Transcript Processes - Historic Cemeteries Conservation Trust of New

A presentation for students that looks at
the types of weathering, that can be
found in our cemeteries.
 The breaking down of rock is called weathering.
 There are two types of weathering.
 Most weathering is the result of both kinds.
 Different rocks weather at different rates depending on how hard
they are. How hard they are depends on what they are made of.
 Chemical weathering is the chemical breaking down of rocks.
 When chemical weathering happens, a new substance is made.
 There are many different minerals in rocks. When water and air react with
these minerals the new minerals formed are usually softer and the rock begins
to crumble.
 Limestone and marble are made of calcium carbonate. A special
type of weathering occurs with limestone and marble.
 Carbon dioxide in the air reacts with rainwater to form carbonic acid
Carbon Dioxide + water
carbonic acid.
 Limestone and marble react with acid so when this acidic water
seeps into cracks and fissures it can dissolve quite large amounts
eventually turning into caves and archways.
 Limestone is a soft sedimentary rock, prone to chemical weathering.
 Example 1 shows delamination on the surface of the stone.
The surface is lifting and flaking around the inscription.
 Example 2 shows weathering on the carved edges of one
side of the inscription plaque.
2
1
 Images show a combination of chemical and
physical weathering on this memorial in Dunedin’s
Southern Cemetery.
 The northern face of this
monument may have been
worn away with water
turning the calcium
carbonate into carbonic
acid.
 It is now a warm
roosting site for sparrows
who are contributing to
the wear.
 Slowly over time the calcium carbonate reacts with
carbon dioxide in the air or in rainwater to form
carbonic acid.
 The acid wears away the
marble from around the
inserted lead letters gradually
leaving them slightly proud of
the surface stone.
 More and more water
gradually wears the marble
away around the lead pegs
and the letters gradually come
loose.
Can you work out what factors affect how quickly some rocks are
attacked by acid?
Choose a factor to investigate. This might be temperature or perhaps
how strong the acid is. Are there other factors to consider?
How will
How will
How will
How will
you measure the rate of reaction?
you make it a reliable test?
you make sure your tests are safe?
you record your results?
Temperature of
acid (0C)
Concentration of
acid (M)
Time to collect
gas
Time to collect
gas
What equipment will you need?
Limestone chips /or marble chips (size of chips
might be a factor to investigate?)
Dilute hydrochloric acid
Bunsen burner, tripod, gauze & heatproof mat
Thermometer
Balance or digital scales
Timer
Flask with stopper and delivery tube
Basin and Measuring cylinders
 Granite is formed by the solidification of magma underground. It is
called an igneous rock.
 Granite often contains cracks and joints.
 Water seeps into the cracks and some of the minerals in the granite
gradually turn to clay causing the granite to crumble.
 This is a very slow process and there is little evidence that
headstones made from granite are starting to weather chemically.
 When the physical weathering of rocks happens no new substances are
made.
 Rocks instead are broken into smaller and smaller pieces in a variety
of ways that include temperature changes, freezing and thawing and
plants.
 Repeated heating and cooling of rocks can cause cracks to form.
 During the day the part of the stone facing the sun heats up more quickly than
the side facing the shade, and expands. At night the sunny side of the stone
cools and contracts. The continual expansion and contraction eventually causes
cracks in the rock.
 This kind of weathering is sometimes called onion skin weathering or
exfoliation and can also be a cause of the delamination seen in slide 5 on the
limestone headstone.
 Water gets into cracks in rocks. When water freezes it forms ice.
When water begins to freeze it expands. Water between 4oC and 0oC
expands. A lump of ice has a greater volume than the liquid water that it was
made from.
Freezing causes expansion of cracks and splitting or spalling of headstones
which can cause even greater damage in future freezing and thawing.
What happens to water when it freezes?
You can try this experiment at home.
Fill the bottle with water right to
the top. Screw on the lid tightly.
