Weathering Versus Erosion

Weathering and erosion sort of sound like the same thing, but geologists make a fairly simple distinction.
With weathering, we are merely speaking of the in situ breakdown of rock material without transport.  In other words, weathering involves rocks breaking apart along fractures but not moving from the site of disaggregation.  As you’ll find below, we then categorize weathering as either mechanical or chemical (depending on whether or not the rock material changes chemical composition).

In the case of erosion, rock material is actually being moved from the site of weathering.  It can be moved by gravity (fall, roll), or by moving water (probably the most common) or by air (wind).

What Is Weathering?

The footprints that astronauts left on the Moon will be there forever.
Well, unless a big space rock comes and whacks that part of the moon!

The Moon has no atmosphere and no running water, and as a result, has virtually no weathering.
Weathering on Earth is a primary factor in the modification and destruction of rocks– and therefore in the generation of most landforms.

Weathering is the process that changes solid rock into sediments. With weathering, rocks break into smaller pieces.

Erosion carries these sediments (smaller pieces) via four primary mechanisms– water, wind, glaciers, and gravity.

• Water is responsible for most erosion. Water can move most sizes of sediments, depending on the strength of the force.
• Wind moves sand-sized and smaller pieces of rock through the air.
• Glaciers move all sizes of sediments, from extremely large boulders to the tiniest fragments.
• Gravity moves broken pieces of rock, large or small, downslope.

While the forces of plate tectonics can build huge mountains, weathering and erosion gradually tears landscapes away.

The pothole pictured here ruined my tire!
OK, just kidding…
But this is a case of a human-made rock (asphalt) undergoing weathering and erosion.  The asphalt has disaggregated (chemically and perhaps due to freeze/thaw) and then the material carried away by rain and water during storms.

 

Mechanical Weathering

Mechanical weathering (also termed physical weathering) breaks rock into smaller pieces. The rock has changed physically without changing its composition. The smaller pieces have the same minerals, in just the same proportions as the original rock.

Examples of mechanical weathering–

Ice wedging is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point (figure 2). Ice wedging works quickly in both polar regions and mid-latitudes, and of course also in mountainous locations.

 

Above we see the north portion of Half Dome in Yosemite Valley, CA.  This is where the cable-route allows a relatively simple ascent of the peak.  From a distance, the rock formation looks a bit like a poorly-cut onion, with layers peeling off all over it!  The onion skin peeling is termed “exfoliation”– it is a form of mechanical weathering, largely caused by the release of pressure within the rock.  This igneous rock formed deep under the earth, but over millions of years, uplift and erosion have removed the immense pressures associated with overlying rock.  Half Dome expands a bit and generates exfoliation!

Abrasion is another form of mechanical weathering. In abrasion, particles (of any size) bump against each other.

• Gravity causes abrasion as rock material moves down-hill.
• Moving water causes abrasion as particles in the water collide.

Abrasion makes rocks with sharp or jagged edges smooth and round, e.g. beach glass or cobbles in a stream.

 

 Plants and animals can also generate  mechanical weathering– roots expanding into rock fissures or animals burrowing into soil and rock.

We are about to consider CHEMICAL WEATHERING– whereby rocks undergo modification and breakdown associated with chemical changes/reactions.  Interestingly, mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases figure 5. With more surfaces exposed, there are more surfaces on which chemical reactions can occur.

Chemical Weathering

Chemical weathering is different from mechanical weathering because the rock actually changes chemical composition via reaction with things like water, acid rain, and air itself.
Mineral changes take place.

Some minerals are particularly unstable under surface conditions– those are typically minerals that initially form at high pressure or high temperatures deep in the earth.   Examples include olivine and pyroxene.

When these rocks reach the Earth’s surface, they are now at very low temperatures and pressures. This is a very different environment from the one in which they formed and the minerals are no longer stable.

Chemical weathering changes minerals that were stable inside the earth to ones that are  stable at Earth’s surface.   For example, clay minerals are stable at the surface and chemical weathering converts many minerals to clay (figure 6).

Chemical Weathering by Water

A water molecule has a very simple chemical formula, H₂O, two hydrogen atoms bonded to one oxygen atom. But water is pretty remarkable in terms of all the things it can do. Remember from the Earth’s Minerals chapter that water is a polar molecule. The positive side of the molecule attracts negative ions and the negative side attracts positive ions. So water molecules separate the ions from their compounds and surround them. Water can completely dissolve some minerals, such as salt. Follow this link to check out this animation of how water dissolves salt.

Hydrolysis is the name of the chemical reaction between a chemical compound and water. When this reaction takes place, water dissolves ions from the mineral and carries them away. These elements have undergone leaching. Through hydrolysis, a mineral such as potassium feldspar is leached of potassium and changed into a clay mineral. Clay minerals are more stable at the Earth’s surface.

http://geologylearn.blogspot.com/2015/10/types-of-weathering.html

Highly altered granite, shown in this image=> called “grus”

It’s great example of chemical weathering– feldspar and micaceous minerals (e.g. biotite) have altered to clay, and increased in volume, and therefore broken the rock apart!

 

 

Chemical Weathering by Acid Rain

Carbon dioxide (CO₂) combines with water as raindrops fall through the atmosphere. This makes a weak acid, called carbonic acid. Carbonic acid is a very common in nature where it works to dissolve rock.

Pollutants, such as sulfur and nitrogen, from fossil fuel burning, create sulfuric and nitric acid. Sulfuric and nitric acids are the two main components of acid rain, which accelerate chemical weathering (figure 7). Acid rain is discussed in the Human Actions and the Atmosphere chapter.

The climbers here (Joshua Tree National Park, CA) are in a sort of pocket, or cave region, on a granite face.  The cave is characteristic of self-accentuating weathering and erosion.  Once an overhang develops, more water tends to accumulate, and this water creates chemical weathering

 

Chemical Weathering by Oxygen

Oxidation is a chemical reaction that takes place when oxygen reacts with another element. Oxygen is very strongly chemically reactive. The most familiar type of oxidation is when iron reacts with oxygen to create rust (figure 8). Minerals that are rich in iron break down as the iron oxidizes and forms new compounds. Iron oxide produces the red color in soils.

 

 

A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece of sandstone was found in glacial drift near Angelica, New York

Source: english wikipedia, original upload 17 July 2005 by en:User:Pollinator

Influences on Weathering

Weathering rates depend on several factors. These include the composition of the rock and the minerals it contains as well as the climate of a region.

Rock and Mineral Type

Different rock types weather at different rates. Certain types of rock are very resistant to weathering. Igneous rocks, especially intrusive igneous rocks such as granite, weather slowly because it is hard for water to penetrate them. Other types of rock, such as limestone, are easily weathered because they dissolve in weak acids.

 

 

Different minerals also weather at different rates. Some minerals in a rock might completely dissolve in water but the more resistant minerals remain. In this case, the rock’s surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock.

Climate

A region’s climate strongly influences weathering.

Chemical weathering increases as:

• Temperature increases: Chemical reactions proceed more rapidly at higher temperatures. For each 10^oC increase in average temperature, the rate of chemical reactions doubles.
• Precipitation increases: More water allows more chemical reactions. Since water participates in both mechanical and chemical weathering, more water strongly increases weathering.

So how do different climates influence weathering? A cold, dry climate will produce the lowest rate of weathering. A warm, wet climate will produce the highest rate of weathering. Warm wet climates also enhance vegetation growth, which can in turn increase weathering rates.

 In tropical climates, intense chemical weathering carries away all soluble minerals, leaving behind just the least soluble components. The aluminum oxide, bauxite, forms this way and is our main source of aluminum ore.