EARTH INTERIOR

SOURCES OF INFORMATION ABOUT INTERIOR OF THE EARTH

Earth’s radius is 6,370 km. It is rather difficult to make observations or collect samples of the material from inside of the earth due to its huge size and the changing nature of its internal composition. Only a part of the information is obtained through direct observations and analysis of materials. Most of our understanding about the interior of the earth is based on estimates and inferences.

1. Direct Sources of Information
   • Surface rock: Laboratory experiments on surface rocks and minerals provide important information about the interior of the earth.
   • Mining- rocks: Through mining and drilling operations, we have been able to observe the earth’s interior directly only up to a depth of a few kilometers. World’s deepest mining is limited only to the depth of fewer than 5 kilometers. Going beyond this depth is not possible due to excessive heat at this depth.
   • Deep Ocean Drilling Projects: Scientists have undertaken some major projects to penetrate the surface of oceans to assess the conditions in crustal portions. Many deep drilling projects have provided a large volume of information through the analysis of materials collected at different depths.
     • For Example: The deepest drill at Kola, in the Arctic Ocean, has so far reached a depth of 12 km.
   • Volcanic Eruptions: Analysis of the molten material (magma) that is thrown onto the surface of the earth during a volcanic eruption. However, it is difficult to find out about the depth of the source of such magma.

1. Indirect Sources of Information
   • Meteors: Meteors at times reach the earth and are an important source of information about the interior structure of the Earth. Although they may not be part of the interior of the earth, they provide valuable information as the structure observed in meteors is similar to that of the earth.
   • Gravitation: The reading of the gravity at different places is influenced by many factors viz. distribution of mass, distance from the center of the Earth. Such a difference is called a gravity anomaly. Gravity anomaly gives us information about the distribution of mass of the material in the crust of the earth.
   • Magnetic Field: Magnetic surveys provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
   • Seismic Activity: Seismic activity is one of the most important sources of information about the interior of the earth. Body waves, generated by an earthquake, especially S-waves, which travel only through solid material, have helped in understanding the interior structure of the Earth.
   • Temperature: We find temperature differences at successive layers of the earth’s interior. Due to which we are able to understand the interior structure of the earth.
     • For example: Temperature increases at the rate of 1° per 32m of depth reaching 4000° Celsius at its core where no rock exists due to such high temperature.
   • Density: The density of the rocks increases going down the earth; the outer shell consists of sedimentary rocks up to about 1.6 km with a density of 2.7g/cm³. Below sedimentary rocks lie crystalline rocks with higher density and at the core the density reaches up to 11g/cm³, such density difference gives the information about the involvement of different material inside the Earth interior.
   • Pressure: Just like the temperature, the pressure is also increasing from the surface towards the center of the earth. It is due to the huge weight of the overlying materials like rocks. It is estimated that in the deeper portions, the pressure is tremendously high which will be nearly 3 to 4 million times more than the pressure of the atmosphere at sea level.

INTERNAL STRUCTURE

The interior of the earth is made up of several concentric layers of which the crust, the mantle, the outer core, and the inner core are significant because of their unique physical and chemical properties. The crust is a silicate solid, the mantle is a viscous molten rock, the outer core is a viscous liquid, and the inner core is a dense solid.

1. Crust
   • Thickness: It is the outermost solid part of the earth, normally about 8-40 km thick.
   • Nature: It is brittle in nature.
   • Volume and mass: Nearly 1% of the earth’s volume and 0.5% of earth’s mass are made of the crust.
   • Composition: Major constituent elements of crust are Silica (Si) and Aluminium (Al) and thus, it is often termed as SIAL (Sometimes SIAL is used to refer to Lithosphere, which is the region comprising the crust and uppermost solid mantle, also).
   • Discontinuity: The discontinuity between the hydrosphere and crust is termed as the Conrad Discontinuity.
2. Mantle
   • Thickness: The portion of the interior beyond the crust is called the mantle. The mantle is about 2900 km in thickness.
   • Nature: Mantle is denser and tightly packed at depth.
   • Volume and mass: Nearly 84% of the earth’s volume and 67% of the earth’s mass is occupied by the mantle.
   • Composition: The major constituent elements of the mantle are Silicon and Magnesium and hence it is also termed as SIMA.
   • Discontinuity: The discontinuity between the crust and mantle is called the Mohorovich Discontinuity or Moho discontinuity.
3. Core
   • Thickness: It is the innermost layer surrounding the earth’s center. Core radius is about 3500 km.
   • Discontinuity: The core is separated from the mantle by Guttenberg’s Discontinuity.
   • Composition: It is composed mainly of iron (Fe) and nickel (Ni) and hence it is also called NIFE.
   • Volume and mass: The core constitutes nearly 15% of earth’s volume and 32.5% of earth’s mass.
   • The Core consists of two sub-layers: The inner core and the outer core. The inner core is in solid state and the outer core is in the liquid state (or semi-liquid).

