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Structure of the Earth
The Earth’s Layers: The Core
The core is in the centre of the Earth. The core has two layers – the inner core and the outer Core.
The inner core is 1 216 km thick, has a temperature of up to 4 000°C and is made of solid iron with some nickel. The outer core is 2 270 km thick, has a temperature of up to 3 600°C and is made of liquid iron with some nickel. Between these two layers is the liquid-solid boundary. The outer core is liquid because there is less pressure pushing on it than on the inner core. The solid inner core rotates within the liquid outer core. This giant ball of rotating solid iron and nickel generates the Earth’s magnetic field. The core is also the Earth's source of internal heat because it contains radioactive materials that release heat as they break down into more stable substances.
The Earth’s Layers: The Mantle
Around the core is the mantle. The mantle also has two main parts – the lower mantle and the upper mantle, which includes the asthenosphere. The mantle is made up of hot, dense, semi-solid rock.
The lower mantle is 2 885 km thick. The volume of this region is about 84% of the Earth’s total volume! The temperature of the mantle is cooler than that of the core, only reaching temperatures up to 3 000°C. The lower mantle is denser than the upper mantle. Between the lower and upper mantle lies a transition zone (400–660 km below the Earth’s surface). Above the transition zone is the upper mantle. This area extends from the Earth’s crust layer down to about 400 km.
Did you know?
The upper most part of the upper mantle is called the asthenosphere. The asthenosphere (from the Greek words “a” + “sthenos” meaning without strength and “sphaira” meaning ball) is made of soft, flowing rock.
The Earth’s Layers: The Crust
Floating on top of the asthenosphere is the solid, rigid part of our planet – the crust. The crust is a cool layer of rigid rock, which has a thickness that varies from 5 to 80 km in depth. Together, the upper part of the asthenosphere and the crust make up the lithosphere (from the Greek word “lithos” meaning rocky) which extends down to a depth of about 100 km.
The thinnest portions of the crust are found on the ocean floor and form the ocean crust, which can be less than 5 km thick. The thickest parts of the Earth’s crust lie under our continents and form continental crust, which can be as much as 80 km thick. The temperature of the crust closest to the mantle is about 500°C whereas the surface temperature of the crust approaches that of the temperature outside.
Human Explorations of the Earth’s Crust
Humans know more about the edge of the Universe than what lies beneath the Earth’s crust. Much of what we know about the world beneath our feet comes from seismic studies. Seismic studies give us indirect evidence of the composition of the layers of the Earth by studying how waves of energy produced by earthquakes travel through the Earth. These energy waves are called seismic waves.
Direct evidence of the composition of the layers of the Earth can only come from digging deep into the Earth itself. In Jules Verne’s 1864 classic fiction novel, Journey to the Centre of the Earth, the scientists discovered dinosaurs and other prehistoric life living in the centre of the Earth. We now know that this is not true. However, the composition of the Earth’s layers is not known for certain because it is very difficult to journey into the depths of the Earth – it’s even more difficult than journeying to the Moon!
This doesn’t mean that people have not tried to explore this mysterious world beneath our feet. During the early 1960s, at the same time as the American-Soviet race to the moon, there was a project to dig down through the Earth’s crust. This project, Project Mohole, was named after the boundary between the crust and mantle, called the Mohorovicic Discontinuity (or Moho for short), named after its discoverer, geologist Andrija Mohorovicic. The project was the deepest offshore drilling that had ever been attempted up to that time. In the first experimental part of the project, five holes were drilled into the ocean floor, in 3 600 m of water, with the deepest hole being 183 m. Although the project didn’t continue onto the next phase (in which deeper holes were planned), scientists have learned much about how to do deep water drilling.
Geologists on land were going much deeper. In 1970 Soviet geologists started drilling into the Kola Peninsula, near Finland, hoping to drill down further into the Earth than anyone had drilled before. Their project, the Kola Superdeep Borehole, involved the digging of a deep vertical tube, or bore, into the ground. Their deepest borehole, which was 12 262 m deep, was completed in 1989. It is considered to be the deepest artificial point on Earth. At that depth, the crust was hotter than they expected (180°C instead of the expected 100°C). The rock was turning gooey, so the project had to be stopped.
More recently, in 2017, an oil and gas project off the Russian island of Sakhalin drilled the world's longest extended-reach well (long, diagonally slanted well). The length of the well with horizontal completion is 15 000 m, which as of 2019 is a world record.
With enough money and dedicated scientists, there is the belief that people may be able to drill into the Earth’s mantle by the early 2020s. It may be by Japanese researchers aboard the Integrated Ocean Drilling Program (IODP) research ship Chikyu, the largest research ship ever built, which set a new record for drilling the deepest ocean research hole (2 466 m) in 2012.
Why is it important to explore the Earth’s crust and beyond through drilling? The main reason is because the assumptions we make from seismic data may not be accurate. It is possible that the indirect evidence from seismic data could be as inaccurate as the assumptions about other planets when earthbound telescopes were our only source of observation. We live on a dynamic and unpredictable planet. A greater understanding of the Earth’s internal activities would allow scientists to predict more accurately geological events such as earthquakes and tsunamis.