Sources of study of interior of earth:

I. Direct sources –

1.    Deep Ocean drilling reveals humongous information through analysis of materials collected at different depth.

2.    Volcanic eruption delivers information by the means of molten magma that comes out of Earth’s interiors. But, it's difficult to determine the depth of such magma’s origin.

3.    Surface rocks are readily available earth material.

4.    Gold mines go to a depth of 5 km on an average, these serve as good opportunities for studying the depths of earth.

II. Indirect sources –

1.   Temperature and pressure patterns through mining activity: An increase in temperature and pressure with depth means an increase in density as well. Hence it becomes possible to determine the rate of change of characteristics of material of earth. This has lead to the knowledge of the layers of earth.

2.    Meteors : These are extra-terrestrial masses reaching the earth’s surface. They have material and structure similar to earth and give information about the materials of which earth is formed of.

3.    Gravitation force(g) : The force exerted by the Earth on all things in its range is not same along all latitudes, it is variable over different places. Observations suggest that gravitational force is greater at poles, and lesser at equator. This is due to increased distance from the core. This difference in (g) is also attributed to the uneven material mass distribution.

4.    Magnetic surveys: The distribution of magnetic materials gives idea of magnetic field of earth which indicates density and type of material present in the interior of earth.

5.    Seismic activity: This gives most important evidences of interior of earth. Earthquakes give a fair idea of interior of earth. We shall look into the details of Earthquake later on.

More than 90 percent of the Earth’s mass is composed of iron, oxygen, silicon, and magnesium, elements that can form the crystalline minerals known as silicates. However, in terms of chemical and mineralogical composition, as in physical properties, the Earth is far from homogeneous. Apart from the superficial lateral heterogeneities near the surface (i.e., in the compositions of the continents and ocean basins), the Earth’s principal differences vary with distance toward the centre owing to increasing temperatures and pressures and due to the original segregation of materials into a metal-rich core, a silicate-rich mantle, and the more buoyant, highly refined crustal rocks. The Earth is geochemically differentiated to a great extent. Crustal rocks contain about twice as much of the rock-forming element aluminum as does the rest of the solid Earth and nearly 50 times as much as uranium. On the other hand, the crust, which accounts for a mere 0.4 percent of the Earth’s mass, contains less than 0.1 percent of its average abundance of iron. Virtually all the iron is concentrated in the Earth’s core.

The increasing pressure with depth causes phase changes in crustal rocks at depths of roughly 60 kilometres, marking the boundary of the upper mantle. This transition area between crust and mantle, called the Mohorvicic discontinuity, is prominently revealed by seismic wave analysis. It is believed that most basaltic magmas are generated near the base of the upper mantle at a depth of about 400 kilometres. The upper mantle, which is rich in the greenish mineral olivine, shows significant lateral in homogeneities. Nearly 50 percent of the body of the Earth, down to a depth of 2,890 kilometers, consists of the lower mantle, which is composed chiefly of magnesium- and iron-bearing silicates, including high-pressure phases of olivine and pyroxene. The mantle is not static but rather slowly convects. One important feature is the production of temporary super plumes (huge, rising jets of hot, partially molten rock), which may originate as deep as the heterogeneous layer near the core-mantle interface. Much larger than ordinary thermal plumes, such as that associated with the Hawaiian Island chain in the central Pacific, super plumes may have had profound effects on the Earth’s geologic history and even on its climate. One outburst of global volcanism that began about 125 million years ago and lasted through most of the Cretaceous Period may have been associated with melting at the tops of one or more giant plumes that rose in the mantle beneath the Pacific and Indian plates.

About one-third of the Earth’s mass is contained in its core, most of which is liquid iron alloyed with some lighter, cosmically abundant components (e.g., sulfur or oxygen). Its liquid nature is revealed by the failure of shear-type seismic waves to penetrate the core. However, a small central part of the core, below 5,150-kilometre depth, is solid. Temperatures in the core are extremely hot, ranging from 4,000 to 5,000 K at the outer part of the core to 5,500 to 7,500 K in the Earth’s centre, probably hotter than the surface of the Sun. (Uncertainties in temperature arise from questions as to which compounds form alloys with iron.) The core’s reservoir of heat may contribute as much as one-fifth of all the internal heat that ultimately flows to the surface of the Earth.

