What is on the inside?

A lemon tree
A lemon tree

When you see a beautiful lemon tree, you might start thinking about holiday. You’ll want to take a lemon from it, slice it up and add it to a nice meal or drink. But have you ever taken a good look at the inside of a lemon? Before you slice it, you do not know where the seeds are, or how thick the skin is. Imagine that you could look inside without slicing! What would it look like?

This might seem a strange question. Why would anybody like to know that? You can slice it up whenever you want! However, in some cases it can be important to look inside something without slicing. Think about a hospital scan to examine your bones. How convenient that they can look at them without an operation! In geology, we have a similar view. Sometimes, we want or need to look at the inside of a rock without disturbing its inner texture by cutting it. So how can we do this?

The solution is similar to what is done in the hospital. We use a CT-scanner like the one below. X-rays are shot through your body, or the rock, and the material inside it will stop a certain percentage of the X-rays. When the output of X-rays is measured on the other side, it gives you information on what has slowed them down or even stopped them on the inside. If we do this while rotating the rock specimen (or by rotating the source of the X-rays around your body) we can get a three-dimensional idea of the internal structure. This allows us to us to show slices through the rock in different orientations and zoom in to see details, just like if you sliced a lemon in different directions.

The µ-CT scanner at University of Bremen. The specimen chamber is opened, and the X-ray detector is visible in the middle of it.
The µ-CT scanner at University of Bremen. The specimen chamber is opened, and the X-ray detector is visible in the middle of it.

These slices will show different shades of gray, depending on how much X-rays are transmitted. To continue the lemon example, below you can see that the skin of the lemon is quite thick; the light color indicates that X-rays have some trouble going through it. The same occurs for the seeds that you will see inside the lemon. On top of that, the different segments of the lemon can be observed easily, separated by thin lines of material. We can even zoom in further, showing a network of small segments that cannot be seen with the naked eye when you slice the piece of fruit. If we want to see a cross-section from a specific angle, there is software to allow us to make slices. Also, it is possible to make segments of portions that block the X-rays to the same amount. This shows the distribution of similar material within the lemon. That last feature is very helpful when looking at rock samples. By segmenting different gray values, we can separate different mineral types from each other and make an internal ‘map’ showing what mineral grows where.

Cross sections through a lemon made using a µ-CT scanner. (Images: Wolf-Achim Kahl)
Cross sections through a lemon made using a µ-CT scanner. (Images: Wolf-Achim Kahl)

Another important thing to look at in rocks is open space or pore space. When we want to know how fluids can migrate through a rather solid piece of stone, the CT-scanner can show us where the voids are. These are the darkest patches, where the X-rays are not stopped but can move through the material easily. As you can see in the next image, such a space often occurs around grains. Fluids can thus be stored inside a rock at the part where different grains meet, because they do not fit perfectly together as squares. If we now follow these grain boundaries through the material, we see elongated pockets where fluid can occur inside the rock. When separate pores are connected, we speak of permeability. The rock has become permeable when a fluid can go through from one end to the other, however long it may take. Using the same software mentioned before, we can give all the open spaces that are connected the same color. This is very helpful to see through how much space a fluid can move, and whether it is possible to go all the way through.

Figure 3: A cut across this rock cylinder shows how the light colored minerals are distributed through the sample, and that porosity is present around these grains. (Image: Wolf-Achim Kahl)
Figure 3: A cut across this rock cylinder shows how the light colored minerals are distributed through the sample, and that porosity is present around these grains. (Image: Wolf-Achim Kahl)

Sometimes we see that needle like crystals have grown inside a pore. Because these needles do not at all fit together with the rest of the rock and do not touch the rock, they are said to form later in the pores, and might have clogged the system. So we can examine how the sample is altered over time, where pores are now and where they used to be, and what type of minerals grew inside. These things are very useful tools to develop an idea of what rock we see and how it has formed.

All of this has shown how useful it can be to see what the internal texture of rocks look like. So next time you find a rock, or cut a lemon or find something else you cannot look into, wonder about what is inside it and think about the possibilities we have to analyze the inside without destroying the object we are looking at.

Written by Karin Los.

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