The Layers of the Earth and Its Composition [EARTH SCIENCE]

• Earth is composed of different layers – crust, mantle, and core. Each layer has distinct characteristics from each other in terms of its state, thickness, and composition.
• Scientists determined that our planet is composed of many different layers through the use of seismic waves. Each seismic wave travels at different velocities. The state of the medium (where the waves travel) and its density affect the velocity of the waves.
• Crust

  This is the outermost solid layer of Earth with a relative thickness of 5 km up to 70 km in depth. It is made up generally of granitic and basaltic rocks, aluminum, magnesium, and silicon. The temperature in this layer varies from 200 degrees Celsius up to 400 degrees Celsius.
• Crust

  There are two types of crust: continental crust and oceanic crust. The continental crust, where the continents are found, is much thicker than the oceanic crust. It is composed of granitic rocks (silicon and aluminum) which make it less dense than oceanic crust. On the other hand, the oceanic crust can be found at the bottom of the oceans or beneath the continental crust. It is made up of basaltic rocks (denser) which make it denser than the continental crust even if the latter is relatively thicker.
• Crust

  • Mohorovicic discontinuity: the boundary between the crust and the mantle
  • Gutenberg discontinuity: the boundary between the mantle and the core
• Crust

  The lithosphere is the rigid outer part of the earth, consisting of the crust

  and the uppermost solid portion of the mantle.
• Mantle

  This is the middle portion of Earth which makes up 84% volume of the planet. It extends up to approximately 2,900 km deep beneath the crust making it the thickest layer of Earth. It is made up of silicate rocks, magnesium, and iron. The temperature increases with depth from 1000 °C near its boundary with the crust, to 3700 °C near its boundary with the core.
• Mantle

  The mantle behaves like a fluid, that, unlike the completely solid crust, this layer is partially melted. The upper portion of the mantle is solid which is part of what we call the lithosphere and the lower mantle is completely viscous. The fluid-like behavior of the mantle is responsible for the movement and shifting of tectonic plates.
• Mantle

  At the base is where the lower mantle meets the outer core. This is called the core-mantle boundary (CMB) or the Gutenberg discontinuity.
• Core

  The core is the densest, hottest innermost layer that is located 2,900 km below Earth’s surface and is mainly composed of nickel, iron, and sulfur. It has three distinct sub-layers: the liquid outer core and the solid inner core and the newly discovered inner-inner core. Temperatures in the core may range from 4,500 degrees Celsius up to 6,000 degrees Celsius.
• Core

  The liquid outer core, which is approximately 1,180 km in diameter, spins faster than the rotation of Earth and undergoes turbulent convection which generates electric currents and sustains the planet’s magnetic field. The Bullen discontinuity separates the outer core from the inner core. However, the inner core, about 1,220 km in radius, is a completely solid metal ball which experiences immense amounts of temperature and pressure.
• Core

  Recent discoveries showed that the inner core is growing slowly and that some parts of the liquid outer core cooled down and became part of the inner core. The inner-inner core is an oddly crystallized structure at the center of Earth, with iron crystals oriented east-west instead of north-south (as with the inner core). Growth is increased as subduction occurs, cooling a surrounding area, therefore more chances for crystallization or solidification at an approximate rate of 1 mm per year.
• State

  Crust: solid

  Mantle: semi-solid

  Core: solid and liquid
• Thickness/Location

  Crust: 5 km - 70 km in depth

  Mantle: 2,900 km deep

  Core: 2,900 km below Earth’s surface
• Composition

  Crust: granitic and basaltic rocks, aluminum, magnesium, and silicon

  Mantle: silicate rocks, magnesium, and iron

  Core: nickel, iron, and sulfur
• Temperature Range

  Crust: 200°C - 400°C

  Mantle: 1000°C - 3700°C

  Core: 4,500°C - 6,000°C
• The Lithosphere consists of 9 large plates and 12 smaller ones. The continents are embedded in continental plates; the oceanic plates make up much of the sea floor. The study of tectonic plates - called plate tectonics - helps to explain continental drift, the spreading of the sea floor, volcanic eruptions and how mountains are formed. The force that causes the movement of the tectonic plates may be the slow churning of the mantle beneath them. Mantle rock is constantly moved upwards to the surface by the high temperatures below and then sinks by cooling. This cycle takes millions of years.
• DIASTROPHISM – the process which involves movements of the earth’s crust such that a portion is pushed up, push down or forced sideways
• Diastrophism

