Earth’s Interior: Structure, Dynamics, and Geological Processes

Posted on Dec 13, 2024 in Geology

Methods of Studying the Earth’s Interior

Direct observation of the Earth’s materials provides very limited information. The rocks extracted are comparable to those that emerge on the surface of the Earth.

  • Study of mines: 2,000 m deep.
  • Polls: 7,000 m deep.

Indirect observations provide more data on the Earth’s interior.

  • Erosion of mountains brings out rocks that originated in depth.
  • Lavas emitted by volcanoes are made of materials originating at depths of up to 100 km.
  • Meteorites fallen on Earth are considered parts of the interior of other planets.

Other features of the Earth that also serve to investigate its structure:

  • Density of the Earth, which increases from the surface from 2.8 g/cm3 to the interior where it reaches 14 g/cm3.
  • The pressure, which varies from a few atmospheres on the surface to more than 3,000,000 in the center of the Earth (theoretical data deduced by indirect methods).
  • Calculation of the temperature inside the Earth. On the surface, the geothermal degree is one degree for every 33 m depth, although this increase cannot continue into the interior, as it would reach temperatures that would make the land unstable.
  • Gravity varies in different parts of the globe. It demonstrates that in the mountains the value of gravity is lower than in the plains, as the width of the cortex (the less dense layer) is higher.
  • The study of magnetism in the core indicates that there are two different layers: a solid inner core and a fluid outer core.
  • The study by seismic waves provides the most comprehensive data on the structure and composition of the Earth.

Study of the Earth’s Interior Using Seismic Waves

Earthquakes are sudden movements due to breakage and displacement of rocks in the deep crust or upper mantle as a result of lithospheric plate motion. They usually occur at a depth of 50 km.

Seismographs are in charge of recording equipment seismic waves.

Source Plate Movement

The movement of the plates is probably due to the release of heat from the Earth. The hot spots inside the Earth heat away to the outside, and in turn, the cold zones of the cortex are introduced into the mantle, perhaps similar to the convection that occurs in fluids when heated. Thus, movement occurs in the plastic mantle rock, which seems to be causing the displacement of the plates. Some scientists believe that there is only one system of circulation in the mantle, while others believe it to be so. Earth would cool very rapidly; therefore, they think that there should be a double circuit of convection for the Earth to lose its heat more slowly. Other theories claim that the subduction of the edge of the plate drags behind it the entire board, due to its weight.

Origin and Evolution of the Seafloor

Most of the seafloor, regardless of the continental shelf, has arisen as a result of the fracturing of the lithospheric plates and their separation. The ocean floor is created in the areas of the ocean ridges and destroyed at subduction zones. Around the continents are the oldest rocks of the sea; this age of the rocks decreases as we approach the ridge, where we find only volcanic rocks.

Building Edges

In the ridges (plate boundaries, where separation occurs), the hole left by the plates is continuously filled by volcanic material from the mantle. This is the material that forms the oceanic crust and is the one who causes the seafloor spreading and continental drift. The ridges are seamount chains with a very rough profile and a length of thousands of kilometers. At the center of many ridges, there is a depression that runs, known as a graben or rift. The rift valley is presented as a series of stepped sunken areas, the result of a fault system. In the center of this valley, volcanic activity is constant. On these rifts, there is no sediment, which shows that there has been no time to collect on it. Along with the lavas are abundant volcanic vents that emit fluids at high temperatures. All these observations indicate that the ridges are the growth areas of the oceanic crust, so they are called constructive margins. Typical examples are Iceland or the area of the African Rift.

Passive Edges

The ridges do not have continuity along their axis of peaks. They are broken or constantly interrupted by lines of fracture perpendicular to the axis, called transform faults. In them, seismic and volcanic activity is important. Strictly, it is considered a transform fault line that ruptures the ridges. A typical example is the San Andreas Fault in California (USA).

