How does the type of the rock influence how fast a surface weathers?

Question

Why does the type of rock influence how fast a surface weathers?

Answer 1

Hi Jessika

The physical and chemical make up of rocks affect how easily they are weathered.

Chemical - how stable is the rock, and if it is a sedimentary rock, how stable is the cement holding the grains together. Limestone dissolves in weak acidic solution, that is what gives us all those lovely caves and the typical features of a Karst landscape. A carbonate cemented sandstone will break down if the carbonate cement is weakened.

Granites wil break down, especially in tropical environments as the feldspars break down into clay minerals ( the source of the China Clay deposits in Cornwall, UK.)

Physical characteristics are important too.

A porous rock like a sandstone will allow groundwater to flow through it which may deposit cement ( like iron minerals) or may allow the cement to disolve. Porous rock will also be more susceptable to freeze-thaw as the pore water freezes and breaks up the grains.

A well jointed rock - sedimentary or igneous will also be susceptable to freeze-thaw as the water can penetrate the joints, and when it freezes lever the joints apart. A massive, unjointed rock far less so.

If there are a mix or rocks then you have a complex situation, clay under a more massive rock may bulge (valley bulging) causing the rock above to crack. An impervious rock under a pervious one will confine the groundwater, etc etc etc....

Go to your local graveyard. Seriously! Look at the different types of rock used for the headstones and compare how each type has weathered. You even have a good guide to when the stone was first exposed to the weather. We often run field trips to the local graveyard to see both a variety of rock types, and a superb experiment in weathering