The atmosphere and the Earth's surface are directly accessible to observation. But the interior of the planet is much less !
The investigations in the internal structures of the Earth are therefore by indirect measures, especially through seismology.
By studying the velocity and the paths taken by the waves during earthquakes , seismologists have concluded that the earth is made up of concentric layers : heart , mantle and crust.
Cut into the Earth and the atmosphere 1.Crust - 2 . Moho - 3. Upper mantle - 4.Zone low speed - 5. Lower mantle - 6. D layer 7. external Core - 8. solid/ liquid border - 9. Internal core
A.Troposphère - B.Stratosphère - C.Mésosphère - D.Thermosphère - E. Exosphère
Doc. Earth -crust - cutaway - japanese.svg : Washiucho / Wilimedia
In 1909, Andrija Mohoroviči , a Croatian seismologist (photo) , has identified a brutal augmentation of speed of the compression wave , which marks the boundary between the crust and mantle . This seismic discontinuity was called discontinuity of Mohoroviči , simply Moho .
The Moho is present under the continents and oceans, at depths of 35 km in the first 6 km in the latter.
Scientists suspect that the cause of this change in wave velocity reflects a transition from basalt above the Moho towards the peridotite , olivine -rich under the Moho .
Drilling to verify this hypothesis have been made at sea, where the Moho lies at a more accessible depth. At mid-ocean ridges, the seafloor is shallower, but the newly formed near the dorsal crust is hot, a major drawback for drilling equipment ... therefore remains to find a place where the Moho is cool enough to drill, but not too deep .
In 1961, an attempt is made off Mexico, with a low penetration of 183 m. In the following years , four holes will go up to more than 1000 meters in the oceanic crust, and up to 2,100 meters off Ecuador coast.
The Japanese scientific drilling vessel Chikyu , contribution to the IODP - Integrated Ocean Drilling Program, will be used to drill into the mantle through the Moho after testing above this depth in September 2012.
Deep sea drilling vessel CHIKYU - Photo jamstec.go.jp
Three former drilling sites are thus in competition: Mexico (drilled in 1961), a site off of Nicaragua already used , and North Arch in Hawaiian Islands, north of Maui ( project abandoned in 1960 ) ... provided that funds allocated following , which is problematic given the circumstances !
The volcanic Hawaiian archipelago and volcanic fields North Arch and South Arch - Map 2201 MBARI
The site of North Arch / Hawaii is interesting for several reasons:
The enormous weight of the Hawaiian volcanoes on oceanic crust strenght the lithosphere to sink. The islands are surrounded by a moat deep of 5500 meters, the Hawaiian Through, and an enhancement to distance ... it is meant as a sheet of plywood resting on two trestles reacts when you go up between them : the edges the plywood are raised higher than the trestles.
Hawaii - subsidence of the underlying crust and enhancement of North and South Arch - Doc. Moore 1987 / Oregonstate Univ.
Large basaltic bodies , found on the North and South Arch, are intriguing as by their position away from the Hawaiian hot spot, and by their huge volumes of very fluid lava. The flows of North Arch are extending 100 km on either side of the axis of the arc. The lavas are full of holes, due to CO2 degassing and geochemically similar to " lava stages of rebirth " of Kaui and Oahu, generated by partial melting of magma accumulated at the base of the lithosphere, or close to it in a structure created by a lithospheric bulge.
- Hawaii 24/7 - Volcano watch : after 50 years, a renewed effort to drill to the Moho
- JAMSTEC - Center for deep earth exploration - Deep sea drilling vessel Chikyu.
- MBARI - Monterey Bay Aquarium research inst. - Hawaiian flexural arch