first is due to mechanical weathering and the second is an example of chemical weathering.  I'll only deal with karst in this post.  You can probably find the term you are looking for and the reference to a specific event with a careful web search.

Karst landscapes form where the dominant rock type is limestone. Calcite (calcium carbonate) is the main mineral constituent of limestone.  Calcite can be dissolved in acid.  Rainfall and shallow groundwater contain dissolved carbon dioxide from the atmosphere making them a weak acid.  The dissolution of rock and removal of mineral constituents in solution is called chemical weathering.

Chemical weathering doesn't proceed at the same rate everywhere limestone is exposed to water.  In general, limestone is not very porous so water slowly lowers the land surface by dissolving the top of the limestone.  Where there are cracks in the limestone, water can penetrate and enlarge the fractures by dissolving the walls.  The result is chemical weathering in three dimensions.

Lets do a thought experiment or if you want to try this at home go ahead.  Imagine a tightly packed box of sugar cubes.  If you take a spray bottle and begin to mist the top of your open box with water  the sugar cubes become saturated then slowly dissolve.  The boundaries between the cubes will dissolve a little faster than the tops so these narrow cracks grow wider.  At some point if you keep misting the top, a surface of regular trenches and hills develops.  This is what happens during karst formation over the course of thousands or tens of thousands of years.

If the fracture density is low and the porosity of the limestone is low, the result can be spectacular hills with vertical sides.  Some of the karst terrain in Viet Nam

http://upload.wikimedia.org/wikipedia/commons/1/1a/TamCoc.jpg)

and southern China

http://www.relaischateaux.com/IMG/jpg/Publie-Karst-2.jpg

has evolved this way.  Usually though, the rate of removal is more even and a lumpy landscape of rounded hills and shallow valleys develops.  Often, surface streams in limestone terrain dissappear down a crack and flow underground.  The network of cracks in the limestone becomes enlarged and this steady supply of slightly acidic surface water "hollows out" the cracks to form caves.  This process can also create vertical caves where water has enlarged a zone where two cracks intersect each other.  If a bridge or roof of undissolved limestone develops and then collapses, a sinkhole is formed.

A web search using the single keyword "karst" yields a wealth of information on this subject.