This Earthcache will highlight the Cathedral to discuss the geology of materials in brickmaking and the cause and effects of an earthquake.
Geology of Bricks
- The common brick is one of our greatest inventions, an artificial stone. Brickmaking transforms low-strength mud into strong materials that can endure for centuries when properly cared for.
- The Cathedral is constructed of 700 000 bricks.
- The main ingredient in clay bricks is a group of surface minerals that arise from the weathering of igneous rocks.
- Adding sand in the mix prevents them from cracking. Clay when dried in the sun becomes stone.
- Bricks can deteriorate from the weather or earthquakes.
- When bricks are fired, the brick clay becomes metamorphic rock.
- Clay minerals break down and release chemically bound water to change it to mixture of two minerals, quartz and mullite.
- Quartz remains in a glassy state as it crystallises very little in that time.
- The key mineral mullite (3AlO3 · 2SiO2), a blended compound of silica and alumina that is quite rare in nature.
- It is named for its occurrence on the Isle of Mull in Scotland.
- Mullite is hard and tough, but it also grows in long, thin crystals that function like the straw in adobe, binding the mix in an interlocking grip. (Adobe bricks consist of a mixture of clay, sand, straw).
- Iron oxide is an important mineral in clay for giving colour.
- Iron oxidises into hematite that gives the red colours.
- A calcareous clay contains more lime and gives a yellow colour.
- The presence of manganese oxide, by contrast, gives a brown colour to the brick.
- Other elements including sodium, calcium and potassium help the silica melt more easily—that is, they act as a flux.
The Newcastle Earthquake
- At 10.27am on Thursday, 28 December 1989, Newcastle was devastated by an ML 5.6 (Richter magnitude) earthquake. This was one of the most serious natural disasters in Australia's history. It lasted approx 10 sec duration.
- One aftershock (M 2.1) was recorded on 29 December 1989
- The earthquake claimed 13 lives, hundreds were injured - 160 people hospitalised
- 50,000 buildings were damaged (including 40,000 homes) within a 9000 sq. km region
- 300 buildings were demolished
- 300,000 people were affected and 1,000 made homeless
- Damages cost $4 billion
- The effects were felt over an area of about 200,000 sq. km, with isolated reports of movement up to 800km from Newcastle
- The earthquake epicentre was at 14 km south west of Newcastle's city centre at Boolaroo, approx 11km depth.
Source info: https://newcastle.nsw.gov.au/library/newcastle-stories/online-collections/the-newcastle-1989-earthquake
Damage to the Cathedral
- High set stone crosses and other decorations fell to the ground
- Flying buttresses were dislodged
- Cracking occurred in all walls and windows were broken / dislodged.
- Structural damage required restoration works from 1995 to 1997 which aimed to resist future earthquakes and kept the aesthetic of the building preserved.
- An Australian first technique employed from the European “Cintec” anchoring system was used to reinforce walls and piers, horizontally and vertically, with a total of over 3700 metres of high strength stainless steel bars.
- The project pioneered many building techniques in Australia thereafter.
- All the reinforcing was inserted in holes drilled in the brickwork and techniques were developed which allowed the holes to be drilled without cooling water, as the escaping water could have damaged other areas of the Cathedral. The horizontal reinforcing of the nave walls, at 32 metres long, was a world record for this type of work.
Cause of the Earthquake debated
- In 2007, Dr Christian Klose, from Columbia’s Lamont Doherty Earth Observatory presented a new theory that the earthquake was prompted by two hundred years of coalmining.
- Dr Klose theorised that because of coalmining, stress in the Earth’s crust started to change and over time this caused the tremor. He said “dewatering" mines removed millions of tonnes of groundwater from Newcastle over the years, adding to "destabilisation in the Newcastle fault."
- A senior seismologist at the Federal Government's research unit, Geoscience Australia, Dr Phil Cummins, said Dr Klose's research was interesting but there was still a lot of uncertainty about the exact cause of the Newcastle earthquake.
- Australian Geo-scientists disagreed with Dr Klose’s theory and said that because the epicentre of the earthquake was too deep underground, it was not likely to be caused by extensive coalmining.
- An Australian report highlighted other factors that contributed to the earthquake - there were minor tremors in the Hunter Valley region prior to the quake that might have created a fault line that led to the earthquake.
What causes an earthquake ?
- Our earth’s crust is broken up into pieces called Tectonic Plates.
- These plates are constantly moving towards, away from and past each other. Our continents are a part of these plates too, so they are also moving.
- The result of this tectonic plate movement are earthquakes- caused when the rocks break and move as a result of stresses built up in the plates.
- When this stress is released, this is what causes earthquakes.
- Most earthquakes occur on the edge of the plates.
- Australia is in the middle of these plates, so its earthquakes here work differently. They are "intraplate" movements.
- Australia’s earthquakes are caused by the movement (7cm per year) of the Indo-Australian Plate, which is at the centre of where the continent lies. It collides with the Eurasian, Philippine and Pacific plates causing a build up of stress in the 25km thick crust.
- Hunter Valley region has a long history of earthquakes in the written record, with the earliest occurring in 1842. Since this time, the region has experienced five earthquakes of magnitude 5.0 or larger; the most recent being a magnitude ML 5.3 earthquake near Cessnock (Ellalong) in August 1994.
To log the EarthCache:
Answer the following 3 questions to send to CO but do not write answers in the log:
1. Describe the colours you see in the bricks of the cathedral and what minerals they would originate from?
2. From the view at GZ, you can see Stockton sand dunes. What is the mineral that would be found here to use in brickmaking?
3.What is your prediction for another earthquake happening in Newcastle again and justify your answer ?
To include in your field log:
4.Take a photo of your GPSr or personal item including a part of the cathedral in the background.
5. (Optional) Share your memories of the day of the Newcastle Earthquake.
For info about National Earthquake Alerts Centre (NEAC) at Geoscience Australia go to https://earthquakes.ga.gov.au/