Earth’s outer core is a fluid layer about 2,260 km (1,400 mi) thick, composed of mostly iron and nickel that lies above Earth’s solid inner core and below its mantle. The outer core begins approximately 2,889 km (1,795 mi) beneath Earth’s surface at the core-mantle boundary and ends 5,150 km (3,200 mi) beneath Earth’s surface at the inner core boundary.
Contents
Properties
The outer core of Earth is liquid, unlike its inner core, which is solid. Evidence for a fluid outer core includes seismology which shows that seismic shear-waves are not transmitted through the outer core. Although having a composition similar to Earth’s solid inner core, the outer core remains liquid as there is not enough pressure to keep it in a solid state. Seismic inversions of body waves and normal modes constrain the radius of the outer core to be 3483 km with an uncertainty of 5 km, while that of the inner core is 1220±10 km.
Estimates for the temperature of the outer core are about 3,000–4,500 K (2,700–4,200 °C; 4,900–7,600 °F) in its outer region and 4,000–8,000 K (3,700–7,700 °C; 6,700–14,000 °F) near the inner core.[8] Modeling has shown that the outer core, because of its high temperature, is a low-viscosity fluid that convicts turbulently. The dynamo theory sees eddy currents in the nickel-iron fluid of the outer core as the principal source of Earth’s magnetic field. The average magnetic field strength in Earth’s outer core is estimated to be 2.5 milli Tesla, 50 times stronger than the magnetic field at the surface.
As Earth’s core cools, the liquid at the inner core boundary freezes, causing the solid inner core to grow at the expense of the outer core, at an estimated rate of 1 mm per year. This is approximately 80,000 tonnes of iron per second.
Light elements of Earth’s outer core
Composition
Earth’s outer core cannot be entirely constituted of iron or iron-nickel alloy because their densities are higher than geophysical measurements of the density of Earth’s outer core. In fact, Earth’s outer core is approximately 5 to 10 percent lower density than iron at Earth’s core temperatures and pressures. Hence it has been proposed that light elements with low atomic numbers compose part of Earth’s outer core, as the only feasible way to lower its density.
Although Earth’s outer core is inaccessible to direct sampling, the composition of light elements can be meaningfully constrained by high-pressure experiments, calculations based on seismic measurements, models of Earth’s accretion, and carbonaceous chondrite meteorite comparisons with bulk silicate Earth (BSE). Recent estimates are that Earth’s outer core is composed of iron along with 0 to 0.26 percent hydrogen, 0.2 percent carbon, 0.8 to 5.3 percent oxygen, 0 to 4.0 percent silicon, 1.7 percent sulfur, and 5 percent nickel by weight, and the temperature of the core-mantle boundary and the inner core boundary ranges from 4,137 to 4,300 K and from 5,400 to 6,300 K respectively.
Outer Core
The outer core, which is liquid, is about 1300 miles (2092 km) thick. Both the inner and outer cores consist primarily of iron and nickel and are extremely hot with temperatures ranging from 7200–9000℉ (4000–5000℃).
Accretion
An artist’s illustration of what Earth might have looked like early in its formation. In this image, the Earth looks molten, with red gaps of lava separating with jagged and seemingly-cooled plates of material. An artist’s illustration of what Earth might have looked like early in its formation.
The variety of light elements present in Earth’s outer core is constrained in part by Earth’s accretion. Namely, the light elements contained must have been abundant during Earth’s formation, must be able to partition into liquid iron at low pressures, and must not volatilize and escape during Earth’s accretionary process.
CI chondrites
CI chondritic meteorites are believed to contain the same planet-forming elements in the same proportions as in the early Solar System, so differences between CI meteorites and BSE can provide insights into the light element composition of Earth’s outer core. For instance, the depletion of silicon in BSE compared to CI meteorites may indicate that silicon was absorbed into Earth’s core; however, a wide range of silicon concentrations in Earth’s outer and inner core is still possible.
