NAVIGATION:  BACK TO MODULE TWO INTRODUCTION

Weathering and Erosion are DIFFERENT!

Before we get too far along in this introduction series of introduction Web pages, we need to understand a few basic definitions and concepts.  We need an "introduction to the introduction."  First of all, weathering is not weather (for you geographic information science (GIS) types, just like topology is not topography)!  Sure, the weather (sunshine, temperature, rain, snow, storms, wind...) does play a HUGE role in weathering - but the terms have different definitions.  Weathering relates to decomposing, disintegrating, or changing rocks at or near the earth's surface whereas weather is... well, weather!  Again, here's a good definition of weathering:  Simply stated, weathering is the in-situ (in place) breakdown of material.

Your bike chain rusts.  Your grandma's birdbath cracks.  Newspapers turn yellow.  Names and dates on tombstones become more and more difficult to read.  These are examples of weathering.  For this module and the rest of the course, we're going to ignore your bike chain, your grandma's birdbath, and even the yellow newspaper.  I guess we can NOT ignore the tombstone, because it is rock, just like the rock (slickrock sandstone surface) in the image below.  We are going to talk exclusively about how ROCK weathers, and about FORMS produced by weathering processes for the rest of the module.

Here's the definition of weathering from a USDA glossary.  Weathering  "All physical disintegration, chemical decomposition, and biologically induced changes in rocks or other deposits at or near the earth's surface by atmospheric or biologic agents or circulating surface waters with essentially no transport of the altered material. These changes result in disintegration and decomposition of the material. Compare - regolith, residuum, saprolite".  Notice how the USDA definition above specifies that there is essentially no transport of the material?

Weathering, then, differs from erosion in terms of its lack of ability to move material!  Erosion involves removal (transport) of material away from a site.  There are several erosion transport mechanisms including wind, water, waves, and gravity. 

This might help.  Imagine a sandstone tombstone in a Tempe graveyard.  If you press the palm of your hand against the sandstone tombstone, and then pull your hand away and look at your palm, you will probably see a few grains of sand "stuck" to your hand.  The sand grains stuck on your palm weathered (in place) on the surface of the tombstone - they had not moved, they were just sitting there, waiting for some form of energy to come and move them.  When your hand inadvertently removed the grains from the stone, the grains were "eroded".  

A rock or tombstone or building or highway surface "weathers" by breaking down particles into smaller pieces.  Potholes form as particles from the road bed are removed (erosion) by tires crashing down on the road; gravity pulls down (erosion) loose rocks from mountain slopes; rain water becomes concentrated and funneled into channels picking up sediment as it travels (erosion)...  These "movements" describe erosion. 

To summarize, chunks or weathered material (broken up pieces of rock) just sit there in place (in-situ)...  the moment a car tire or running water or gravity or perhaps a strong wind gust has enough energy to move or transport the weathered particles, the process becomes erosion.  Erosion is the movement of weathered particles!  Simple, right?

The real challenge of this module is to understand the processes responsible for "breaking down" the particles in the first place!  That is, the weathering processes!  Because weathering and erosion are such good friends, it is difficult to talk about or teach weathering without mentioning or referring to erosion too.

Here (Figure One above) we have an interesting form that illustrates weathering AND erosion.  The depression you see above that is filled with water is a combination "weathering pit" and "pothole".  The water trapped inside the pit accelerates weathering by dissolving the matrix holding the sand grains together.  A positive feedback cycle helps this hole get bigger and deeper. A positive feedback is a situation where a process enhances itself.  In a weathering pit, the depression itself traps water... the water, in turn (through weathering) dissolves the rock making the depression deeper and deeper...  So, the deeper the weathering pit, the longer the water remains trapped in the pit, and the more time the rock is wet, the deeper the pit. 

Material can be removed from these pits in a couple ways...  When the pit is dry, wind probably scours out the sediment that collects in the pit.  During a cloudburst, water probably runs through this hole (it is located in a topographic low along a drainage) and flushes out the weathered material (sand).  When the material (sand grains) that are trapped in this depression become "mobile" as they are swept away by running water or blown away by wind, they are being eroded.