How do Rocks Break Down? 

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Leveling Forces 

WEATHERING AND EROSION ARE BOTH KNOWN AS LEVELING FORCES - WHICH MEANS THEY WEAR DOWN AND REMOVE ROCKS THAT ARE EXPOSED ON EARTH'S SURFACE.

WEATHERING IS THE BREAKUP OF ROCKS AT EARTH'S SURFACE DUE TO EXPOSURE TO THE ATMOSPHERE.

In particular, the two substances that do the most damage to rocks are:

  • Oxygen
  • Water

The presence of just water and oxygen in the atmosphere is enough to break down all rocks given enough time.

 

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EROSION IS THE REMOVAL AND TRANSPORTATION OF WEATHERED ROCK BY NATURAL AGENTS SUCH AS:

  • wind
  • glaciers
  • rivers
  • rain
  • waves
  • currents

 

Weathering

There are many different processes that cause weathering, but all of them can be broken up into two main types:

  1. Mechanical Weathering
  2. Chemical Weathering

 

Mechanical Weathering

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MECHANICAL WEATHERING (AKA PHYSICAL WEATHERING) OCCURS WHEN A ROCK BREAKS APART INTO SMALLER PIECES.

  • THIS IS A PHYSICAL CHANGE ONLY - THE PIECES ARE EXACTLY THE SAME AS THE WHOLE, JUST SMALLER

 

 

MECHANICAL WEATHERING CAN HAPPEN IN SEVERAL DIFFERENT WAYS: 

Ice Wedging  

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 WATER IS A VERY UINQUE SUBSTANCE IN THAT IT EXPANDS WHEN IT FREEZES.  

Almost all other liquids contract when frozen, but when water freezes it becomes 10% bigger, or takes up 10% more space.

As water expands it puts great pressure on the walls of anything containing it, including any rocks which happen to be surrounding it. 

When water falls into the cracks of rocks and freezes, the force of the pressure exerted on the rock surrounding it is enough to wedge the walls of the crack farther apart, thus expanding and deepening the crack.

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Ice wedging is what causes pot holes in city streets.  Water freezes in the cracks under the pavement.  As it expands it forces the pavement up, and when the water thaws the pavement falls back down and creates a pot hole. 

 

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Wetting and Drying

WETTING AND DRYING BREAKS UP ROCKS THAT CONTAIN CLAY PARTICLES.

WHEN CLAY GETS WET IT EXPANDS JUST LIKE ICE DOES.  

WHEN ROCKS MADE OF CLAY GET WET THE CLAY EXPANDS PUTTING PRESSURE ON THE ROCK, CAUSING IT TO CRACK AND BREAK.

 

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Plant and Animal Action

Some plants like mosses and lichens are capable of growing without soil on bare rock.  When they do this their roots penetrate pores and crevices and cause rocks to split apart as the roots force their way down through the rocks. 

PLANT ROOTS GROW DOWN INTO ROCK.

AS THEY GROW THEY WIDEN PUTTING PRESSURE ON SURROUNDING ROCK, CAUSING IT TO CRACK AND BREAK.

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Animals can indirectly cause mechanical weathering by digging holes in the soil that allows water to reach down to the bedrock and cause weathering there.

 

 

 

Chemical Weathering

CHEMICAL WEATHERING IS DIFFERENT FROM MECHANICAL WEATHERING IN THAT THERE IS A CHEMICAL CHANGE THAT CAUSES THE ROCK TO BREAK.  AS A RESULT, THE BROKEN PIECES ARE CHEMICALLY DIFFERENT FROM THE ORIGINAL ROCK.  Just like mechanical weathering, chemical weathering can occur via several different methods:

 

Hydrolysis

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HYDROLYSIS IS A TYPE OF CHEMICAL WEATHERING THAT RESULTS FROM A REACTION BETWEEN WATER AND ANOTHER SUBSTANCE.

Some minerals such as feldspar, amphibole, and augite, react with water to form clay.

 

Oxidation

OXIDATION IS A TYPE OF CHEMICAL WEATHERING RESULTING FROM A  REACTION BETWEEN OXYGEN AND ANOTHER SUBSTANCE.

Oxidation reactions are the reason that many things rust over time.  Minerals and metals that contain iron react with oxygen to form a reddish color of rust.  When both oxygen and water are present the iron containing minerals react to form a more brownish rust.

 

Acids

ACIDS REACT WITH MANY DIFFERENT MINERALS CAUSING THEM TO DISSOLVE.  ROCKS THAT CONTAIN THESE MINERALS CRACK AND BREAK WHEN EXPOSED TO ACIDS. 

One common way occurs when CO2 is dissolved in water, which creates carbonic acid, the acid in soda.  It can also enter the soil as decaying plants dissolve in rainwater. 

The video to the right shows the reaction between the sedimentary rock Dolostone and a weak acid.