ASTHENOSPHERE

• Lithosphere: Uppermost solid part of the mantle and the entire crust constitute the Lithosphere.
• Depth: Below the Earth’s surface from about 70 km down to about 300 km is the soft zone in the upper mantle called the asthenosphere.
• Source of heat: This zone contains pockets of increased heat from radioactive decay. It is susceptible to slow convectional current in its hot material.
• Nature: It is a highly viscous, mechanically weak, and ductile, deforming region of the upper mantle which lies just below the lithosphere.
• Follow up of tectonic plates movement: The asthenosphere has the characteristics of a plastic solid meaning it can easily deform and follow up a few cm per year. The hot material in the asthenosphere flows both vertically and horizontally, dragging the lithospheric plates/continental plates (plate tectonics) along with it.

Paleomagnetism

1. Meaning: Palaeomagnetism refers to the earth’s past magnetism preserved in a rock, i.e., a rock acting as a tape recording of Earth’s past Magnetism.
2. Material: The trace of magnetic polarization surviving in igneous rocks and those sediments rocks which contain magnetite are referred to as Palaeomagnetism.
3. Findings:
   • The magnetic studies have revealed that the ocean floor consists of parallel bands of oceanic crust which have alternating magnetic polarity. These magnetic bands are symmetric and are mirrored around the mid-oceanic ridge.
   • Alternating magnetic polarity or reversal of magnetic polarity refers to a change in Earth’s magnetic field, that is, the north magnetic pole becomes the south magnetic pole and vice versa.
   • These alternating magnetic bands have happened because the new rocks which are formed near the ridge, while the older rocks, which formed millions of years ago when the magnetic field was reversed, have been pushed farther away.
   • Hence, this further explains the seafloor spreading.

THEORIES

1. Continental Drift Theory

• Theory proposes: This was regarding the distribution of the oceans and the continents proposed by Alfred Wegener. According to Wegener, all the continents formed a single continental mass, a mega ocean surrounded by the same.
• The super continent was named PANGAEA, which meant all earth. The mega-ocean was called PANTHALASSA, meaning all water.
• Arguments:
  • He argued that, around 200 million years ago, the supercontinent, Pangaea, began to split.
  • Pangaea first broke into two large continental masses as Laurasia and Gondwanaland forming the northern and southern components respectively.
  • Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today.
  • A variety of evidence was offered in support of the continental drift.

Evidence in support of the continental drift

• The matching of Continents (Jig-Saw-Fit): The shorelines of Africa and South America facing each other have a remarkable and unmistakable match.
• Rocks of same age across the oceans: The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa. The earliest marine deposits along the coastline of South America and Africa are of the Jurassic age.
• Tillite: It is the sedimentary rock formed out of deposits of glaciers. The Gondwana system of sediments from India is known to have its counterparts in six different landmasses of the Southern Hemisphere.
• Placer deposits: The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact. The gold-bearing veins are in Brazil and it is obvious that the gold deposits of Ghana are derived from the Brazil plateau when the two continents lay side by side.
• Distribution of fossils: Similar distribution of fossils of plants and animals across marine barriers. E.g. Lemurs occur in India, Madagascar, and Africa.
• Force for drifting: Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force. Wegener believed that these forces would become effective when applied over many million years.