Therefore the earth is almost a spherical body, made up of concentric zones. Physical composition of the different zones are

Core

The centre of the earth is occupied by a spherical zone called core, about 3475 km in radius. The innermost part of the core may be solid or crystalline, with a radius of about 1255 km, while the outer core has properties of a liquid. The liquid core is considered to comprise of iron and small proportion of nickel. The temperatures in the earth’s core are between 6000 ⁰C and 7000 ⁰C. Pressures are as high as three to four million times the pressure of atmosphere at sea level.

Mantle

The mantle lies outside the core, a layer about 2895 km thick, composed of mineral matter in a solid state. It is probably composed largely of magnesium iron silicate, which comprises an ultrasonic rock called dunite. This rock exhibits great rigidity and high density in response to earthquakes that pass through it. It can also adjust to unequal forces acting over great periods of time. The rigid part of upper mantle adjoining the crust is known as Lithosphere. The soft part below that is called Aesthenosphere.

The Crust

It is the outermost and thinnest layer of the earth’s surface, about 8 to 40 km thick. The base of the crust is sharply defined where it contacts the mantle. This surface of separation between the mantle and the crust is called Moho (Mohorovicic Discontinuity). The crust varies greatly in thickness and composition as small as 5 km thick in some places beneath the oceans, while under some mountain ranges it extends up to 70 km in depth. The rocks of this layer can be subdivided in it (i) basaltic rocks, underlying the ocean basins, containing much iron and magnesium (ii) the rocks that make up the continents which are rich in silicon and aluminum and are higher in color and density.

THE COMPOSITIONS OF THE EARTH’S CRUST (by WEIGHT)
(i)  BY ELEMENTS :
Oxygen	47%
Silicon	28%
Aluminium	8%
Iron	5%
Calcium	3.5%
Sodium	2.5%
Potassium	2.5%
Magnesium	2.2%
Titanium	0.5%
Hydrogen	0.2%
Carbon	0.2%
Phosphorus	0.1%
Sulphur	0.1%
(ii)  BY OXIDES :
Silica	59.1%
Alumina	15.2%
Iron	6.8%
Lime	5.1%
Soda	3.7%
Magnesia	3.5%
Potash	3.15
Water	1.3%
Titania	1.0%

Composition of Earth’s Crust

The other constitutes in order are titanium, hydrogen, phosphorus, barium and strontium. Elements like copper, lead; zinc, nickel and tin are present in very low percentage and are scarce elements.

OTHER ZONING OF THE EARTH’S INTERIOR

1.       Prof. Edward Suess divides the Earth’s interior into three parts : -

(i)         BARYSPHERE – This is the Core or the Central Region of the Earth.  The rocks of this layer are composed of Iron and Nickel. The relative densities of these rocks are about 11 or 12. Suess calls this layer as NIFE, this is, NI = Nickel + Fe = Ferum.

(ii)        PYROSPHERE – This is the Middle Layer, surrounds the core on all sides.  Its relative densities range from 2.90 to 4.75.  It is composed of Silicon and magnesium.  Suess calls it as SIMA (SI – Silicon + MA – Magnesium).  An example of rocks having such density and other properties is Basalt.

(iii)    LITHOSPHERE – This is an Outer Layer, surrounds the SIMA.  Its relative density ranges from 2.75 to 2.90.  It is composed of Silicon + Aluminum. It is called SIAL by Suess (SI – Silicon +AL – Aluminium.  This layer is mainly composed of GRANITE.

2.         Prof. R.A. Daly divides the Earth into four layers –

(i)      CENTROSPHERE – It is crystalline and rigid with density of 11.0.

(ii)     MESOSPHERE  - It is comparatively rigid and 400 km deep.  The average density of this zone is calculated from 9 to 11.0- a mixed zone of iron and silicate material.

 (iii)   ASTHENOSPHERE – It is the exterior, rigid and about 80 km deep, mostly made up of silicate material, with a density of 3.0.

3.       Sir Harold Jeffrey’s divides the Earth’s interior into four parts –

(i)      OUTER LAYER – Mostly composed of sedimentary rocks.

(ii)     SECOND LAYER  - Made up of Granite rocks.

(iii)    THIRD LAYER OR INTERMEDIATE – Mostly composed of Thachylyte or Diorite rocks.

(iv)    LOWEST LAYER  - A mixed layer of Dunite, Peridolite and Eclogite rocks.

4.       Prof. Vonder Gracht divided the Earth’s interior into four parts –

 (i)     OUTER SIALIC CRUST – Depth varies : 60 km under continents and 20 km under Atlantic Ocean.  Density 2.75 to 2.9.