Destructive Edges

Destructive edges match the deeper areas of the ocean (underwater ocean trenches). The oceanic plate is denser than continental edges, and upon bumping into them, the oceanic lithosphere penetrates into the asthenosphere. The introduction into the land enables the plate to melt due to the temperature increase that occurs with depth. Thus, the lithospheric plate, which has been created in the ridge, is recycled in these areas, which has been called subduction zones. At the edges originate great relief, as the Rockies and the Andes. In addition, it is at the destructive edges where most of the earthquakes and volcanoes on Earth originate.

Types of Convergence Between Plates (Types of Orogens)

Clashes between plates give rise to the elevation of mountain ranges or orogens. There are three possibilities of collision:

  • Collision orogens: Result from the crash and subduction of two continental plate boundaries, resulting in the elevation of the orogen (e.g., the Himalayas).
  • Island arcs: They form when an oceanic plate subducts under another ocean plate. This subduction produces a very intense volcanic activity leading to volcanic islands arranged in a curve (e.g., the islands of Japan).
  • Andean-type orogens: They are formed in a continental margin when under the subducting oceanic crust.

The Pyrenees: An Example of a Collision Orogen

The Interior of the Plates

Certain geological structures that originate from inside the slabs, sometimes very far from the edges, are difficult to explain by the theory of plate tectonics. The most characteristic is the existence of oceanic islands aligned. The hot spot theory proposes the existence of a particularly hot area in the interior of the Earth, located deep underground, sending molten material to the surface, which gives rise to volcanoes and volcanic islands. By moving the plate and the point remaining fixed and hot, islands and seamounts will rise aligned. The older islands that are already out of the hot spot have no active volcanoes, while the youngest, which are situated just above this, have active volcanism.

Earthquakes

The ridges create new crust, causing a constant movement of the plates. When it reaches the edges of subduction, the constant motion becomes discontinuous (such as when moving something pulls). The non-continuous release of energy that occurs when the plate enters the mantle causes earthquakes. An earthquake begins in the inner bark at a point called the focus or hypocenter and is subsequently transmitted to the surface of the Earth. The surface point closest to the outbreak, which is what is in its vertical, is called the epicenter. The magnitude is a measure of the energy released. It is an absolute value and is derived from the waves recorded on seismographs. For its measurement, the Richter scale is used.

Intensity is a subjective measure. It measures the effects caused by the earthquake. It is given by the Mercalli scale.

Seismic Precursors: The Prevention of Earthquakes

The accumulation of energy due to plate motion produces small changes in local ground characteristics. These variations are called signs of earthquake precursors. Some of them are:

  • Elevation.
  • Changes in the transmission of electric current in the field.
  • Changes in the magnetic field in the area.
  • Increased radon (radioactive gas produced in the wells).
  • Increasing the amount of local microseisms.

However, the current seismic prediction is full of failures. We still do not have a precise methodology for detection.

Volcanoes

Volcanism is also associated with lithospheric plate boundaries, although it can occur within it. In the peninsula, there is no volcanic activity, although remains of geologically recent volcanism exist. The Canary Islands have had eruptions until very recently (the last on the island of La Palma in 1971, the Teneguía volcano). The danger of an eruption is related to the explosion; the higher the viscosity of lava, the higher its explosiveness.

Risk Volcanic Surveillance

The explosion of magma depends on its viscosity and its gas content. If the magma is viscous and very rich in gas, it explodes violently, blowing fragments of magma fluids and rocks torn from the tube that comes out. There are two types of volcanic eruptions:

  • Gushing: Releasing the material sliding down the walls of the volcano (lava), which are not dangerous to people because of their slow speed.
  • Explosive: They are much more dangerous and can produce a hot cloud (moving at 100 km/h toward the base of the volcano) or a shower of ash and fine material called lapilli.

The heat generated inside the Earth is responsible for its internal dynamics. This internal heat, combined with pressure, causes molten material or transforms it into other rocks so that their minerals are more in line with the new thermodynamic conditions.