2. Sea Floor Spreading

• Given by: The concept of seafloor spreading was put forward by H. Harry Hess, an American geologist.
• Theory proposes: He suggested that new sea-floor forms at the oceanic ridges and spread outwards from the line of origin. Further, he claimed that continents would be pushed aside by the same forces that cause the ocean to grow. That is, constant eruptions at the crest of oceanic ridges cause the oceanic crust to rupture and new lava to wedge out of it, pushing the oceanic crust on either side. The ocean floor thus spreads.
• Theory of seafloor spreading solved many of the unsolved problems
  • It solved the problem of younger age crust found at the mid-oceanic ridges and older rocks being found as we go away from the middle part of the ridges.
  • It also explained why the sediments at the central parts of the oceanic ridges are relatively thin.
  • The sea-floor spreading also proved the drifting of continents as propounded by Alfred Wegener and helped in the development of the theory of plate-tectonics.

3. Convectional Current Theory

• Given by: Convectional current theory was proposed by Arthur Holmes.
• Generation of heat: First of all, it is to be noted that the heat which is generated from the radioactive decay of substances deep inside the Earth (the mantle) creates magma which consists of molten rocks, volatiles, dissolved gasses among other materials.
• Theory proposes: The Convectional current theory states that this magma, heat, and gasses seek a path to escape which leads to the formation of convection currents in the mantle.
  • According to the theory of Seafloor spreading, convectional cells are the force behind the drifting of continents.
  • Also, please note that the ocean plates get subducted under the continental plates (since ocean plates are denser than continental plates), when these two types of plates converge.
  • The collision of plates is followed by earthquakes and volcanoes.

4. Plate Tectonic Theory

• Contradiction: Alfred Wegener in his theory of continental drift had thought that continents move, but this is incorrect. He further believed that all continents were initially existent as a supercontinent, Pangea. However, later discoveries have revealed that continental masses resting on plates have been moving, and Pangea was a result of the convergence of different continental masses that were part of one or the other plates.
• Need of PT theory: We have till now covered concepts like continental drifting, seafloor spreading, and convectional current theory which gives the explanation for many geomorphological features present on the earth. But still, there remain some unanswered questions like—how do fold mountains form, what are the causes behind the occurrence of earthquakes, what are the reasons behind volcanic activity on land, and so on. For answering these and other similar questions, we have the theory of Plate Tectonics.
• Formulation of theory: With the emergence of the concept of seafloor spreading, and a wealth of new evidence at the beginning of the 1950s and 1960s, the interest in the problem of distributions of oceans and continents was revived. Also, the following six developments were instrumental in the formulation of the theory of plate tectonics:
  • Development of mid-oceanic ridges and seafloor spreading
  • Palaeomagnetism
  • The findings of the age of ocean floors
  • Discoveries of island arcs and submarine trenches
  • The precise documentation of volcanoes and earthquakes, identification of susceptible seismic zones, and spots vulnerable to volcanic activity
  • Identification of hotspots, their strengths, size, and retrospective ejections.
• Theory:
  • Tectonic Plate: A tectonic plate is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere as rigid units.
  • Description of plate: On the basis of size, a tectonic plate may be a major plate or a minor plate. On the basis of nature, a plate may be referred to as a continental plate or oceanic plate.
  • Tectonic activity: The earth’s crust is continuously experiencing movements in horizontal as well as vertical directions resulting in breaking and bending of crustal rocks, and this process of deformation is known as tectonic activity.
  • Propose: The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and several minor plates.
  • These plates have been moving very slowly across the globe throughout the history of the earth. Moreover, it may be noted that all the plates without exception have moved in the geological past, and shall continue to move in the future as well.
  • The movement of the plates results in the building up of stresses within the plates and the continental rocks above, which leads to folding, faulting, and volcanic activity. The major plates are surrounded by folded mountains, ridges, trenches, and faults.
  • Convictive magma theory encourages PTT: Previously, we have studied that intense heat is generated from the radioactive decay of substances deep inside the Earth (the mantle) which creates magma consisting of molten rocks, volatiles, and dissolved gasses. These produce convection currents when the magma, heat, and gasses seek a path to escape in the mantle. The force behind the movement of the plates are these convection currents generated by the upwelling of hot magma which causes the overlying lithospheric slabs to uplift and stretch.