(ii)     INNER SILICATE AND MANTLE SIMA  - The thickness is 60 to  1140 kilometres with a density of 3.1 to 4.75.

(iii)    MIXED METALS AND SILICATE ZONE  - The depth of this zone is 1140 km to 2900 km with a density of 4.75 to 5.0.

(iv)    METALLIC NUCLEUS  - 2900 km to 6378 km density 11.0.

5.         Prof. Gutenberg divides the Earth’s interior into five parts –

(i)      CORE  - The depth of the core is between 2902 to 6370 km.  The density of the core is calculated at 11.0.

(ii)     ULTRABASIC LAYER  - This layer extends below MOHOROVICIC discontinuity at the depth of 2900 km.

(iii)   BASIC ROCKS  - This zone extends between Mohorovicic discontinuity the velocity of the seismic waves increases rapidly.  The existence of several such discontinuities of the second order is, for instance, at 1200, 1900 and 2150 km depth.

(iv)    GRANITE LAYER – Average depth is 15 to 30 kilometer.

(v)     SEDIMENTARY LAYER – This layer is unequally distributed below the continents and oceans.  This is also known as upper SIALIC layer with a density of 3.5.

Mohorovicic discontinuity

The discontinuity between the Earth’s Crust and MANTLE is known as the MOHOROVICIC DISCONTINUITY, so called after the scientist A. Mohorovicic, discovered it in 1909 while studying a Balkan earthquake.  The discontinuity greatly affects the speed at which earthquake waves travel. American scientists made preliminary trials towards drilling a hole through the crust into the mantle (Operation Mohole’, 170 miles north-east of Hawaii, abandoned 1966). The Mohorovicic discontinuity lies at a depth of about 20 miles under the continents, but at only 4 to 6 miles under the oceans.

Gutenberg Discontinuity

The discontinuity at a depth of about 2900 km form the surface of the Earth, between the MANTLE  and the Core, is called the GUTENBERG DISCONTINUITY, after the scientist B Gutenberg, who discovered it in 1914.  At this depth the S-waves of earthquake disappear, while the P-waves travel on at a reduced speed, that is, it is likely that Gutenberg Discontinuity marks a change from a solid to a liquid medium, though of much greater density and under enormous pressure.

	Discontinuity
	Boundary line	Between
1.	Moho or Mohorovicic Discontinuity	Crust and Mantle
2.	Cornard Discontinuity	Outer and Inner Crust
3.	Weichert-Gutenberg Discontinuity	Mantle and Core
4.	Repetti Discontinuity	Outer and Inner Mantle
5.	Transition Discontinuity	Outer and Inner Core

 

The Lithosphere and Plate Tectonics

The Earth’s outermost rigid, rocky layer is called the lithosphere. It is broken, like a slightly cracked eggshell, into about a dozen separate rigid blocks, or plates. There are two types of plates, oceanic and continental. An example of an oceanic plate is the Pacific Plate, which extends from the East Pacific Rise to the deep-ocean trenches bordering the western part of the Pacific basin. A continental plate is exemplified by the North American Plate, which includes North America as well as the oceanic crust between it and a portion of the Mid-Atlantic Ridge, an enormous submarine mountain chain that extends down the axis of the Atlantic basin, passing midway between Africa and North and South America.

The upper layer of the lithosphere is termed the crust. It is composed of low-density, easily melted rocks; the continental crust is predominantly granitic, while the oceanic crust is basaltic. Analysis of seismic waves, generated by earthquakes within the Earth’s interior, show that crustal compositions extend beneath the continents to depths of about 50 kilometres, but only 5 or 10 kilometres beneath the ocean floors. The denser lithospheric plates (60 kilometres thick beneath the oceans and ranging from about 100-200 kilometres beneath the continents) ride on a weak, perhaps partially molten, layer of the upper mantle called the Aesthenosphere.

Slow convection currents deep within the mantle generated by radioactive heating of the interior are believed to drive the lateral movements of the plates (and the continents that rest on top of them) at a rate of several centimeters per year. The plates interact along their marginal zones, and these boundaries are classified into three general types on the basis of the relative motions of the adjacent plates: divergent, convergent, and transform (or strike slip).

In areas of divergence, two plates move in opposite directions. Buoyant upwelling motions force the plates apart at rift zones (such as along the middle of the Atlantic Ocean floor) where magmas from the underlying mantle rise to form new oceanic crustal rocks.

