Another in our series looking a bit laterally at terms we’re generally familiar with – this time to do with basements.
Basement rock is probably a term not many people outside the field of geology will be familiar with so for those of you who aren’t geologists or budding geologists let me explain. Basement rock is the foundation of the oldest metamorphic and igneous rock that form the crust of the continents.
More often than not basement rock is granite based, which makes it a noticeable contrast to the rock types that are typically found on top of the basement rocks. These overlying sedimentary rocks are on average around 3 miles thick while the basement rocks that actually form the crust of the continents are a great deal thicker, they can easily be between 20 – 30 miles in density.
As you can no doubt surmise from this, basement rock is very old – ancient in fact. Geologists have estimated a section of basement rock visible at the bottom of the Grand Canyon is between 1.7 – 2 billion years old.
Basement Rocks are Complex
Basement rock isn’t any old kind of simple rock like you might see in your garden or at the bottom of a beach. They can be a complex blend of many kinds of rock. The age of basement rock also varies greatly. As already mentioned basement rock is very old, but some basement rocks are much older than others, with some being relative youngsters at only a few hundred million years old.
Usually basement rock found around the continental crust is much older than basement rock found around the oceanic crust. The rock found around the oceanic crust is also usually much thinner too. This is because the rock around the oceanic crust is more prone to being subducted when the tectonic plates shift under one another.
It’s also been found that basement rock around the oceanic crust is usually made up of more basaltic rocks than those found on the continental crust. While it’s the norm that basement rock is younger the closer you get to the edge of a continent, this isn’t always the case. Exotic terrains, for example, are made up of fragments from other continents that have broken of their original continent and joined another.
This is one of the reasons why basement rock is so highly metamorphosed and complex. It can be made up of volcanic, igneous and metamorphic rocks. And like with the basement rock found on the oceanic crust the blend of rocks that form the basement rocks can depend on the location of the continent.