Implications for Earth’s magnetic field
A diagram of Earth’s geodynamo and magnetic field, which could have been driven in Earth’s early history by the crystallization of magnesium oxide, silicon dioxide, and iron oxide. Convection of Earth’s outer core is displayed alongside magnetic field lines.
A diagram of Earth’s geodynamo and magnetic field, which could have been driven in Earth’s early history by the crystallization of magnesium oxide, silicon dioxide, and iron oxide.
Earth’s magnetic field is driven by thermal convection and also by chemical convection, the exclusion of light elements from the inner core, which float upward within the fluid outer core while denser elements sink. This chemical convection releases gravitational energy that is then available to power the geodynamo that produces Earth’s magnetic field. Carnot efficiencies with large uncertainties suggest that compositional and thermal convection contribute about 80 percent and 20 percent respectively to the power of Earth’s geodynamo.
Traditionally it was thought that prior to the formation of Earth’s inner core, Earth’s geodynamo was mainly driven by thermal convection.However, recent claims that the thermal conductivity of iron at core temperatures and pressures is much higher than previously thought imply that core cooling was largely by conduction not convection, limiting the ability of thermal convection to drive the geodynamo.This conundrum is known as the new “core paradox.
An alternative process that could have sustained Earth’s geodynamo requires Earth’s core to have initially been hot enough to dissolve oxygen, magnesium, silicon, and other light elements. As the Earth’s core began to cool, it would become supersaturated in these light elements that would then precipitate into the lower mantle forming oxides leading to a different variant of chemical convection.
From Core to Crust: Defining Earth’s Layers
What happens on Earth’s surface is directly related to its interior. About 4.6 billion years ago, Earth formed from a hot cloud of dust orbiting a blazing sun. As the planet cooled, dense elements became concentrated in the core of the planet, while lighter elements formed the mantle. A thin, rigid crust formed at the surface. A constant heating and cooling cycle in the mantle drives plate movement on Earth’s surface. Heat working its way out from the core of the planet fractured the crust into irregular tectonic plates that are constantly in motion.
Inner Core: The innermost part of Earth is the core and is about 1500 miles (2414 km) thick. Both the inner and outer cores consist primarily of iron and nickel. They’re extremely hot, with temperatures ranging from 7200–9000℉ (4000–5000℃). The inner core is under intense pressure, which keeps it solid despite high temperatures.
Outer Core: The outer core, which is liquid, is about 1300 miles (2092 km) thick. Both the inner and outer cores consist primarily of iron and nickel and are extremely hot with temperatures ranging from 7200–9000℉ (4000–5000℃).
Mantle: Most of Earth’s volume is in the mantle. This layer is about 1800 miles (2880 km) thick. It’s composed of dark, dense rock, similar to oceanic basalt. The deeper you go inside the Earth, the hotter it gets. Mantle material near the cold outer crust is about 1300℉ (700℃) while rock near the Earth’s core heats up to about 7200℉ (4000℃).
Crust: Two types of crust make up Earth’s outermost layer: continental and oceanic. Continental crust is composed of silica-rich rocks and is an average of 44 miles (70 km) thick. Ocean crust is made of dark, silica-poor rocks like basalt. It is thinner and more flexible than the continents, only about 3 miles (5 km) thick.
Frequently Asked Questions – FAQs
Q1 Name the different layers of the Earth.
The inner core, the outer core, the mantle and the crust are the four layers of the earth.
Q2 Which is the centre and the hottest layer of the Earth?
The Inner Core is the centre and the hottest layer of the Earth.
Q3 The Outer Core is composed of which Metals?
The Outer Core comprises metals such as iron and nickel.
Q4 What is the temperature of the Outer Core of the Earth?
The temperature of the Outer Core of the Earth is around 4000 oF to 9000 oF.
Q5 Which is the widest section of the Earth, and what is its thickness?
Mantle is the widest section of the Earth. Its thickness is approximately 2,900 km.