Lithospheric plates move toward each other along convergent plate boundaries. When a continental plate and an oceanic plate come together, the leading edge of the oceanic crust is forced beneath the continental plate (i.e., is subducted). Only the thinner, denser slabs of oceanic crust will subduct, however. When the thicker, more buoyant continents come together at convergent zones, they resist subduction and tend to buckle, producing great mountain ranges. The Himalayas, along with the adjacent Plateau of Tibet, were formed during such a continent-continent collision when India was carried into the Eurasian Plate by relative motion of the Indian-Australian Plate.

At the third type of plate boundary, the transform variety, two plates slide parallel to one another in opposite directions. These areas are often associated with high seismicity, as stresses that build up in the sliding crustal slabs are released to generate earthquakes. The San Andreas Fault in California is an example of this type of boundary, which is also known as a fault or fracture zone.

Most of the Earth’s active tectonic processes, including nearly all earthquakes, occur near plate margins. Volcanoes form along zones of subduction, because the oceanic crust tends to be remelted as it moves into the hot mantle and then rises to the surface as molten lava. Chains of active, often explosive, volcanoes are thus formed in such places as the western Pacific and the west coasts of the Americas. Older mountain ranges, eroded by weathering and runoff, mark zones of earlier plate-margin activity. The oldest, most geologically stable parts of the Earth are the central cores of some continents (such as Australia, southern parts of Africa, and northern North America) where little mountain-building, faulting, or other tectonic processes have occurred for hundreds of millions to billions of years. Because of the stability, erosion has flattened the topography, and geologic evidence of crater scars from the rare, often ancient impacts of asteroids and comets is preferentially preserved. In contrast, much of the oceanic crust is substantially younger (tens of millions of years old), and none dates back more than 200 million years.

It is not known when the original continental cores formed or how long ago modern plate-tectonic processes began to operate. Certainly the processes of internal convection, thermal segregation of minerals by partial melting and fractional crystallization, and basaltic volcanism were operating even more extensively and thoroughly in early epochs. But the assembling of continental landmasses had to compete with giant impacts, which tended to disaggregate them until the impact rate decreased nearly four billion years ago. It is thought that a single super continent that had been created by the amalgamation of many smaller continental cores and island arcs was broken up approximately 500 million years ago into at least three major continents: Gondwana (or Gondwanaland), Laurentia, and Baltica. These three landmasses were widely separated by the so-called Iapetus Ocean (a precursor to the Atlantic). By about 250 million years ago, the continued drifting of these continents resulted in their fusion into a single super continental landmass called Pangea. Some 70 million years later, Pangea began to fragment, gradually giving rise to today’s continental configuration. The distribution is still asymmetric, with continents predominantly located in the Northern Hemisphere opposite the Pacific basin.

The entire conceptual framework in which geologists and geophysicists now understand the evolution of the Earth’s lithosphere is termed plate tectonics (see the article plate tectonics). Analogies from plate tectonics have been applied to understanding surface features on Venus and Mars, as well as to some of the icy satellites of the outer solar system, but with only moderate success.

Earth’s Magnetic Field and Magnetosphere

Earth’s Magnetic Field

The Magnetic Field of the Earth is generated by the motion of molten iron alloys in the Earth’s outer core.

The solid inner core is too hot to hold a permanent magnetic field, but the outer core gives rise to Earth's magnetic field. The geomagnetic field extends from outer core to where it meets the solar wind. At the surface of Earth, the magnitude of Earth’s magnetic field ranges from 25 to 65 microteslas (0.25 to 0.65 gauss).

How magnetic field protects life on Earth?

The magnetic field deflects most of the charged particles emanating from the Sun in the form of solar winds. If there were no magnetic field, the particles of the solar wind would strip away the ozone layer, which protects the Earth from harmful ultraviolet rays. One of the reasons that there is no atmosphere at Mars is that its magnetic field is turned off which led to the loss of carbon dioxide due to scavenging of ions by the solar wind.

How it is formed?

The Earth’s magnetic field is believed to be caused by electric currents in the liquid outer core, which is composed of highly conductive molten iron. The motion of the fluid is sustained by convection, motion driven by buoyancy. At the core, the pressure is so great that the super hot iron crystallizes into a solid. The higher temperature of the fluid lower down makes it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is played to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This Is called compositional convection.