  Folding – the process when the sideward forces acting on rocks deform the rocks into wavelike folds after tilting, bending of wrinkling.

  Faulting sliding or moving over of rock layers over one another along the break or fracture, may occur vertically or horizontally.
• The Earth can be subdivided into layers based on two criteria: (1) composition (density) differences and (2) physical properties.
• Based on compositional differences

  • crust
  • mantle
  • core
• Based on physical properties

  • lithosphere
  • asthenosphere
  • mesosphere
  • outer core
  • inner core
• crust

  This is the thinnest and outermost layer of the Earth. There are two types of crust– the continental crust and the oceanic crust
• The continental crust is the older and more buoyant type of crust. It has an average thickness of 35 km, but can be more than 70 km thick in mountainous regions. It has an average composition consisting of granite with a density of 2.7 g/cm^3.

  The oceanic crust is the younger and denser type of crust. It has an average thickness of 7 km, much thinner than the continental crust. It is composed of basalt, a dark igneous rock with a density of 3.0 g/cm^3
• The mantle comprises the majority of the Earth’s volume (more than 80%) and begins where the crust ends, down to a depth of 2,900 km.
• The boundary between the crust and mantle is called the Mohorovicic discontinuity, which is marked by a change in chemical composition.

  It can be divided into two parts: the upper mantle and the lower mantle, separated by the Repetti discontinuity.
• The core begins at the mantle-core boundary, the Gutenberg discontinuity, located at the 2,900 km depth.

  Although no one has ever been to or sampled the core, scientific investigations led to the conclusion that its composition is made up of a Fe-Ni (iron and nickel) alloy.

  Due to its composition and the pressure conditions at depth, it is calculated to have a whopping density of around 11 g/cm^3.
• The lithosphere (from the Greek word lithos meaning “stone”) is a thick and brittle layer that comprises the entire crust and uppermost layer of the upper mantle. It has an average thickness of 100 km but can reach up to 300 km in the thickest portions of continents.
• The asthenosphere (from the Greek word asthenēs meaning “weak”) is a mechanically weak layer consisting of the lower portion of the upper mantle, extending down to 660 km.

  Contrary to popular belief, it is not a “sea of molten rock”. The upper mantle is actually composed of an Mg- and Fe-rich rock called peridotite.
• At this depth, the temperature and pressure conditions are high enough that rocks become ductile and deform easily. Because of this, the asthenosphere flows more like very, very viscous fluid (but remember: it is not liquid!) and moves independently from the overlying lithosphere. This is a very important mechanism for plate tectonics (which will be discussed later in more detail).
• Beneath the asthenosphere is the mesosphere (from the Greek word mesos meaning “middle”), made up of the lower mantle, and reaches down to the 2,900 km depth.

  The dominant rock type in this layer is a silicate rock called perovskite.
• Unlike all the other mechanical layers, the outer core is the only one made out of liquid— melted Fe-Ni alloy, to be exact.

  The liquid nature of this layer can be attributed to extremely high temperatures (more than 3000°C!) that melt Fe, Ni, and all other elements. The flow of the liquid metals is responsible for the Earth’s magnetic field.

  The outer core terminates at a depth of 5,150 km, where the solid inner core begins. The outer-inner core boundary is also known as the Lehmann discontinuity.
• Despite the extreme temperature, the overwhelming pressure in this layer forces the inner core to be a solid ball of mostly Fe. Temperatures in the inner core are similar to the temperatures of the surface of the Sun— around more than 5400°C.