Weathering

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This cement bollard is being weathered mechanically

Weathering is defined as the process by which rock materials are broken down by the action of physical or chemical processes. Physical weathering is known as mechanical weathering, where rocks breakdown into smaller pieces by mechanical means. Agents of mechanical weathering include ice, wind, water, gravity, plants, and even, yes, animals [us]!

Mechanical Weathering[edit]

Mechanical Weathering is defined as "the process of breaking big rocks into little ones". Remember the agents of mechanical weathering:

  • Ice
  • Wind
  • Water
  • Gravity
  • Plants
  • Animals

Let's go through the first of these six agents of mechanical weathering.

Ice[edit]

Ice wedging taking its time in Spitsbergen, Norway
A diagram of Ice Wedging

A form of mechanical weathering, that is of/related to ice, is frost action (Alternate freezing and thawing of soil and rock). A famous type of frost action is the process of ice wedging.

Ice wedging starts when water seeps into the cracks [in the rocks] and, when the temperature drops to freezing temperatures, freezes. When it freezes, it also expands. The expanding ice pushes against the sides of the crack. This causes the crack to widen, and eventually (when repeated several and several times) breaks the rock apart.

Abrasion[edit]

Abrasion is the grinding and wearing away of rock surfaces through mechanical means of other rock/sand particles. This process can be understood with how a piece of chalk works. As you scrape a piece of chalk against a board to draw/write, particles of the piece of chalk rub off to make a line on the board and the piece of chalk wears down and gradually becomes smaller.

Abrasion can happen in many ways. Let's discuss those ways.

Water, Wind, and Gravity[edit]
A ventifact

As mentioned before, Abrasion can happen in many ways. Here, we are going to briefly list three forms of Abrasion that happen to rocks (to give you an idea of Abrasion):

  1. If you go into a river, and see that the rocks are rounded, that means those rounded rocks in the riverbed have been tumbling repetitively by fast moving water for years and years. The water carrying the sand hits the rocks and makes the rock wear away and may change texture and maybe color if you pull the rock out of the water. [Water]
  2. If you see rocks, or ventifacts, on the side of a beach, then you should know that those ventifacts are rocks that have been shaped by blowing sand. [Wind]
  3. If you see heaps of small and small rock fragments on-top of each other, then you should know that these fragments used to be big rocks that would grind against each other. [Gravity]

You have just read three examples of rocks that have gone through abrasion in three forms: Water, Wind, and Gravity. Water can wear down rocks due to the tremendous power of water pushing against the rock. Have you ever wondered why some beaches have extremely steep mountains on their shorelines? It is because of years and years of harsh waves [of water] pounding on those rocks, eventually forming steep mountains (due to rock particles wearing away due to the force of water). This is why some beaches have high-gradient mountains at their shorelines.

Plants[edit]

This sidewalk is at the wrath of tree roots

Though plants seem weak, plants can be super strong! To the point that plants can, indeed, break rock into pieces! Root pry is when trees send in their roots into rocks [through cracks] and break them apart. As the tree grows, the roots grow, and thus, break the rock [in which the roots have been sent into] apart. This is the process that might be happening in the sidewalks near your houses!

Animals[edit]

Even animals can cause mechanical weathering! Earthworms are a good example of this. Earthworms, and other animals that burrow (ants, worms, mice, coyotes, rabbits, etc.), move sand particles around, causing the exposure of fresh surfaces to weathering. Animals are essential to weathering, so much so, that they even contribute to another form of weathering, known as Chemical Weathering.

Chemical Weathering[edit]

Chemical Weathering is defined as "the process by which rocks are broken down by chemical reactions". The common agents of chemical weathering are:

  • Water
  • Weak Acids
  • Air

Water[edit]

This gargoyle has been severely damaged by acid rain

Water can break down rocks. It is somewhat like how a sugar cube dissolves in a glass of water.

When rain, weak acids, and air chemically weather granite, the bonds between the mineral grains [in the granite] weaken as the agents of chemical weathering are being played [on the rock]. When granite is weathered after thousands and thousands of years, it makes sand and clay (also called "sediment").

When rain, sleet, or snow contains a high concentration of acids (For the record, precipitation is naturally acidic), this is known as acid precipitation. When acid precipitation takes place, it can cause rocks to weather pretty quickly!

The contributors to acidic precipitation could be natural sources (such as volcanoes). When the sulfuric and nitric acids, from natural sources, come in contact with precipitation (rain, sleet, snow, etc.), then those acids could make precipitation highly acidic. Acid precipitation can also be formed by the burning of fossil fuels (coal, oil), which contributes to pollution. The pollution may contain [the gases of] sulfur oxides, nitrogen oxides, and carbon oxides, because these gases form from the burning of fossil fuels. Not only does acidic precipitation weather rock, but it also can cause harm to plants and animals (including us, humans)! Acidic precipitation also can harm statues and relics.

Acids[edit]

Groundwater[edit]

Chemical weathering also takes place under our feet! Some groundwaters contain acids such as carbonic/sulfuric acid. Groundwaters, containing these acids, sometimes seep into stones, like limestone, and weather them down. After weathering, they become karat features, such as caverns.

Living Things[edit]
You've probably seen this before if you're an outsider

An example of living things that produce acids are lichens (which is a composite organism that consists of fungi and algae living together). Lichens give off acids that can gradually weather down rocks. Lichens can be found in parks, forests, deserts, arctic areas, and in high areas (where even trees don't grow!).

Air[edit]

One word, Oxidation.

Oxidation is when oxygen combines with an element, such as iron or silver, to form an oxide. This is what happens when you leave cars, bikes, tractors, or trucks out in the rain and air too long. Oxidation is what causes rusting, which is a chemical process.

Rates of Weathering[edit]

Differential Weathering[edit]

Devils Tower in Wyoming

Differential Weathering is when softer, less-weather resistance rocks wear away and leave off harder, more weather-resistance rocks behind. A good example of this is Devils Tower in Wyoming. This once was a mass of molten rock deep inside an active volcano. When the molten rock cool and hardened, it was surrounded by the outer rock of the volcano, thus, protecting it from weathering. After thousands and thousands of years, the soft outer parts of the volcano have worn away and now harder, more resistant rock is all that remains [to this day].

Weathering takes its course on the outer surface of rocks. Thus, the more surface area this is opened to weathering = the faster the rock will be worn down. But, a large rock also has a large volume, thus, the large rock will take a long time to wear down. But, if this large rock breaks into smaller fragments/pieces, the rate of weathering increases because a smaller rock has more surface area to volume than a larger rock has. So, more of a smaller rock is exposed to the weathering process.

Gravity and climate affects weathering:

Gravity
  • When mountains have steep slopes, gravity allows the water to quickly run off the mountain, carrying sediments along with it [the water]. When this event takes place quite a number of times, this exposes fresh rock surfaces to weathering.
Climate
  • The rate of chemical weathering happens faster in warm, humid climates.
    • Example: A mailbox in a cold, dry climate with experience less weathering than a mailbox in a warm, humid climate ("oxidation")
    • Example: In Arizona, even though it is definitely hot, rain does not happen quite often, and since it is always hot, the climate in Arizona is not cold enough to freeze water [that does come], so ice wedging doesn't take place. But in Virginia, the climate is warm and humid (and can get cold to freezing temperatures), so chemical and mechanical (ice wedging, etc.) weathering takes place quite a lot more than in Arizona.