It’s very easy in geosciences to get caught up with the details of whatever you’re investigating and forget the big picture. How does this strontium isotope ratio relate to the movement of the continents? After all, the history of the planet is what we’re trying to unravel, so I want to talk about the interconnectedness of scales in geology.
The movement of continents over the last 4.54 billion years of Earth history is what I’m studying. But obviously the data to reconstruct this has to come from somewhere. You can’t just work backwards, as the continents have been arranged, made, and destroyed countless times before they got to how they are today.
This is a sketch map of relative geographic proximities before the Atlantic opened. I study the Caledonian rocks of Shetland, because they were pretty close to the Norwegian, East Greenland, and Scottish sectors of the Caledonian Orogeny. So it’s a pretty important little archipelago, up there in the North Sea. So a palaeotectonic map. How did that come into being?
This is where scale becomes important.
We collected some rocks from Shetland with a sledge hammer. Typical outcrops look like this:
Macro scale is really important in geology, as a lot can be told about how the rocks have behaved under temperatures and pressures. This outcrop has undergone a considerable amount of shearing, and a direction of movement can be extrapolated from this. Cool, huh?
Every geologist loves a good thin section. They’re pretty, have cool colours and the micro-textures you can see in thin-section are just as important as those seen at macro scale.
This is the thin-section of the rock collected from the above outcrop. It beautifully illustrates how the textures can be seen at different scales. I even photographed it in the same orientation to highlight this!
How does this relate to the nano scale? Well, notice how the brightly coloured mica grains are touching the grey coloured feldspar grains? That suggests that at the time this rock was cooling, the isotopic systems will have been in equilibrium, which is really important when thinking about isotopes and geochronology.
This is where I start to get overly excited. Not unlike the ions I measure. bad geochemistry joke, I’m sorry (I’m not sorry).
0.732917 ± 0.000010
The question is, how do you get down to the nano scale? Well, we measure isotopic ratios. Which is what the above number is, actually measuring ions that have been deflected through a magnetic field! How cool is that?
But whats the point?
Measuring isotopes means you can calculate the age of rock and how it fits together with other rocks in the grand old scheme of things. Bringing us back to the mega scale of how the continents fitted together hundreds of millions of years ago.
This is the reason geology is cool. You have to draw ideas from all over to see the bigger picture. But being able to piece together how the Earth looked millions of years ago, will never cease to amaze me.