The Deformation of Rocks

When a rock is subjected to stress, it can react in three ways:

  • Deform elastically: Once the effort disappears, giving rise to the strain, it returns to the starting position.
  • Plastically deformed: When deformed and does not return to the starting position.
  • Breaking: Separating into fragments.

The deformation depends on several factors, including the intensity of effort, the nature of the rock, the pressure it is under, the temperature, and time.

Fundamental efforts that are under the rocks are of two types:

  • Compression, which makes the rock formations may be shortened.
  • Distension, which makes the rock formations become longer.

Folds

They are the result of a compressive plastic deformation that occurs, which causes wrinkles in strata and a series of undulations like waves.

Elements of a fold:

  • Axial plane or surface: Imaginary surface lines passing through the hinge.
  • Hinge: Line of maximum curvature of a fold.
  • Flank: Areas of fold hinges located between two consecutive, are the sides of the folds.
  • Crest: Area that contains the high or low points of a fold.
  • Trace axial (axis or long axis of the fold): Intersection of axial surface with the topographic surface.
  • Vergence: Axial plane inclination.

Classification of the folds:

Depending on the position of the layers of the fold:

  • Anticline: When the older layers are the core of the fold. (Mnemonic: The fold-shaped anticline A).
  • Syncline: When younger layers appear in the core of the fold.

Depending on the degree of compression flanks have:

  • Isopach: The sides show no thickening or thinning.
  • Ansiopacos: They have a thinning on the flanks.

According to the convergence:

  • Rectum.
  • Apt.
  • Lying.
  • Recumbent.

Faults

When the effort that rocks undergo is enough to break and move the broken pieces, it creates a fault.

Parts of a fault:

  • Fault plane: Surface displacement of the two blocks is usually a curved surface and is inclined with respect to the so-called vertical dip.
  • Break fails: Navigating between the two blocks. It can be measured as the vertical direction or as the fault plane.

Types of faults:

  • Normal fault: When the fault plane dips or leans toward the downthrown block.
  • Reverse fault: When the fault plane dips or leans towards the block lifted.
  • Failure to address or tear: When the displacement of the blocks is not performed vertically and there is neither a raised nor downthrown block; blocks move horizontally.

Association of Folds and Faults

  • Anticlinorium: Associations of folds that cause a positive relief (Association of anticlines).
  • Synclinorium: Association of folds that cause a negative relief (Association of synclines).
  • Mantos Folding: Association of lying and lying folds that are in big mountain formations produced during the collision of two tectonic plates.
  • Pillars (Beds) tectonic faults: Association of blocks that originate and produce raised relief.
  • Grabens: Association of faults that cause blocks and produce sunken reliefs.

Joints

Are those fractures that occur in the rocks when the relative displacement that arises between the blocks is zero or very small (a fault is not developed, in which there has been displacement).

Origin of the Rocks

  • Magmatic or igneous
  • Metamorphic
  • Sedimentary

Phenomena Which Cause the Rocks

Magmatism

Magma is a molten mixture of minerals and chemical compounds composed mainly of silicon. This mixture is composed of elements, solids, liquids, and gases. It originates minerals called silicates. 80% of magma originates at the edges of construction (in the back), and the rest in subduction zones. A small portion is formed in the center of the plates, the so-called hot spots.

Bowen Series Crystallization

The minerals crystallize from a magma not all at once. The first mineral to crystallize is olivine, and later, as the temperature decreases, the remaining will crystallize.

Once formed, the minerals have two choices: change their composition in a progressive (continuous reaction series) or react with the liquid magma and cause another mineral with a more complicated structure (discontinuous reaction series).