Reversal of the fields

Based on data from ancient and new rocks, it has been observed that Earth’s north and south

magnetic fields have reversed polarity many times. This is because the polarity of the Earth’s magnetic field is recorded in sedimentary rocks. The switching from north to south (an individual reversal event)seems to take around a couple thousand years to complete; once the reversal takes place, periods of stability seem to average about 200,000 years. Nobody has been able to explain why the poles reverse, but theories range from the changes in lower mantle temperatures to the imbalance of land masses on our world (most of the continental landmass is in the Northern Hemisphere).

The last magnetic reversal was 780,000 years ago, which gives us current northern and southern magnetic poles.It is believed that geomagnetic field is slowing weakening, so Earth might be heading for a long-overdue magnetic reversal. Reversals tend to occur when there is a wide divergence between the magnetic poles and their geographic equivalent

Supercontinent.

A supercontinent is a single continental landmass made of all or most of the continental lithosphere at the time. There seems to be a cycle of supercontinents that form and split up every 400 or 500 million years, driven by plate tectonics. Scientists estimate that there was a cycle of at least seven supercontinents on Earth. The last supercontinent, the famous Pangea, formed around 300 million years ago and broke up about 100 million years later. Within several hundreds of million years all the continents should join together to form once again a new supercontinent.

Continental Drift and Plate Tectonics

The concept of continental drift was first put forward in 1915 in a book entitled The Origin of Continents and Oceans by the German meteorologist Alfred Wegener, who recognized that continental plates rupture, drift apart, and eventually collide with one another.

According to Wegner, all the continents were compact and one in the Paleozoic era. This Super and compact continent was called Pangaea .This land was surrounded by a giant ocean known as Panthalassa.  The Pangea was composed of light Sial material while the oceans consisted of heavy Sima. Wegner does not subscribe to the view that the whole earth had a covering of sial. The continents had a sea in its middle part. This sea was called Tethys by Wegner. Laurentia land mass was situated to the north of Tethys and consisted of North America, Europe and Asia. Gondwana land was another landmass situated to the south of the Tethys and comprised Australia. Africa, South America, Antarctica and Peninsular India.

In those days, the landmasses were covered with shallow peninsular seas. The South Pole existed in the middle of Pangea near the South African coast (Natal). About 200 million years ago, Pangaea began to break up into two large masses called Gondwanaland and Laurasia, which in turn separated into the continents as they are today, and which have drifted to their present locations.  According to Wegner, the Pangea broke up in carboniferous period (Mesozoic Era) and began to drift.

The continents had two directions of drift:

            (i) Towards the Equator.

            (ii) Towards the West.

The movement towards the Equator is considered to be caused by the relation of the forces of gravitation and Buoyancy. The centre of gravity and the centre of buoyancy existed in such a position that the continents began to drift towards the Equator. The movement towards the west is ascribed to the tidal forces affecting the Earth.

The drift in this way caused the redistribution of continents and ocean basins resulting in the present form and shape. According to Wegner, the drift has not come to a stop but is still continuing.

Development of Plate Tectonics Theory

In the early 1960s scientists discovered that most earthquakes occur along lines parallel to ocean trenches and ridges, and in 1965 the theory of plate tectonics was formulated by Canadian geophysicist John Tuzo Wilson; it has now gained widespread acceptance among earth scientists who have traced the movements of tectonic plates millions of years into the past. In 1995 US and French geophysicists produced the first direct evidence that the Indo-Australian plate has split in two in the middle of the Indian Ocean, just south of the Equator. They believe the split began about 8 million years ago.

Plate Tectonics

Theory formulated in the 1960s to explain the phenomena of continental drift and seafloor spreading, and the formation of the major physical features of the Earth's surface. The Earth's outermost layer, the lithosphere, is regarded as a jigsaw puzzle of rigid major and minor plates that move relative to each other, probably under the influence of convection currents in the mantle beneath. At the margins of the plates, where they collide or move apart, major landforms such as mountains, volcanoes, ocean trenches, and ocean ridges are created. The rate of plate movement is at most 15 cm per year.

The concept of plate tectonics brings together under one unifying theory many previously unrelated phenomena observed in the Earth's crust. The size of the crust plates is variable, as they are constantly changing, but six or seven large plates now cover much of the Earth's surface, the remainder being occupied by a number of smaller plates. Each large plate may include both continental and ocean crust. As a result of seismic studies it is known that the lithosphere is a rigid layer extending to depths of 50-100 km/30-60 mi, overlying the upper part of the mantle (the asthenosphere), which is composed of rocks very close to melting point, with a low shear strength. This zone of mechanical weakness allows the movement of the overlying plates. The margins of the plates are defined by major earthquake zones and belts of volcanic and tectonic activity, which have been well known for many years. Almost all earthquake, volcanic, and tectonic activity is confined to the margins of plates, and shows that the plates are in constant motion.

