Fading Fast EarthCache

If one is buried after death, it is customary to place some sort of grave marker over the deceased's remains. Today, many people assume that modern grave markers will last forever. With few exceptions, the evidence suggests otherwise. A survey of grave sites in cemeteries will reveal many deteriorating grave markers. Some grave markers are so deteriorated that the inscriptions on them are difficult to read. This is because of a process called weathering.

Weathering is the breakdown of rocks at the Earth’s surface, by the action of rainwater, extremes of temperature, and biological activity. Weathering is the process where rock is dissolved, worn away or broken down into smaller and smaller pieces. There are mechanical, chemical and organic weathering processes. Once the rock has been weakened and broken up by weathering it is ready for erosion. Erosion happens when rocks and sediments are picked up and moved to another place by ice, water, wind or gravity.

Mechanical Weathering: weathering due to wind, rain, and changing temperatures
Chemical Weathering: weathering due to chemical reactions
Biological Weathering: weathering due to fungi, lichens, plants, and animals

Cemeteries make for a good location to study weathering, because a death date on a grave marker gives researchers an approximate date for when the grave marker was placed. Researchers can then identify the extent of weathering that has taken place on that grave marker, and check if the amount of weathering is average/expected, or if other factors are accelerating the weathering process.

Mechanical Weathering physically breaks up rock. Mechanical, also known as physical weathering, can be divided into two main categories: fracturing and abrasion. Over time, pieces of rock can split off a rock face and big boulders are broken into smaller rocks and gravel.

--Frost Wedging or Freeze-Thaw: Water expands by 9 percent when it freezes into ice. As it expands, it exerts up to 4.3 million pounds per square foot of pressure, enough to open cracks and fissures in rocks. Repeated freezing and thawing allows water to seep deeper into these crevices and enlarge them. Cracks may also allow entry of roots, agents of biological weathering that can also pry apart rock.

--Crystal Formation or Salt Wedging: Crystal formation cracks rock in a similar way. Most water contains dissolved salts. When water in rock fissures evaporates, salt crystals form that, like ice, can force open fissures. This “salt wedging” tends to be most pronounced in arid regions given the high evaporation rates; it also occurs along sea coasts.

--Unloading and Exfoliation: Granitic rocks formed by cooling magma underground and later exposed by uplift and erosion may “exfoliate”: The release of pressure causes strips or sheets of rock to peel away. Rock once compressed under the weight of glaciers may also exfoliate due to unloading: When the glacier finally melts – for example, at the start of an interglacial period – the rock expands from the reduction of pressure. This causes fracturing between the layers parallel to the Earth’s surface. The top layer breaks apart in sheets, having no load above it at all. As the rock below is exposed, it too exfoliates.

--Thermal Expansion and Contraction: Heating causes rock to expand. Cooling causes it to contract. The resulting cracking looks similar to frost wedging, though it tends to take a much longer time. Areas with extreme swings in daily temperature may see higher rates of this kind of wear. The moon has almost no atmosphere and no tectonic activity to weather the rock, and the temperature variation between day and night is 536 degrees F (280 degrees C). Thermal expansion and contraction may, therefore, be the only form of weathering that occurs.

--Rock Abrasion: In dry regions, wind-driven sand abrades exposed rock in a natural form of sandblasting. In streams, rivers and ocean surf, water turbulence causes particles of rock to collide with one another and grind against larger bodies of rock: abrasion that ultimately whittles them into smaller particles. Boulders, stones, and grit embedded in glaciers also abrade the rock surfaces over which the ice flows.

--Gravitational Impact: Rocks toppling off cliffs or steep slopes due to the tug of gravity or swept up in landslides split into smaller pieces, another form of physical weathering by abrasion and impact. The actual gravitational transport of rocks and sediments is known as mass wasting, which isn’t itself a form of weathering but rather a process by which weathered material moves from one place to another.

Organic weathering happens when plants break up rocks with their growing roots or plant acids help dissolve rock. Organic or biological weathering can result from plant or animal activity. Such weather can be quite subtle but can cause significant change over time. Tree roots, because of their size, cause a significant amount of biological weathering. But even much smaller plant-related actions can weather rocks.

--Weeds pushing through road surfaces or cracks in boulders can expand gaps in the rock. These gaps fill with water. When the water freezes, the roads or boulders crack.

--Lichen (fungi and algae living together in a symbiotic relationship) can cause a great deal of weathering. Chemicals produced by fungi can break down the minerals in rocks. Algae consume the minerals. As this process of breakdown and consumption continues, rocks start to develop holes. As described above, holes in rocks are vulnerable to physical weathering caused by the freeze/melt cycle.

Human beings have a dramatic weathering effect. Even a simple path in the woods has an impact on the soil and rocks that make up the path. Major changes affected by humans include:

--Construction moving, scoring, and smashing rock for construction of buildings and transportation systems

--Mining massive projects involve stripping entire hillsides or making major changes to or removing rock from under the surface of the Earth

--Agriculture in addition to moving rocks to make farming possible, human beings also change the composition of the soil through fertilization and application of herbicides.

Chemical weathering decomposes or decays rocks and minerals. An example of chemical weathering is water dissolving limestone. Water and many chemical compounds found in water is the main agent of chemical weathering. Feldspar, one of the most abundant rock-forming minerals, chemically reacts with water and water-soluble compounds to form clay. Water contains many weak acids such as carbonic acid. This weak, but abundant, acid is formed when carbon dioxide gas from the atmosphere mixes with rainwater. Sulfur dioxide and nitrogen gases create other types of acid rain that act as chemical weathering agents. Some sources of sulfur dioxide are power plants that burn coal; as well as volcanoes and coastal marshes. Sulfur gases react with oxygen and rainwater to form sulfuric acid. Although relatively weak, acid’s abundance and long-term effects produce noticeable damage to vegetation, fabrics, paints, and rocks. There are different types of chemical weathering, the most important are:

--Solution: removal of rock in solution by acidic rainwater. In particular, limestone is weathered by rainwater containing dissolved CO2, (this process is sometimes called carbonation).

--Hydrolysis: the breakdown of rock by acidic water to produce clay and soluble salts.

--Oxidation: the breakdown of rock by oxygen and water, often giving iron-rich rocks a rusty-colored weathered surface. Oxidation is another kind of chemical weathering that occurs when oxygen combines with another substance and creates compounds called oxides. Rust, for example, is iron oxide. When rocks, particularly those with iron in them, are exposed to air and water, the iron undergoes oxidation, which can weaken the rocks and make them crumble.

Question Time!

The posted coordinates will bring you to The Fairmont Wesleyan Church and Cemetery, parking is available nearby. Please be respectful on Sundays and do not cache after dark. Listed below are four waypoints visit each of the locations below and in an email, tell me what kind of weathering is present at each to claim credit for this Earthcache. In addition, answer the questions below.

Extra Credit: Locate another headstone with evidence of geological weathering. Share a photo and the coordinates with your log.