  • Melanocratic discontinuous reaction series (black minerals)
  • Leucocratic continuous reaction series (white mineral)
  • Olivine
  • As the magma cools, the end of the forming mineral begins to form the next.
  • First the calcium plagioclase and, as the magma cools, it will appear until the mixture causes the sodium plagioclase
  • Calcic plagioclase
  • Pyroxene
  • Amphibole
  • Biotite
  • Sodium plagioclase

Origin, Formation, and Dynamics of Magmas

  • Magmas originate from upper mantle rocks or deep crust.
  • Temperatures measured in the lavas of the volcanoes are located between 700 and 1,200 ºC.
  • The asthenosphere (molten zone located in the upper mantle) is very near the surface at the ridges and has temperatures of 1000-1500 ºC.
  • The increase in pressure that occurs as the greater the depth causes the melting points of the minerals to increase.
  • In many cases, a temperature increase partially melts rocks, and the magmatic fluid has a different composition than the initial rock.

Evolution of a Magma

The main processes that can change the initial composition of the magma and generate various other rocks are:

  • Differentiation by gravity: When the crystals formed fall to the bottom of the magma chamber.
  • Migration of fluid or magmatic gases: Due to the pressure being put on the magma, some liquids or gases may evolve to higher areas.
  • Assimilation: The magma in its migration to the surface is contaminated and changes its composition to melt and incorporate rocks of different composition to theirs.

Types of Magmas

Geologists believe that there are two basic types of magmas, from which all igneous rocks originate.

  • Basaltic magma (acid): Originates from the fusion of the mantle. They are poor in silica.
  • Granitic magma (basic): Formed by the merger of the deep crust. They are rich in silica.

Magmatic Rocks

Plutonic Rocks

They form when a magma rising from inside the Earth cools and solidifies before reaching the exterior.

  • Granite
  • Granodiorite
  • Greenstone
  • Gabbro

Volcanic Rocks

It is caused when magma goes through all the bark and leaves without solidifying abroad. The magma cools quickly.

  • Basalt

Metamorphic Rocks

When a rock of any kind (plutonic, volcanic, or sedimentary) is subjected to intense pressures, high temperatures, or both factors simultaneously, it experiences significant changes in its composition, structure, or both at once. This change produces minerals according to the new thermodynamic conditions. The new rock formed by different minerals is called metamorphic rock. A prerequisite for the metamorphic process to take place is that the pressure and temperature are not so large that they produce the fusion of rock, as in this case, it would originate plutonic rocks.

Metamorphic Minerals

Generally, major minerals of metamorphic rocks are the same as those of the igneous, although they have some peculiar minerals:

  • Andalusite, sillimanite, and kyanite: They have the same chemical formula (aluminum, silicon, and oxygen, but originate at different temperatures and pressures).
  • Garnets: Are a set of iron silicates.
  • Staurolite: It is a hydrated iron aluminosilicate.
  • Chlorite: It has a structure similar to biotite.

Intensity of Metamorphism

Metamorphic zones are the interior regions of the Earth that are defined by temperature ranges, often coinciding with increased depth. They are distinguished:

  • The epizone (low-grade metamorphism), with temperatures ranging from 200 to 450 ºC.
  • The mesozone (medium-grade metamorphism), with temperatures ranging between 450 and 650 ºC.
  • The catazone (high-grade metamorphism), with temperatures ranging from 650 ºC to the melting point of the rock.

Types of Metamorphism

The classification is done based on the pressure and temperature and the intensity with which each operates.

  • Burial metamorphism: The determining factor is the pressure acting on the vertical due to gravity.
  • Or dinamometamorfismo pressure metamorphism: It arises when the pressure is the dominant factor addressed.
  • Contact or thermal metamorphism: The limiting factor is temperature. It arises when a hot rising magma rocks surrounding recrystallizes its minerals.
  • Regional metamorphism: It is the most abundant. Acting factors are time, pressure, and temperature. It affects large areas of the Earth’s crust, located both in subduction zones and any collision orogen.
  • Metasomatism: It is caused when water loaded with various items, especially hot areas, flows inside the rocks, causing reactions that change the composition of the rock.