It is important to place the filled
bottle in the ice-cream or plastic
container and fit the lid.
Place in the freezer over night.
What equipment will you need?
Small glass bottle with tight fitting
lid
Water
Freezer
Ice cream or other strong plastic
container with tight fitting lid.
Describe how
the bottle and
water look
next morning.
 The base of this sandstone memorial is beginning to fracture and
flake on the sunny side of the
memorial. Cracks are running
parallel and at right angles to the
surface.
 As the cracks open up water
enters and undermines the stone
especially in freeze thaw cycles.
 Plants cause physical weathering.
 Some of the seeds may fall, be blown, or dropped by birds
into cracks in rocks and develop into seedlings.
 As the tree grows, the roots act like a crowbar wedging rocks apart.
 Plants and lichens also exude chemicals that enter cracks in the
rock.
 Windblown or bird
dropped seeds often
grow unnoticed in
cracks and can remain
safe from weed killers.
 When the trees
become larger the
roots can undermine
the headstone
foundations, and
trunks can push and
displace the
headstone, or even
subsume it completely.
 Left: Foetid iris and holly are growing in
cracks between and behind headstones.
 Above: Agapanthus is a major
nuisance at an Auckland cemetery.
 Lower left: Ivy clings by growing into
the surface of softer stones and can
never be completely cleaned.
 A wide variety of mosses, lichen and fungi can
be found growing over cemetery headstones.
 Although lichens are
usually the first living
organisms to colonise rocks
the process of wearing
down the stone is very slow
and they do less actual
damage to the stone than
some cleaning processes.
This is a species of encrusting lichen called Xanthoria ligulata
commonly found on coastal rocks.
 Mosses can be found colonising
a range of different stone types
especially in areas where water
accumulates or pools.
 Limestone is a porous rock that
retains water. Limestone
headstones that are lying with their
backs in contact with the ground
are often covered in a thick layer of
moss in damper areas.
 Red or black staining on white marble detracts from the look of the
headstone.
 The red discolouration is caused by
lichen staining which emanates from the
microscopic growths which inhabit the
spaces behind the lead letters.
 The black
discolouration is caused
by a fungus which grows
on the surface of the
marble.
 Xanthoparmelia scabrosa is the botanical name for a pale green lichen that
thrives in damp urban environments. It can be found growing on rocks, tree trunks
and roof tiles, as well as on asphalt and is a common lichen found in our
cemeteries.
 Various lichens including the very common
Xanthoparmelia scabrosa grow over the surfaces of
headstones making it difficult to read inscriptions.
The stones in the cemetery are also open to other types of wear and tear. One of
the most common is the gradual movement of soil down a steep slope. Sometimes
the headstone and the corner posts will all shift to the same degree.
 Another is uneven soil compaction.
After a burial soil was used to refill the
hole but this was not compacted. Over
time this un-compacted soil settles and
creates sinkage.
In many places cemetery headstones are affected by being located in low lying
swampy places.
 Some older cemeteries were not developed with
adequate field drains. Some of the soil creep and
compaction problems can be due to a lack of, or poor
drainage arrangements.
 Deliberate vandalism is a problem in many cemeteries. Examples from left
include…
Whole or parts of memorials can be smashed.
Graffiti painted or incised on headstones
Marble decorations deliberately chipped off
Stelae pushed over
 Examples of a range of chemical and physical weathering of rocks can be found
in almost all of our cemeteries. The natural processes of wearing away and
breaking down of cemetery materials is occurring all the time.
 Maintenance and conservation of our cemeteries is an important and ongoing
task if the historic cemetery is to remain for future generations. For homework visit
http://www.cemeteries.org.nz/ and click on the left hand link to “Cemetery
Conservation.” Assign problems from the slide show or have groups or individual
students choose to investigate and find a possible solution from the site. Revisit
each of the slides and encourage the class to discuss solutions to some of the
problems in this slide show.
Discussion: What are the solutions
to some of these problems?