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Castle Hill

You are now standing on top of an isolated pink granite monolith, which you probably know as Castle Hill, or Cootharinga to the local indigenous people. For many in Townsville, this hill plays a role in the everyday. We walk up it in the morning and look up at it in the evening as the sun is setting. And yet, how many people in Townsville know anything about how it was formed, what it is made out of or how old it is? Well, soon you are going to be one of the few. So buckle up, because we are going back in time… really really far back.


The earth is about 4.6 billion years old. To fully understand how massive this number is, we first need to be able to understand how big 1 billion is. If you counted 1 million seconds, you would be counting for 11.5 days. But, if you counted 1 billion seconds, you would be counting for 31.5 years. So, the earth is really old… Animals have been on the earth for almost 600 million years, while homo sapiens have only been here for about 200,000 years. That is seriously just a blip of earths history. In fact, if we condensed all of earths history into just 1 year, modern humans would have evolved with just 8 minutes to go until midnight on new years night.


So, what has been happening on earth for all of the time before we got here, well, actually a lot. Essentially, when Earth formed, it was just a big giant ball of molten rock. It was super hot, a big lava ball. But then, it started to cool. And, when lava cools, it makes rocks. So, for a lot of earths history it was cooling and making rocks. All the rocks that you have ever stood on. Including this one!


This big chunk of rock formed about 320 million years ago. It rises to about 286 metres above current sea level, making it just 4 metres too short to be called Castle Mountain! It is an igneous rock, which means that it formed from the cooling of molten rock. And, it is a granite!



Take out your microscopes if you have them, or, just have a close look at the granite which we call castle hill. See if you can identify the 4 different minerals present in the granite. To do this, try and look for four different colours.


When you look at Castle Hill from afar, it looks pink or orange! Can you see this mineral? That’s called Potassium Feldspar. 

The clear/grey looking mineral is quartz!

The white mineral is another feldspar, called a plagioclase feldspar.

And the small black stuff is likely biotite!

So, now you know that castle hill is a granite, made up of feldspar, quartz and biotite!


Most of the rocks around Townsville are granites, so you can practice this exercise wherever you go! We are going to see a lot more granite today throughout this tour.


So castle hill formed 320 million years ago when it cooled from a molten state. When rocks form by cooling, we call them igneous rocks. There are two types of igneous rock, intrusive and extrusive igneous rocks. An intrusive igneous rock cools BELOW the earths surface, inside the earth, whereas an extrusive igneous rock cools ABOVE the earths surface, outside of the earth. Where the rock cooled is important, as it effects the size of the crystals. When you look at this granite, you can SEE with your eye the different crystals. You can identify the different minerals just by looking. That is because the crystals are big enough to see. This tells you that this rock cooled slowly, giving the crystals enough time to grow to this size. It was able to cool slowly as it cooled inside the earth, and so, Castle Hill is an intrusive igneous rock. If this were an extrusive igneous rock, that cooled as it exploded out of a volcano and was thrust into the earths atmosphere, the crystals would be way smaller, as they formed very quickly. You would not be able to see them with just your eye, you would have to use a very powerful microscope.


So, if Castle Hill formed deep underground, how can it be that we are standing on it? Well, this is because of erosion. Erosion is a process where rock is removed from where it formed. There are many ways that erosion can happen. Right now as we walk on Castle Hill and kick up rocks with our feet, we are contributing to erosion. All of the rocks in the entire Queensland region have been slowly eroding over the last 66 million years. So basically, every rock above and around here has eroded, uncovering the once buried Castle Hill. Because Castle Hill is made of granite, it has not been eroded as much as everything else. The rock granite is very resistant to erosion, so while everything else eroded away, the granite remained!

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