Explaining Highway-Speed Magma Ascension

Geologists think magma travels from where it originates beneath the oceanic floor to where it erupts in only 1000 years. While 70km / 1,000 years is orders of magnitude slower than the speed we drive, or even walk, in terms of geologic speeds this is quite rapid. Many geologic processes, such as the uplift of mountains, takes millions of years, not thousands! This ascension is, fundamentally, driven by pressure; much in the way air leaves a balloon when squeezed, or what drives water up a well.

How magma can segregate and ascend so rapidly has been a question geologists have been working to answer for decades. There must be processes which explain the limited observations that exist of the volcanic features underwater where juvenile oceanic floor is created, and where life could have originated billions of years ago. Not only could these underwater volcanic regimes have impacted the evolution of primitive bacteria billions of years ago, but they play an active role in Earth's climate, oceanic and atmospheric chemistry, and the ability for Earth to continue sustaining life. 

Photograph showing volcanic gas venting near the Mid-Atlantic Ridge, where some of the most primitive life forms are thought to have evolved. Image from New York Times (https://www.nytimes.com/2016/01/12/science/midocean-ridges-volcano-underwater.html)

Explaining magma movement here is not as simple as explaining other surficial or sub-aerial geologic processes, largely because observations are rather limited. The magmatic regions beneath these volcanic settings are deeper than anything humans can directly sample. The observations that exist are largely indirect, and mostly fall under three groups of evidence geologists have grown over the past several decades. 

1.) Rocks that form from the magma that travels upward beneath the sea floor can provide estimates on how quickly the magma that forms these rocks traveled. The chemical composition of these rocks suggest magma moves very quickly, yielding the estimate of ~1000 years from magma origin to eruption along the oceanic floor.

2.) Waves generated by the vibration from earthquakes that can inform scientists about how much magma is beneath the ocean surface. Much in the same way that your voice sounds different underwater as it does otherwise, the waves generated from an earthquake behave differently depending on the material they travel through. This distinction of how waves behave differently through various materials can provide an estimate of how much magma is in a specific area of the Earth. 

3.) The observation of electric currents can also provide estimates of how much magma is beneath the sea floor. Did you know how an electric thermometer works? Slight fluctuations in temperature impact how quickly an electric current can travel, and from this change in speed of the current, the thermometer can yield a precise measure of temperature. Material properties also affect the speed of currents; otherwise, we would not make wires out of gold and copper, if a cheaper metal would work just as well. Using this relationship, scientists can estimate the amount of magma beneath the sea floor.

         However, while the geochemical evidence suggests magma travels extremely quickly, the waves generated by earthquakes suggest the regions beneath the sea floor contain less than two percent magma. How magma ascends this quickly with such little magma present is not clear. Meanwhile, the percent of magma estimated in these regions is different depending on if you look at the vibrational waves from earthquakes, or the data from how quickly an electric current can travel in this region. 

Simple mid ocean ridge diagram, where magma shown in orange, rises from a large region of melt origin at the bottom of the diagram, to a very narrow region at the surface of the sea-floor (shown in blue). Image from https://divediscover.whoi.edu/mid-ocean-ridges/types-of-ridges/

           Geologists have suggested several ways magma might achieve rapid ascension in these regions. One could be that, at shallow depths, the oceanic floor is cracked, and magma travels quickly through these cracks. Another, is that some regions beneath the sea floor are chemically distinct. This could enable some regions to melt more magma initially, which could promote rapid magma movement. There are several other factors that impact rapid magma ascension, but all suggest magma would have to ascend at geological highway-like speeds.

          Better experimental constraints that simulate magma movement and how magma might develop "highways", and more highly resolved mid ocean ridge observations, will help explain these geologic structures which are the interface between the Earth’s surface and the deep Earth. These are interfaces where processes occur that directly influence oceanic sea life, the ocean’s chemistry, and Earth’s climate, and as such, are paramount in understanding Earth as an entire system.