Importance of the theory of Plate Tectonics

• For geologists, it is the unifying theory of geology, which further explains large-scale geological phenomena, such as earthquakes, volcanoes, and the existence of ocean basins and continents.
• PTT explains why there are lots of volcanoes in Iceland and Japan, but far fewer in Russia and Africa. This is because Iceland was created by a mid-oceanic ridge. Similarly, Japan is located on a fault line. The constant pressure around the fault line causes many earthquakes and volcanic eruptions.
• For geographers, the theory of Plate tectonics aids in the interpretation of landforms. It ultimately explains why and where deformation of Earth’s surface occurs.
• Further, the concept of plate tectonics explains mineralogy. New minerals pour up from the core along with the magmatic ejections. The plate boundaries are the pathways through which rocks from the mantle come out as deposits on the lithosphere. These rocks are the source of many minerals. The famous Pacific Ring of Fire known for its violent volcanic activity is also a ring of mineral deposits.

INTERACTION OF TECTONIC PLATES

Major geomorphological features such as fold and block mountains, mid-oceanic ridges, trenches, volcanism, earthquakes, etc. are a direct consequence of the interaction between various Tectonic Plates (lithospheric plates).

Types of plate boundary interaction

1. Divergent Plate Boundaries
   • A divergent boundary is formed by tectonic plates pulling apart from each other. They are known as constructive boundaries.
   • For Example: The Mid-Atlantic Ridge where the American Plates are separated from the Eurasian and African Plates.
   • There are 3 processes:
     • Rift valley: Divergent boundaries are the site of seafloor spreading and rift valleys.
       • For example: On land, giant troughs such as the Great Rift Valley in Africa form where plates are tugged apart.
     • Sea floor spreading: Magma upwells and releases forming new crust.
       • For example: The East Pacific Rise is a site of major seafloor spreading in the Ring of Fire.

• New Sea: Lithosphere uplift and stretches, and if the divergent plates are kept by continental crust, fractures will be developed, rift valleys will be created and finally a new ocean is created.
  • For example: Red Sea, Gulf of California

1. Convergent Plate Boundaries
   • When two plates move towards each other. They are also known as destructive boundaries.
   • There are three ways in which convergence can occur:
     • Oceanic-Oceanic Plate Convergence: The older plate descends in the subduction zone along the trench, carrying water-filled sediments into the mantle. Magma forms, rises, and volcano is created.
       • For Example: An example of the oceanic-oceanic convergence is the Mariana Trench, the deepest point on Earth.
     • Oceanic-Continental Plates Convergence: The oceanic lithosphere is dense and it consumes first, and the continental plate is almost never destroyed.
       • For Example: The Washington-Oregon coastline of the USA is an example of oceanic-continental convergent plate boundary. Here, the Juan de Fuca oceanic plate is subducted beneath the North American continental plate.
     • Continental-Continental Plates Convergence: When two continental plates meet head-on, none is subducted and both are light and resist downward well, so the crust buckles and either moves sideways or moves upward.
       • For Example: The Himalayan Mountain Range is the best active example of continental convergent plate boundaries.
2. Transform Plate Boundaries
   • A transform boundary is formed as tectonic plates slide horizontally past each other but parts of these plates get stuck at the places where they touch.
   • These boundaries are conservative because plate interaction occurs without creating or destroying crust.
   • Hence, they don’t produce spectacular features like mountains or oceans, but the halting motion often triggers large earthquakes.
     • For Example: The 1906 earthquake that devastated San Francisco.
   • In these areas of contact, stress is built which causes the rocks to break or slip, suddenly lurching the plates forward and causing earthquakes. These areas of breakage or slippage are called transform faults.
     • For Example: The San Andreas Fault in California is an example of a transform boundary, where the Pacific Plate moves northward past the North American Plate.

Types of Continental Margins

Continental margins are of two types:

• Active type margins: Are those which in the present geological time period exhibit boundary interactions. Since at present Pacific margins are very active, therefore active type are also called Pacific type.
• Passive type margins: Are those which in the present geological time period do not exhibit boundary interaction. Since at present Atlantic margins are very passive, so passive margins are also called Atlantic type.