Structure of the Metamorphic Rocks

They are made of crystallized minerals, so they are crystalline rocks. They can be found in two types of structures:

  • Non-foliated (sheets are not): These rocks are easily confused with the plutonic and even with the sediments.
    • Quartzite
    • Marbles
  • Foliated (sheet form): It is the most characteristic.
    • The very fine-size crystals give rise to a leafy structure, the blackboard.
      • Slates
    • Medium-grained crystals give a coarser banded called schistosity.
      • Schists
    • The coarse crystals are grouped in bands or strips thick gnésicas of light and dark colors.
      • Gneiss

Seismic Waves

There are three main types of seismic waves:

  • P waves: These are the fastest. They vibrate in the same direction as the direction of propagation. Waves are also called primary or main.
  • S waves: They are slower than P. They vibrate perpendicular to the direction of propagation. They do not cross fluids. Also known as secondary waves.
  • Surface waves: These reach the surface to cause disasters. There are two types of surface waves:
    • Waves R (Rayleigh): They move up and down the substrate particles in a circular motion, causing a shift forward and backward.
    • Waves L (Love): They give rise to a lateral movement, perpendicular to the direction of propagation.

Characteristics of Seismic Waves

  • The higher the density of the material traversed, the lower the wave velocity, and the greater the rigidity, the greater its speed.
  • S waves do not propagate in fluids.
  • The curves P and S have a curved path in certain areas of land and are refracted by changing from one medium to another with very different characteristics (e.g., the core mantle).

Discontinuities

The study of seismic waves shows that Earth has a layered structure. Each step change in the speed of wave propagation indicates that they enter into a new type of material or a material with a different viscosity state. These variations are called sharp discontinuities.

  • The first is located at 10 km in the oceans and 30-40 km under the continents, separating the crust from the mantle, and is called the Mohorovičić discontinuity.
  • The second is located about 670 km deep, is called the Repetti discontinuity, and marks the transition between the upper and lower mantle.
  • The third is located at 2,900 km and is called the Gutenberg discontinuity. It separates the mantle of the core.
  • The fourth discontinuity occurs at 5,100 km and is called the Wiechert-Lheman discontinuity.

Divisions of the Earth

You can consider two criteria to divide the planet into layers:

Geochemical Divisions

92% of the land comprises:

  • Iron (34.6%).
  • Oxygen (29.2%).
  • Silicon (15.2%).
  • Magnesium (12.2%).

These chemicals combine to form minerals and are distributed after major layers:

  • Crust: It is the outermost layer, between 8 and 70 km thick. It concentrates the lighter elements.
  • Mantle: It occupies the greater mass of the earth and reaches 2,900 km depth. The percentage of heavy elements is higher than in the crust.
  • Core: It occupies the center of the earth. It locates the highest percentage of dense elements such as iron.

Dynamic Divisions

According to the behavior that the materials have to the deformations. Under this standard are four layers:

  • Lithosphere: It’s the outer layer of rigid and brittle nature. The estimated average thickness is 100 km. It consists of the crust and upper mantle.
  • Asthenosphere: It has an average thickness of 200 km. It constitutes the bottom of the upper mantle. Also called the low-speed channel in it, the seismic wave velocity drops sharply.
  • Mesosphere: This matches the rest of the mantle.
  • Endosphere: Coincides with the kernel.

Structure and Physical-Chemical Nature of the Interior of the Earth

There are two basic types of crust: oceanic crust and continental crust.

The Oceanic Crust

  • It has a thickness much smaller than the continental crust; it exceeds 12 km.
  • Its outer area is formed by sediments.
  • Below this first zone is a layer of volcanic rocks of basaltic composition, caused by the cooling of the molten part of the mantle to be issued to the surface as lava.
  • The deepest part of the oceanic crust is composed of gabbro, rocks that have undergone a slow cooling inside the earth and, because of that, have the entire volume of the rock consisting of crystals.
  • The oceanic crust is young compared to the continental crust.

