Expert: Keith Patton - 4/10/2009

Question
I have a number of questions i would like you to assist me on:

1.If 2 rocks are waethered, one physically and the onther chemically, how will their product differ.

2.How do mechanical weathering add to the effectveness of chemical weathering.

3 Granite and Basalt are exposed at the surface in a hot, wet region a)what type of weathering will predominate, b) which of the rock will weather rapidly and why?

4.Heat speed up chemical reation, then why is it in the desert chemical reaction is slow

Answer
Tony:

Sounds like homework.  I'll help you out a bit.

1)  Physical or mechanical weathering attacks the rock like a hammer.  It breaks the rock using brute force.  Water pounds rock against rock, ice prys and breaks the rock apart.  Chemical weathering on the other hand attacks the atomic bonds that hold the minerals together eating away at the crystal boundaries disagregating the crystals for one another or dissolving one or more minerals in the rock completely.  So considering both scenarios what size particles would one process give versus the other?


2) Take a block of wood and measure the outside surface area of all the sides.  Now take that same block and cut it into 20 smaller blocks.  Measure each of them again.  Would you expect that the surface area of all the blocks would be greater or less than the large block?  Now which do you think would burn faster, the pile of small blocks or the one large block?  Think of a fire in a fire place.  Does kindling burn faster or slower than a large log?
The same thing applies to chemical weathering.  Increased surface area allows the chemical process to accelerate by giving a larger area for the chemical agents to attack.  Chemical weathering is a chemical process just like combustion.  Both are affected by the same thing.

3) Think about answers 1 and 2.  Hot, wet?  Lots of decaying vegetable matter.  Humic acids in the soil.  There will be some running water, but also lots of vegetation to hold things in place.  No ice.  Thick soil.  Okay, I think you got that.  Now, look at granite, large crystals visible to the eye without a hand lense.  Basalt is a big black blob, no dicerable crystals which tells us that it is made up of very very small wooden blocks, whoops, I mean very very small crystals.  Which rock has the most exposed surface area like in question 2?  I think you can answer the question now. Another thing is the type of minerals that make up the rock.  (High school science teachers tend to simplify things since they have only had a couple of courses in geology at best) Some rocks are composed of minerals that resist weathering, others are composed of minerals that weather rapidly into clays and such.  Granite is over 50% quartz.  The rest is feldspar and mica.  These weather to clay leaving the quartz crystals behind.  On the other hand basalt weathers by oxidation, the iron in the mafic minerals are attacked by water turning to rust basically, the other minerals turn to clay. That is why you don't see many beachs made of of basalt sand.

4) What is missing in a desert?  It is called the univerasl solvent.  It allows most chemical reactions to take place?  We drink it.  In absence of it, the heat does not matter too much.  Heat accelerates molecular movement by exciting them, but if they are not in solution, it doesn't matter too much.