In my previous blog post I promised to write an article detailing the chemistry of mantle plumes and rift systems. However before I go into the detailed chemistry of the inner Earth it occurred to me that many of you (including some of my colleagues) might not know what plumes are. Plumes remain one of the great geological debates, there is little agreement about their mode of formation and some still argue against their existence.
As such the purpose of this blog is to explore where plumes originate and what makes them unique. I will also explain what groundbreaking scientific discoveries we have been made through the study of plumes.
So what is a plume?
Well at its simplest form a plume is a column of mantle material which is heated deep within the Earth and then rises up through the mantle towards the Earth’s Surface not dissimilar to a giant lava lamp. What makes plumes special is that they act independently of plate tectonics and therefore remain stationary whilst the tectonic plates move over them. So having clarified what plumes are we should consider how they form. The formation of plumes is varied and complex: they can represent old subducted crust which has remelted; plumes can be remnants of melts formed after a violent impact with a meteorite; or they can be derived from hot material deep within the Earth next to the core – mantle boundary. The material that forms plumes are thought to have a higher temperature than the surrounding mantle, this will lower the density of the plume source and cause it to rise towards the surface. When a plume approaches the surface, the pressure that it is under will be decreased and the plume will begin to melt and pool under the crust into a mushroom shaped plume. This melt will start to push up the crust and will eventually led to a spell of volcanism as seen in locations such as Hawaii and Yellowstone.
So, even though the formation of plumes is still under debate what can be said is that the information they carry to the surface of the earth is invaluable to geologists and planetary scientists.
So what have plumes told us?
Well, the majority of plumes are believed to have formed deep within the Earth near the core, this represents some of the oldest and least altered material on Earth. When the Earth first formed it separated into layers due to its density. Heavy iron sank to the core and buoyant silica formed the crust and mantle. The upper layers of the Earth (crust and mantle) have undergone lots of alteration due to tectonic processes whilst the lower part of the mantle and the core has remained similar in composition to the original planet. By studying plumes we have discovered that each layer within the Earth has a different chemical signature and therefore we can conclude that the layers must have been separated within the first 100 million years of the Earths existence and have experienced little mixing since then. The noble gases signature contained within plumes support the view that the material that makes up the Earth is extremely similar to that of the most primitive meteorites (Chondrites). The Earth must have been formed from meteorites which existed in the early solar system which clumped together over time to build up our planet. While it is acknowledged that the majority of the Earth was formed from meteorites, more recent research suggests that deep within the Earth the 20Ne isotope signature is similar to that of the early solar nebula. The solar nebula was only around for the first few tens of millions of years, so some part of the Earth must have formed early in the history of the solar system and was then latter added to by the addition of meteorites.
So plumes aren’t just the domain of geologists they are key to understanding how the Earth first formed and what conditions were like during the early solar system.