How much water is in the Earth?
To answer this question the majority of people will turn to the internet, however when “how much water is in the Earth?” is searched, the figures and information returned can be misleading. The top results are describing only the classic water cycle (oceans, atmosphere, groundwater). In fact there are at least three (based on our estimates) Pacific Oceans worth of water locked up within rocks and minerals below the surface. This highlights how scientific information taken straight from the internet should not be used alone, but with thought as to how it fits with existing knowledge of the processes involved. It is important to recognise that the internet does not always provide the right answer, yet is commonly used by students, teachers and the general public to satisfy curiosity and as a research tool and is frequently assumed to be correct. This blog highlights some geological processes that are useful in understanding why we as geologists would expect there to be water in the Earth, and produces an estimate for water in the Earth.
Geological evidence for water in the Earth
1. The existence of mineral deposits - Without water in the Earth there would be no natural resources to exploit. Water has a high dielectric constant and hence can dissolve more ionic substances than any other natural liquid. This is important with respect to ore formation as water is the primary substance responsible for dissolution and transport of elements and compounds through the Earth’s crust (Robb, 2011). Water sources for ore minerals can be both magmatic and metamorphic fluids, both of which are related to the deep water cycle (i.e. not included in the classic water cycle that focuses on surface water).
2. Volcanoes at subduction zones - A large proportion of the world’s active volcanoes are concentrated around the Pacific “ring of fire”. These volcanoes are active due to the subduction of hydrated oceanic crust at convergent plate boundaries that form the edges of the Pacific Ocean. When the cold hydrated oceanic crust and ocean sediments are subducted beneath the continental crust dehydration reactions take place that release water from hydrated minerals. This water rises into the hotter surrounding rocks lowering the melting point causing melting. The water is dissolved into this newly formed melt and the magma rises through the continental crust, to potentially form a volcano at the Earth’s surface.
3. Volcanic eruption styles -Volcanic eruptions emit large volumes of water vapour that is released from the magma when it decompresses as it reaches the surface. Styles of volcanic eruption are affected by water within the volcanic melts. Highly explosive eruptions are indicative of melts with high water contents.
4. Observations of water content in minerals -Observations have recently been made of water contents from minerals in the upper/lower mantle transition zone. This research suggests there may be 1% water in the mantle transition zone (Pearson et al, 2014). Numerous studies of minerals in the crust show a wide variation of water contents, generally going from more hydrous nearer the surface (sediments to greenschist facies rocks) to anhydrous at deeper depths (granulite facies rocks).
An estimate for water in the Earth
Here we use some fundamental geological principles to calculate how much water is in the Earth. To calculate this we use an estimate of the amount of water within the minerals that make up the oceanic and continental crust. The oceanic crust is divided into three sections basalt, sheeted dykes and gabbro. Each of these is assigned a wt% H2O value. A weighted average is then calculated using estimated thicknesses of each section. An estimated wt% H2O for the oceanic crust is calculated as 0.71wt%. This number can now be used with the mass of the oceanic crust to get the mass of H2O.
Figure 1. Oceanic crust section with water percentages from Rupke et al, 2004
The continental crust is divided into five zones based on metamorphic grade. Sediments, zeolite, greenschist, amphibolite, granulite. Each of these is assigned a wt% H2O value based on an estimated mineral composition. A weighted average is then calculated using estimated thicknesses of each section, calculated using a metamorphic facies diagram and normal geothermal gradient. Table 1 shows the H2O value calculated for the continental crust of 3.5 wt%.
Figure 2. Metamorphic facies diagram and table showing estimated water percentages in the continental crust (diagram from Press & Siever, 1998)
The table below shows the value calculated for water in the crust. Rows have been added for estimates of water in the mantle from various authors. The crust and mantle estimates have then been combined to produce a high and low value for amount of water in the Earth. There is a large range primarily driven by the uncertainty of water in the mantle. Even with these levels of uncertainty we have an order of magnitude to use to communicate to a wide audience. Using our estimates there is between three and nine Pacific Oceans within the Earth, that are not considered by the classic water cycle and the top results found in internet searches.
ReferencesHirschmann, M. M. (2006). Water, melting, and the deep Earth H20 cycle. Annual review of earth and planetary science, 629-653.
Mottl, M. J., Glazer, B. T., Kaiser, R. I., & Meech, K. J. (2007). Water and astrobiology. Chemie der Erde 67, 253 - 282.
Pearson, D. G., Brenker, F. E., McNeill, J., Nasdala, L., Hutchison, M. T., Matveev, S., et al. (2014). Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature, 221-224.
Press, F., & Siever, R. (1998). Understanding Earth. New York: W.H Freeman.
Robb, L. (2011). Introduction to ore-forming processes. Blackwell publishing.
Rupke, L. H., Morgan, J. P., Hort, M., & Connolly, J. A. (2004). Serpentine and the subduction zone water cycle. Earth and Planetary Science Letters 223, 17-34.