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STRUCTURE OF THE EARTH
THE CRUST
It is the outermost solid part of the earth.
It is brittle in nature.
The thickness of the crust varies under the oceanic and continental areas.
Oceanic crust is thinner as compared to the continental crust.
The mean thickness of the oceanic crust is 5 km whereas that of the continental is around 30 km.
The continental crust is thicker in the areas of major mountain systems.
It is as much as 70 km thick in the Himalayan region.
It is made up of heavier rocks having a density of 3 g/cm3. This type of rock found in the oceanic crust is basalt.
The mean density of material in the oceanic crust is 2.7 g/cm3.
THE MANTLE
The portion of the interior beyond the crust is called the mantle.
The mantle extends from Moho’s discontinuity to a depth of 2,900 km.
The upper portion of the mantle is called the asthenosphere. The word astheno means weak.
It is considered to be extending up to 400 km.
It is the main source of magma that finds its way to the surface during volcanic eruptions.
It has a density higher than the crust’s density (3.4 g/cm3).
The crust and the uppermost part of the mantle are called the lithosphere. Its thickness ranges from 10-200 km.
The lower mantle extends beyond the asthenosphere. It is in solid-state.
THE CORE
As indicated earlier, the earthquake wave velocities helped in understanding the existence of the core of the earth.
The core-mantle boundary is located at the depth of 2,900 km.
The outer core is in a liquid state while the inner core is in solid state.
The density of material at the mantle core boundary is around 5 g/cm3 and at the center of the earth at 6,300 km, the density value is around 13 g/cm3.
The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.
LAYERING SYSTEM OF EARTH
The earth is made up of several concentric layers.
The outer layer is the earth’s crust– the lithosphere, which comprises two distinct parts. The upper part consists of granitic rocks that form continents – SIAL and the lower part is a continuous zone of basaltic rocks forming the ocean floors – SIMA.
SIAL
It is located just below the outer sedimentary cover and is composed of granites and is dominated by Silica (SI) and Aluminium (AL).
The average density of this layer is 2.9 and its thickness range from 50 to 300 km.
This region is dominated by acid materials and silicates of potassium, sodium, and aluminum.
SIMA
It is located just below the SIAL layer and is composed of basalt and this layer is the source of magma and lava during volcanic eruptions.
Silica (SI) and Magnesium (MA) are the dominant constituents.
Average density between 2.9 – 4.7, whereas thickness varies from 1,000 km – 2,000 km, and having an abundance of basic matter and silicates of magnesium, calcium, and iron are abundantly found.
NIFE
It is located below the SIMA layer and composed of Nickel (Ni) and Fermium (FE).
This layer is made of heavy metals and is the reason for the very high density of this layer.
The diameter of this zone is 6,880 km and the presence of iron indicates the magnetic property of the earth’s interior.
This property also indicates the rigidity of the earth 23.5.
LITHOSPHERE
The word lithosphere is derived from the Greek word lithos, meaning rock.
The lithosphere is the solid outer section of Earth, which includes Earth’s crust, as well as the underlying cool, dense, and rigid upper part of the upper mantle.
The lithosphere extends from the surface of Earth to a depth of about 70–100 km.
This relatively cool and rigid section of Earth is believed to “float” on top of the warmer, non-rigid, and partially melted material directly below.
This motion of the lithospheric plates is known as plate tectonics and is responsible for many of the movements seen on Earth’s surface today including earthquakes, certain types of volcanic activity, and continental drift.
ASTHENOSPHERE
The asthenosphere (from Greek ‘weak’ + “sphere”) is the highly viscous, mechanically weak, and deforming region of the upper mantle of the Earth.
It lies below the lithosphere, at depths between approximately 80 and 200 km below the surface.
The asthenosphere is generally solid, although some of its regions could be melted (e.g., below mid-ocean ridges).
The lower boundary of the asthenosphere is not well defined and the thickness of the asthenosphere depends mainly on the temperature.
Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle.
CONTINENTAL DRIFT THEORY
CONTINENTAL DRIFT – EARLY CONTRIBUTIONS
While observing the shape of the coastline of the Atlantic Ocean, we shall observe the symmetry of the coastlines on either side of the ocean.
Hence, many scientists proposed based on this similarity and considered the possibility of the two Americas, Europe, and Africa, to be once joined together.
CONTINENTAL DRIFT THEORY
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 supercontinent was named PANGAEA, which meant all earth.
The mega-ocean was called PANTHALASSA, meaning all water.
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. Some of these are given below.
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.
A map produced using a computer program to find the best fit of the Atlantic margin was presented by Bullard in 1964 and it was proved to be quite perfect.
The match was tried at 1,000- fathom line instead of the present shoreline.