Continental Crust

  • It is the continents and continental shelves (which are submerged edges).
  • Unlike the ocean, the continent is very old; it can have up to 4,000 million years.
  • There are three distinct areas:
    • Surface Area: Sedimentary rocks folded (or not).
    • Intermediate zone: Metamorphic rocks of medium intensity with plutons (granite).
    • Deep zone: High-grade metamorphic rocks of intensity of pressure and temperature along with basic intrusions.

The Mantle

  • For their study, there is less evidence for the study of the crust.
  • Reaches 2,900 km depth.
  • It consists of ultramafic rocks, which are silica-poor rocks rich in olivine and pyroxene.
  • One of its most important features is its dynamic convection currents, which balance the internal heat to the surface.

The Nucleus

  • The outer core is liquid.
  • The temperature may be about 4000-5000 °C.
  • The core generates a magnetic field that has been recorded in the rocks for about 3,500 million years.
  • It’s supposed to be composed mainly of iron.

Water

The Origin of the Mountains: Contractionary and Mobility

Until the early ’60s, when we began to know better the bottom of the oceans and interpreted the data of terrestrial magnetism, the scientific community was torn between two schools of thought: that of the contraction and cell phones. The contractions are based on the theory of the geosyncline, which were a kind of narrow rows, but many kilometers in length, which were accumulated over time huge amounts of sediments that horizontal compressive forces (contraction) rising and forming folded mountain ranges. However, the idea that the Earth was contracted as a result of cooling was losing weight slowly, the discovery of radioactivity Earth and thus the possibility that there was a source for maintaining the heat inside the planet.

The motorists were based on the old idea of moving continents, although the main difficulty that their ideas were accepted was that they did not provide sufficient specific data to justify the movement of land masses. But in 1911, Wegener, a German meteorologist, raised with an abundance of arguments and facts the idea that the continents had not been fixed but had moved in the course of geological time.

Wegener’s theory, known as continental drift, described how a large primitive continent, which he called Pangea, broke into many smaller continents. These continents, as they move over time, land masses rise and mountains today.

The Theory of Plate Tectonics

According to the theory proposed by Wilson in 1965, the Earth is divided, like a puzzle, into a series of compartments called rigid lithospheric plates. The plates are separated by a network of seismic and volcanic belts, seamount chains of volcanic islands, and archipelagos arranged in an arc, running through the Earth’s surface. Today it is accepted that the Earth is divided into 12 panels of varying size. There are purely oceanic plates, although it is common to have a party formed by continental lithosphere (the continent and the continental shelf) and another consisting of oceanic lithosphere.

Relationship Between Plates

The plates can be separated, colliding with each other or sliding laterally. Areas where this happens are called respectively: mid-ocean ridges, subduction zones, and transform faults. In this connection, plate to plate can recognize three types of boundaries or borders:

  • Borders constructive or ridges: These are areas where oceanic lithosphere is constantly being created. They are usually located in the center of the oceans.

The material comes from the inside

The plates are separated

  • Borders destructive: They are areas where plates are destroyed by penetrating the oceanic lithosphere beneath the continental subduction zone. They are generally situated near the continental margins. Subduction also occurs when two oceanic plates converge.

The plates collide, and one sinks beneath the other.

The material is melted inside

  • Borders liabilities: These are the plate boundaries in which neither created nor destroyed the lithosphere. They are the edges that allow the relative movement of the plates through break lines called transform faults.

Parallel Rods

Evidence of Plate Tectonics

  • Matching coastlines: When continents fit the lines delimiting the continental shelf, about 2000 m depth below sea level, we can see that the coincidence of the masses is very large.
  • Matching rock formations in distant continents: The overlap corresponds to rocks with more than 100 million years, during which time they began the separation of the original single continent (Pangea).
  • Continuity of Alpine chains and trending strand separation: The latest mountain ranges, called Alps, originated about 25-30 million years. It is noted that the distribution of these over the Earth does not stop at any time. However, other oldest mountain ranges, occurring during the Hercynian fold, about 300 million years, are only major disruption, and continuity can be seen if the continent is united.