Understanding Earth’s Layers and Their Dynamics
5. Earthquakes That Provide Information
The speed of propagation of seismic waves in the deep Earth undergoes gradual changes, sometimes abrupt changes; these sudden changes are called discontinuities.
The speed at which seismic waves travel depends on two factors: the composition of materials and the physical state of these materials.
Discontinuities are used to divide the deep Earth into layers.
Main Breaks and Their Interpretation
- Moho, or Mohorovičić Discontinuity: It was the first significant discontinuity described. Its depth varies between 70 km in continents and between 5 and 10 km in oceans. This discontinuity is used to differentiate the thin surface layer called the crust from the layer beneath it, the mantle.
- Gutenberg Discontinuity: Discovered by Beno Gutenberg, it lies at a depth of 2900 km. This discontinuity separates the mantle from the Earth’s core. Since S-waves propagate through all solids but not fluids, it is concluded that at 2900 km depth, there is a continuous layer of molten material.
Discontinuities like the Gutenberg Discontinuity allow for the three layers traditionally distinguished in the deep Earth: crust, mantle, and core.
Other Breaks
Inge Lehmann discovered that the core is not entirely liquid. At a depth of 5150 km, there is a sharp increase in P-wave velocity; this jump is interpreted as the result of a physical change of materials in the core, moving from liquid to solid state. This is known as the Lehmann Discontinuity and differentiates the outer core from the inner core, which is in solid state.
Between 100 and 800 km depth, the velocity increases, with fluctuations of decreases and increases. The greatest of these is produced at 670 km and is used to differentiate the upper mantle.
6. Other Indirect Evidence
Internal Temperature of Earth
The value of the geothermal gradient decreases with depth. It is now considered that the increase in temperature in the mantle is around 0.5 ºC/km. At the base of the crust, the temperature should be near 700 degrees C. At the boundary between the upper and lower mantle, it will have risen to about 2000 degrees C.
The core temperature should be sufficient for the constituent materials, mainly iron and nickel, to be found in the outer core in a molten state and solidified in the inner core. In the outermost part of the core, the temperature must be above 3800 degrees and probably does not exceed 5000 degrees in any area of the deep Earth.
Terrestrial Magnetism
The Earth has a magnetic field, and its existence supports the idea that our planet has a metallic core in constant agitation. The Earth behaves like a self-induced dynamo. Under this theory, the circulating molten iron in the outer core, due to the Earth’s rotation and convection currents generated by internal heat, causes an electric current that in turn produces a magnetic field. In this way, it acts as a self-sustaining dynamo.
Meteorites
Meteorites are small planetary bodies that cross Earth’s orbit and fall on its surface. Most come from the asteroid belt between Mars and Jupiter and have an age of 4500 million years, the same as Earth, arising from the material from which the solar system was formed.
There are three types of meteorites:
- Chondrites: Made of a mixture of minerals found in peridotites, representing 86% of the total.
- Achondrites: Have a composition similar to basalt and represent 9% of the total.
- Siderites: Consist of iron, representing 4% of known meteorites.
7. Layered Earth
The Earth is a layered planet. These layers can be distinguished based on two criteria:
- Geothermal Units: Established based on the chemical composition of the materials. In this case, the different areas are crust, mantle, and core.
- Dynamic Units: Established based on the mechanical behavior of each terrestrial layer. Thus, we differentiate between the lithosphere, asthenosphere, upper mantle, lower mantle, outer core, and inner core.
Geochemical Units
- Crust: The thin outer layer of the Earth, extending from the surface to the Moho. The most abundant chemical elements are O, Si, Al, Fe, and Ca.
- Continental Crust: Between 25 and 70 km thick, it is very heterogeneous and consists of low-density rocks composed essentially of quartz, feldspar, and mica. In the lower half, it is dominated by metamorphic rocks such as gneiss and schist, with large granite massifs and abundant surface sediments and sedimentary rocks.
- Oceanic Crust: Much thinner, with a thickness between 5 and 10 km. It is stratified into three levels: a surface sediment layer, a layer of basalt beneath it, and finally a layer of gabbro, composed of feldspar and pyroxene. The oceanic crustal rocks are younger than those of the continental crust.
- Mantle: The zone between the Moho and the Gutenberg Discontinuity, extending from the base of the crust to a depth of 2900 km. The most abundant elements in the mantle are O, Si, Mg, and Fe, and it is composed of peridotite, a rock similar to that which is most abundant in meteorites, the chondrites, composed of olivine and pyroxene.
- Core: The central area of the planet, located below the Gutenberg Discontinuity. Its high density, behavior when seismic waves pass through, and its role in the creation of the magnetic field support the hypothesis of a core composed mostly of iron with 6% nickel, similar to the composition of siderite.
8. Dynamic Drive
Dynamic behavior is determined according to the physical characteristics of materials, such as their mechanical behavior or physical condition.
- Lithosphere: The rigid outermost layer, including all the crust and upper mantle. The oceanic lithosphere is 50 to 100 km thick, while the continental lithosphere is 100 to 200 km thick.
- Upper Mantle: The layer situated immediately beneath the lithosphere, reaching a depth of 670 km. This portion of the mantle is composed of peridotite and is in solid state. High pressures and temperatures cause these materials to respond differently over time; in the short term, their behavior is rigid, while over very long periods, their behavior is plastic and deformable, similar to a very high viscosity fluid, allowing for convection currents. Traditionally, the upper mantle beneath the lithosphere has been called the asthenosphere because it limits mantle convection currents.
- Lower Mantle: Includes the rest of the mantle situated between 670 and 2900 km depth. Lower mantle rocks are also subject to convection currents. At the boundary with the core, there is a layer known as the double prime layer, a discontinuous and irregular layer with a thickness from 0 to 300 km, composed of the denser materials that have fallen to the bottom of the mantle.
- Outer Core: Below the mantle, reaching a depth of 5150 km, it is in liquid state, moved by convection currents, and plays a key role in the creation of the Earth’s magnetic field.
- Inner Core: As the core evacuates its heat through the mantle, iron crystallizes and collects at the bottom. The solid iron is what constitutes the inner core.