Preservation of igneous textures - 2 billion year old pillow basalts

This is my entry into the first Geopolis photo contest.

The picture is from my visit to the Ungava Peninsula in northern Quebec. On the picture you can see round blebs of rock covering the outcrop. On the picture below I have highlighted these features for you.

These features are known as pillow basalts.

Pillow basalts form when hot lava erupts into water. The water leads to extremely fast cooling of the exterior of the lava, solidifying its surface and forming beautiful pillow shapes. These particular pillow basalts where photographed on the Purtuniq ophiolite, a piece of oceanic crust that has been obducted onto the continent. The age of the Purtuniq ophiolite has been dated to be 2 billion years old^[1]. That means these rocks are representing the ocean floor two billion years ago. So after this humongous amount of time, the igneous textures formed during the creation of these rocks can still be observed. This helps geologists to understand the paleo-environment that was present during the formation of these rocks. An incredibly helpful tool. Additionally you can investigate the shape of these pillows. Do you see how some of them form little ‘tails’? Those are features that form when a new layer of pillows is forming above an older one. The lava fills in the gap between pillows, forming the tail. On the top those flows the beautiful pillow shape themselves are forming. This means we can use these pillows as an indicator of our stratigraphic ‘up’ direction. Geologists can use this to orientate geological formations in respect to each other even if they have undergone deformation.

All in all it is spectacular that these rocks have retained their shape over all this years, but it is the geological tool they provide that makes them even more amazing.

_{Location of the Purtuniq ophiolite.
Modified after commons.wikimedia.org}

If you are not convinced how much detail they kept check out his closer view:

This is a view on a single pillow. These features are known as drainage cavities. While the lava solidifies immediately where it touches the water, the inside of the pillows stays molten and lava continues to flow. When new magma supply ceases the cavity stays empty and is filled later. In this case with secondary calcite. The interesting thing in this case is that magma had to have been delivered in pulses, leaving multiple generations of drainage cavities. Simply spectacular.

There are even older preserved pillow lavas that reach back all the way into the Archean. But I have not yet been able to visit them myself :)

References

• [1] Parrish, R.R. (1989). U-Pb geochronology of the Cape Smith Belt and Sugluk block, northern Quebec. Geoscience Canada, v. 16 (3), p. 126-130.
• https://commons.wikimedia.org/wiki/File:Qu%C3%A9bec,_Canada.svg