Weathered rock

Weathered rocks

These highly weathered rocks probably don't seem particularly interesting, and in truth, they really aren't. But this kind of weathering and erosion of rocks is the important first step in the geologic CO2 cycle. 

What happens is that carbon dioxide in the atmosphere combines with rain water to make a very weak acid rain. Rainfall is, therefore, always slightly acidic. This means nothing for humans and other forms of life as the acidity is nowhere near significant enough to do anything bad to us, but over a long period of time, this weak acid rain can dissolve some of these rocks and break them down into their component molecules. 

These component molecules then get carried into the ocean. Here at Pallarenda that process is pretty quick as the ocean is just metres away. But if the rocks are being weatherd and dissolved further inland, then it can take some time for those molecules to make it to the ocean via rivers and streams. 

Once in the ocean, those molecules are used to form new minerals, such as calcite, or rocks, such as limestone. Both of which are made out of something we call calcium carbonate. A really good example of this stage of the geologic CO2 cycle is the Great Barrier Reef. 

Coral reefs, or more specifically, the skeletons of corals, are made out of calcium carbonate and limestone. Once corals die, their skeletons are left behind just like any other animal, and all of that calcium carbonate and limestone can stay sitting in the ocean for a long time. 

On a good day at low tide, you can see Middle Reef out there between Townsville and Magnetic Island, which is a living example of the geologic CO2 cycle in action.

So from here we can really start to picture how the geologic CO2 cycle works. CO2 in the atmosphere forms a weak acid rain which dissolves rocks on the land over a long period of time, then the component minerals and molecules from the rocks make their way into the ocean and help form limestone and calcium carbonate. That carbon product is still present. 

Soon we will come across examples of the next stage of the geologic CO2 cycle: volcanism!

Geoscience Audio Tour around Townsville
  1. Castle Hill
  2. Dolerite Dyke Intruding into Granite Rock Wall
  3. Faulting in Granite
  4. Xenoliths in Granite
  5. Pallarenda Beach
  6. Pumice and Coal on the beach
  7. Weathered rocks
  8. Lava Flows
  9. Volcanic tuff
  10. Preferentially weathered intrusion
  11. Coarse volcanic tuff