In this lab you will develop an appreciation for how fast rocks can weather (be broken down). We can address this question by examining tombstones at the City Cemetery in Laredo. The goal of this lab is to determine if there is a relationship between rock type, tombstone age and the amount of weathering that has occurred.
Drive in the main entrance to the City Cemetery and drive past the caretaker’s office and park your vehicle in the back of the Cemetery. Please meet me at this spot at the time at which your lab is scheduled to begin.
LET US CONDUCT OURSELF WITH RESTRAINT AND RESPECT. REMEMBER MANY PEOPLE CONSIDER A CEMETERY AS A SACRED PLACE.
Most tombstones at the City Cemetery date back to the at least the 1880’s. It is reasonable to assume that when a tombstone is first erected the rock was fresh and the carving is sharp. Since that time, the surface has been exposed to the elements, so we assume that the older the tombstones the more weathered the tombstones.
Most tombstones at the City Cemetery are made of one of the three following
geologic materials. Be sure to be on
time because Dr. Tobin will show you how to identify these rocks.
· Marble - White to gray in color. Has a smooth surface when fresh, but becomes rougher as it weathers. Consist of coarse interlocking calcite crystals that are the same color. Note that marble is more common in older tombstones.
· Granite – Usually light gray or pink in color, with course-grained crystals of quartz, feldspar, and minor quantities of dark colored ferromagnesium minerals.
· Concrete – Sandy and gravely in appearance and feel with a tan to gray color.
WE WILL NOT EXAMINE THESE TOMBSTONES IN THIS LAB
1. Examine 15 marble and 15 granite tombstones. Try to select at least one tombstone from each decade spanning time from the 1880’s to 2000’s. Find tombstones where letters are engraved into the headstone and not embossed or raised letters. Fill out the data table, which you should have printed out before coming to lab. For each tombstone record the following information
· Rock type (marble or granite)
· Family Name
· Year of Death. Select tombstones that record the death of only one person.
· Degree of Weathering. Assign each tombstone a value from 1 – 5 based on the following scale. You may use half points (i.e. 4.5) if you wish to be more precise.
(5) Unweathered, fresh marble surfaces will be very smooth and white;
granite surfaces will be polished and reflective
(4) Slightly weathered, fainting rounding of the edges of letters (marble
surfaces feels slightly rough and granite has lost some of its polish)
(3) Moderately weathered, letters have a significantly weathered appearance
(2) Highly weathered, letters are not easy to read
(1) Extremely weathered, letters are barely readable
2. Calculate the age of each of the tombstones you examined and write answer on the data sheet.
Age of Tombstone = 2001 - Year of Death
3. Plot degree of weathering (Y-axis) versus age of tombstone (X-axis) on the graph paper you printed out from the web site. Use a different symbol for granite and marble data (example: granite – square; marble – circle).
After all data points are plotted draw two best fit lines (one for marble and one for granite) through each data set. Make one line solid and the other dashed to differentiate them. Be sure to include a key to indicate which line reflects marble vs. granite best fit line
NOTE THAT THESE LINES MUST BEGINNING AT A DEGREE OF WEATHERING = 5 and YEAR = 2001.
HINT: PLOT GRANITE DATA FIRST AND DRAW BEST FIT LINE THROUGH THIS DATA. THEN PLOT MARBLE DATA AND DRAW ITS BEST FIT LINE
4. How many years will it take for both a marble and granite tombstone to become moderately weathered (3); to become extremely weathered (1).
5. Finally, let us use our tombstone data to determine how long it will take Enchanted Rock (granite) to be completely weathered. If we assume that the letters have an average depth of 5 mm deep use the number of years as determined in question 4 to determine a granite weathering rate in mm per year, meters per year, and meters per million years. At this rate, assuming that Enchanted Rock is 425 meters high how many years will it take to weather away.
6. Another large rock in Texas is El Captain, which is made of the mineral calcite. Note that calcite is the primary mineral present in marble. Assuming El Captain is 1000 m high how long would it take for this rock to be weathered away?