The mantle is targeted from the ocean floor as oceanic crust is significantly thinner than continental crust. Oceanic crust is typically 5-6 kilometres thick while continental crust ranges in thickness from about 25-70 km.
The mantle is nearly 3,000 km thick and is a rocky shell that makes up about 84% of the Earth's volume. It is mainly solid and surrounds the iron-rich hot core. Almost all of the sea floor and continents that the Earth’s surface consists of originates from the mantle.
In order to reach the mantle, an exploratory ultra-deepwater well will be drilled using the deep-sea drilling vessel Chikyu. The Chikyu was first launched in 2002 and is able to carry 10 kilometres of drilling pipes; the vessel has set the world record for drilling the deepest hole in scientific ocean drilling history, reaching 2.2 km into the seafloor. The Japanese government has invested in the project already through the construction of Chikyu; hopefully other funding can also be secured.
There are three potential locations for the drilling: the Cocos Plate off Central America; Baja California; and Hawaii. The Hawaii location is very attractive as there is a relatively low bottom-hole temperature of around 150°C.
There are several challenges for the drilling. The crew will be drilling with a riser in ultra-deepwater environments with water depths around 4,000 metres. The drilling and coring itself will be in very high temperature igneous rocks; the bottom hole temperatures are estimated to be as high as 250°C. The total drilled core is expected to be around 6,000 metres below the Pacific Ocean seafloor. The hole drilled will be 30 cm in width throughout the drill, from the ocean floor to the mantle. The drill pipes used are 10 kilometres long and the drill bits have a lifespan of 50-60 hours before needing replacement.
This is not the first attempt to reach the mantle. In the early 1960’s there was a race between the USSR and the USA to be the first to drill to the mantle. The Americans abandoned the effort in 1966, but the Russians, with a drill hole in the Kola Peninsula, took the record for the deepest borehole ever drilled when they reached 12 km into the Earth’s crust in the 1980’s. Read our post on the Kola drill hole here:http://on.fb.me/Qc3o4O.
This drilling will not increase volcanic activity. The drill holes are too narrow to transmit the explosive force of a volcanic eruption. What magma could flow into the shaft would solidify long before reaching the surface.
Drilling toward the mantle could take many years. If the team can start drilling before the end of the decade, the drill may reach the Earth's mantle by the early 2020s.
The feasibility studies and initial reports are available here: http://www.iodp.org/
Image: Kyle Vanhemert