Specially made nylon sling was fabricated and used to lift the sandstone columns
from the wreck of the Zephyr to a barge. (Louis Vallee photo)
These sandstone columns were carved from the Nanaimo Group at a quarry on the north coast of Newcastle Island.Due to the uniform size and dense packing of the sandstone grains, this "foreign" sandstone was certified as "highest character in quality" and was found acceptable for use at the San Francisco Mint. A sample description of the medium grained sandstone discovered that it was an attractive light grey and had a specked salt and pepper appearance. It was noted that buildings using Newcastle Island sandstone was darker than fresh outcrop, suggesting that the stone darkens when exposed. Pitting and peeling take place on exposed surfaces. Thin sections of the rock showed 50 to 60 % of it is comprised of closely packed, fresh, angular to subangular quartz grains, that were commonly 0.5 millimeter in size. During shipment to the Mint in 1872, the Zephyr sank off the coast of Mayne Island. Because of this, original plans for eightcolumns in the design were changed to just six.A diver discovered the wreck in 1976, and in 1987 after spending 115 years under water, both columns were raised. One was returned to the original site on Newcastle Island and the other was transported and placed in front of the Vancouver Maritime Museum.
Please use the message center when answering the questions. Copy and paste the questions. Please do not post answers in your log. After sending answers, you can log it as found. If answers are not received, your found it log will be deleted. I really appreciate it if both the answers and the Found It log happen on the same day.
1.Which type (or types) of weathering best fit the sandstone you see at the posted coordinates?
Back up your answer with an example of biological (plants), physical, or chemical breakdown of this column.
2.What is the average size of the pitting found on the column?
3. What do you think might be the leading cause of this weathering? Include your observations of the climate and environment with your answer.
4.Measure the column, and email me it's diameter and length in feet and inches (or if you prefer in meters)
How sandstone iscreated:
Deposits of sand, by wind or water, accumulate in an area. As more sediment is added the weight that is applied on top compacts the lower deposits. This compaction forces the air and water out from the grains of sand. Mineralsfrom the concentrated solution act as a cement which binds the particles of sand together crating sandstone. The quality of the sandstone is determined by the color, size of the sand grains, minerals present and it's ability to withstand weather. Sandstone in general terms has a specific gravity of 2.32 and a weight of 145 pounds per cubic foot.
Natural weathering of stone is inevitable, but some types have a structure that makes them more durable than others. There are three major causes of deterioration in natural stone; pollutants, frost, and crystallization of soluble salts. Water penetration is the main instigator of decay, and the structure of the rock is the most important factor influencing the ability of stone to resist decay processes.
Weathering is different from erosion.Erosion is the process whereby sediments and rock particles are picked up and moved to another place by gravity, wind, water or ice. Weatheringis the process of weakening and breakdown of rocks at the Earth's surface. This can be caused by extreme temperatures, rainwater, or biological activity and does not involve any movement of the rock material.
There are three main types of weathering.
Mechanical / Physical Weathering
Mechanical weathering (also known as physical weathering) is the physical breakdown of rock into smaller and smaller pieces. Frost shattering (frost weathering, frost wedging, ice wedging, freeze-thaw weathering, and cryofracturing) are one of the most common mechanical actions. It is caused by water entering the pores and cracks of the rock and then freezing. Water within the rock expands by about 10% when it freezes and opens the crack a bit wider. The pressure acting within the rock at -7.6 F (-13.5 C) is estimated at 30,000 pounds per square inch. Over time the pressure and changes in the weather cause the rock to split and the bigger pieces are broken into smaller and smaller chunks.
Salt wedging is another type of mechanical weathering. Wind, water, waves, and rain have a reaction with rocks as they are physical forces that wear away rock particles over long periods of time. These forces release their pressures on the rocks either directly or indirectly which causes the rocks to fracture and disintegrate.
Thermal stress causes mechanical / physical weathering. Contraction and expansion by changes in temperature (extremely hot or cold) results in the rock splitting apart and disintegrating into smaller and smaller pieces.
Physical weatheringis caused by the effects of changing temperatures on rock causing it to break apart. This process is sometimes assisted by water. It can happen through repeated melting and freezing of water in mountains or tundra, or through the expansion and contraction of the surface layer of rocks that are baked by thesunin hot deserts. Another name for this is mechanical weathering. It physically breaks up rock. One example is called frost action or frost shattering. Water gets into cracks and joints in the bedrock.
Organic / Biological Weathering
Organic or biological weathering are different names for the disintegration of rock as a result of living organisms. Trees and other plantswear away rocks. As their roots grow larger, they apply pressure and eventually break the rock apart. Microscopic organisms such as bacteria, algae, moss and lichens, grow on the surface of the rock and produce chemicals which encourage the chemical and physical breakdown of the outer layer of rock.They eat away the surface of the rock (nutrient extraction) and create a moist chemical micro-environment that encourages breakdown at the rocks' surface. Burrowing animals can speed up this process by digging around the stone and exposing rock surfaces to the elements.
Chemical Weathering
Chemical weatheringis caused by rain water reacting with the mineral grains in rock to form new minerals (clays) and soluble salts. These reactions occur more often when the water is slightly acidic. This chemical process needs water and occurs more rapidly at higher temperature, so a warm, damp climate is best. Chemical weathering happens when rock is worn away by chemical changes. Over time, natural chemical reactions inside the rock changesits composition. The chemical process is gradual and ongoing. As the mineralogy of the rock changes it allows the rock to wear away, dissolve, and disintegrate.
Chemical weathering occurs in different ways, and the most important are:
1. Solution - removal of rock by acidic rainwater
2. Hydrolysis - the breakdown of rock by acidic water to produce clay and soluble salts.
3. Oxidation - the breakdown of rock by oxygen and water, often giving iron rich rocks a rusty-colored weathered surface.
Chemical transformations occur when oxygen and water interact with minerals inside the rocks to create different chemical reactions andcompounds (oxidation and hydrolysis). This creates new material formations, pores and fissures which enhance disintegration.
Acid rain reacts with rock mineral particles producing new minerals and salts that can dissolve or wear away rock grains. Chemical weatheringmostly depends on water, temperature and the rock type. Higher temperatures increase the rate of chemical weathering. Sandstone is more prone to chemical erosion that granite.
Pitting
|
Congratulations to kttyquest for the First To Find, January 14, 2019!
|
References:
The Geology of Southern Vancouver Island
by Chris Yorath
British Columbia Geological Survey
Geological Fieldwork 1987
Wikipedia