ROCKS OF THE SAME AGE ACROSS THE OCEANS
The radiometric dating methods developed in the recent period have facilitated correlating the rock formation from different continents across the vast ocean.
The belt of ancient rocks of 2,000 million years from Brazil's 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.
At the base, the system has thick tillite indicating extensive and prolonged glaciation.
Counterparts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica, and Australia besides India.
The overall resemblance of the Gondwana-type sediments clearly demonstrates that these landmasses had remarkably similar histories.
The glacial tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents.
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
DISTRIBUTION OF FOSSILS
When identical species of plants and animals adapted to living on land or in freshwater are found on either side of the marine barriers, a problem arises regarding accounting for such distribution.
The observations that Lemurs occur in India, Madagascar, and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.
Mosasaur was a small reptile adapted to shallow brackish water.
The skeletons of these are found only in two localities: the Southern Cape Province of South Africa and Traver formations of Brazil.
The two localities presently are 4,800 km apart with an ocean in between them.
FORCE FOR DRIFTING
Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force.
The polar-fleeing force relates to the rotation of the earth.
You are aware of the fact that the earth is not a perfect sphere; it has a bulge at the equator.
This bulge is due to the rotation of the earth.
The second force that was suggested by Wegener—the tidal force—is due to the attraction of the moon and the sun that develops tides in oceanic waters. Wegener believed that these forces would become effective when applied over many million years.
However, most scholars considered these forces to be totally inadequate.
POST-DRIFT STUDIES
It is interesting to note that for continental drift; most of the evidence was collected from the continental areas in the form of distribution of flora and fauna or deposits like tillite.
A number of discoveries during the post-war period added new information to geological literature.
Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of the distribution of oceans and continents.
CONVECTIONAL CURRENT THEORY
Arthur Holmes in the 1930s discussed the possibility of convection currents operating in the mantle portion.
These currents are generated due to radioactive elements causing thermal differences in the mantle portion.
Holmes argued that there exists a system of such currents in the entire mantle portion.
This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.
DISTRIBUTION OF EARTHQUAKES AND VOLCANOES
While observing the distribution of seismic activity and volcanoes, we will notice a line of dots in the central parts of the Atlantic Ocean almost parallel to the coastlines.
It further extends into the Indian Ocean.
It bifurcates a little south of the Indian subcontinent with one branch moving into East Africa and the other meeting a similar line from Myanmar to New Guiana.
We will notice that this line of dots coincides with the mid-oceanic ridges.
The shaded belt showing another area of concentration coincides with the Alpine-Himalayan system and the rim of the Pacific Ocean.
In general, the foci of the earthquake in the areas of mid-oceanic ridges are at shallow depths whereas, along the Alpine- Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones.
The map of volcanoes also shows a similar pattern.
The rim of the Pacific is also called the rim of fire due to the existence of active volcanoes in this area.
PACIFIC RING OF FIRE
The Ring of Fire is an area in the basin of the Pacific Ocean where a large number of earthquakes and volcanic eruptions occur. In a 40,000 km (25,000 mi) horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts, and/or plate movements. It has 452 volcanoes (more than 75% of the world’s active and dormant volcanoes). The Ring of Fire is sometimes called the circum-Pacific belt. About 90% of the world’s earthquakes and 81% of the world’s largest earthquakes occur along the Ring of Fire. The next most seismically active region (5–6% of earthquakes and 17% of the world’s largest earthquakes) is the Alpide belt, which extends from Java to the northern Atlantic Ocean via the Himalayas and southern Europe.
CONCEPT OF SEAFLOOR SPREADING
As mentioned above, the post- drift studies provided considerable information that was not available at the time Wegener put forth his concept of continental drift. Particularly, the mapping of the ocean floor and paleomagnetism studies of rocks from oceanic regions revealed the following facts:
It was 193ealized that all along the mid-oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava to the surface in this area.
The rocks equidistant on either side of the crest of mid-oceanic ridges show remarkable similarities in terms of a period of formation, chemical compositions, and magnetic properties. Rocks closer to the mid-oceanic ridges are normal polarity and are the youngest. The age of the rocks increases as one moves away from the crest.
The ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years.
The sediments on the ocean floor are unexpectedly very thin. Scientists were expecting, if the ocean floors were as old as the continent, to have a complete sequence of sediments for a period of much longer duration. However, nowhere was the sediment column found to be older than 200 million years.
The deep trenches have deep-seated earthquake occurrences while in the mid-oceanic ridge areas; the quake foci have shallow depths.
These facts and a detailed analysis of magnetic properties of the rocks on either side of the mid-oceanic ridge led Hess (1961) to propose his hypothesis, known as the “seafloor spreading”.
Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. The ocean floor thus spreads.