Classification of Plates

Plates ranges in size from 10,000 square Km and 100,000,000 square km. There are seven major plates (above 10,000,0000), Eight intermediate sized plates (1,000,000 km to 10,000,000) and 20 minor plates (10,000 km top 1,000,000 km).

Major Plates African Plate. Antarctic Plate. Eurasian Plate. Indo-Australian Plate. North American Plate. Pacific Plate. South American Plate.

Minor Plates Arabian Plate. Caribbean Plate. Cocos Plate. Juan de Fuca Plate. Indian Plate. Nazca Plate. Philippine Sea Plate. Scotia Plate

Plate movement

Plate tectonic theory deals with the movement of rocks structures. These plates are basically moved in three different directions, such as:

Divergent Plates: it is also known as Constructive Margins

Convergent Plates: also known as Destructive Margins

Parallel Plates: also known as Conservative Margins.

Constructive Margins

Where two plates are moving apart from each other, molten rock from the mantle wells up in the space between the plates and hardens to form new crust, usually in the form of an ocean ridge (such as the Mid-Atlantic Ridge). The newly formed crust accumulates on either side of the ocean ridge, causing the seafloor to spread; the floor of the Atlantic Ocean is growing by 5 cm² in each year because of the welling-up of new material at the Mid-Atlantic Ridge.

Destructive Margins

 Where two plates are moving towards each other, the denser of the two plates may be forced under the other into a region called the subduction zone. The descending plate melts to form a body of magma, which may then rise to the surface through cracks and faults to form volcanoes. If the two plates consist of more buoyant continental crust, subduction does not occur. Instead, the crust crumples gradually to form ranges of young mountains, such as the Himalayas in Asia, the Andes in South America, and the Rockies in North America. This process of mountain building is termed orogenesis.

Conservative Margins

 Sometimes two plates will slide past each other - an example is the San Andreas Fault, California, where the movement of the plates sometimes takes the form of sudden jerks, causing the earthquakes common in the San Francisco-Los Angeles area. Most of the earthquake and zones of the world are found in regions where two plates meet or are moving apart.

Causes of Plate Movement

 Before the plate tectonics theory was proposed, it was believed that the earth is a solid motionless body. But the concept of seafloor spreading and plate tectonics have highlighted that the earth’s crust is not static but dynamic and constantly evolving due to relative motion of tectonic plates.

The movement of plates has been assigned various reasons by various scientists. Wegener suggested that the drifting in plates was due to ‘pole fleeing force’ generated due to rotation of earth or due to tidal forces generated by gravitational pull of sun and moon.

However, most of the scientists believe that these forces are too weak to generate any considerable motion in plates. Instead it is the convection currents in the mantle that are responsible for movement of plates. The temperature differences[1] in mantle are responsible for convection currents of hot magma beneath the earth’s crust. The mobile rocks in the mantle beneath the plates are believed to be moving in a circular manner. The heated material rises to the surface, spreads and cools, and then sinks back to the depths. This cycle is repeated and is called convection cell or convection flow. This slow movement of hot soft mantle beneath the plates is the force behind plate movement.

Sea-Floor Spreading

New plate material is generated along the mid- ocean ridges, where basaltic lava is poured out by submarine volcanoes. The theory of sea-floor spreading has demonstrated the way in which the basaltic lava spreads outwards away from the ridge crest at 1-6 cm/0.5-2.5 in per year. Plate material is consumed at a rate of 5-15 cm/2-6 in per year at the site of the deep ocean trenches.

Hess was first to propose this concept of sea floor spreading. He argued that constant eruptions along 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 thereby spreading the oceanic floor.

The younger age of oceanic crust[2] and the fact that spreading of one oceanic floor does not led to shrinkage of other made Hess argue that the oceanic crust gets consumed into the mantle i.e. as the oceanic floor spreads due to volcano at crest, the oceanic crest sinks down to get consumed.

For example, along the Pacific coast of South America. The trenches are sites where two plates of lithosphere meet; the one bearing ocean-floor basalts plunges beneath the adjacent continental mass at an angle of 45º, giving rise to shallow earthquakes near the coast and progressively deeper earthquakes inland. In places the sinking plate may descend beneath an island arc of offshore islands, as in the Aleutian Islands and Japan, and in this case the shallow earthquakes will occur beneath the island arc.