The Earth's Lithosphere-Documentary

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The user has requested enhancement of the downloaded file. THE EARTH’S LITHOSPHERE- Documentary By Prof. A. Balasubramanian University of Mysore 19-11-2011

Introduction

Earth’s environmental segments include Atmosphere, Hydrosphere, lithosphere, and biosphere. Lithosphere is the basic solid sphere of the planet earth. It is the sphere of hard rock masses. The land we live in is on this lithosphere only. All other spheres are attached to this lithosphere due to earth’s gravity. Lithosphere is a massive and hard solid substratum holding the semisolid, liquid, biotic and gaseous molecules and masses surrounding it. All geomorphic processes happen on this sphere. It is the sphere where all natural resources are existing. It links the cyclic processes of atmosphere, hydrosphere, and biosphere. Lithosphere also acts as the basic route for all biogeochemical activities. For all geographic studies, a basic understanding of the lithosphere is needed. In this lesson, the following aspects are included: 1. The Earth’s Interior. 2. The Lithospheric Plates . 3. The Plate tectonics. 4. The Earth’s Internal processes. 5. The Magma and its emplacements.

1. THE EARTH’S INTERIOR

The Earth’s lithosphere is composed of three major solid layers as outermost Crust, middle Mantle and the central Core. The Crust is the outermost layer of the earth on which all living organisms survive. This is a very thin layer. It is ranging from 5 km under the oceans to 100 km under the mountainous areas of continents. Usually, it’s about 40 km thick under the flat continents. The crust is made of many types of rocks and thousands of minerals. These rocks and minerals are made from just 8 elements. They are Oxygen (46.6%), Silicon (27.72%), Aluminum (8.13%), Iron (5.00%), Calcium(3.63%), Sodium (2.83%), Potassium (2.70%) and Magnesium (2.09%). The rocks present in the earth’s crust are solid, rigid and brittle in nature. They are also highly variable, including rocks of molten origin, rocks of sedimentary origin, and rocks that have undergone all sorts of structural and chemical alterations through metamorphism. The crust itself can be divided into two sub-layers. One kind of layer is found everywhere, under the oceans and also below the continents. This is called as the oceanic crust. This layer is dominated by relatively heavy, dark, dense rocks of “mafic” composition. Most of these mafic rocks are of volcanic in origin and are called “basalts.” This dense, heavy mafic layer is sometimes called the “SiMa” denoting its chemistry as silica and magnesium. It tends to be relatively thin, usually from about 5-12 km in thickness. A second layer is normally found in the continents. It is made up of light colored rocks. These rocks are primarily composed of silicates enriched in lighter elements, such as aluminum (Al), potassium (K), and sodium (Na). This layer is called as “SiAl” as it is dominated by silicate rocks with lighter elements mixed with aluminum. These rocks are granitic masses and hence, this layer is called as the granitic layer. This is considerably thicker, around 40 km, than the basaltic lower layer,

Below this crust, the earth’s solid constituents have shown a transition. The density of the mass is very high and very rigid. Seismic soundings have identified a discontinuity between the crust and this layer. This is the mantle layer of the earth. There is a sharp increase in the velocity of seismic waves as they pass into this layer of differing density and rigidity. The Mohorovicic discontinuity (often called, simply, “the Moho”) marks the transition from the bottom of the crust to the top of this mantle layer. Andrija Mohorovicic first noticed this effect in the year 1909. He found that some of the earthquake waves near the surface, moved slower than the earthquake waves that passed through the interior of the Earth. He also noticed that the P (primary, first and strongest) waves that passed through the interior of the Earth, did move in a straight line. These waves were bent or deflected by something. He decided that the outside layer of Crust was made of less dense material (Rock) and the mantle. The Mantle was much denser. Waves of all other kinds move faster and straighter through this denser, more solid layer. Based on this observation, the nature of the mantle was identified.

The Earth’s Mantle

The middle, mantle, layer makes up the largest volume of the Earth’s interior. It is almost 2900 kilometers thick and comprises of about 83% of the Earth’s volume. It is divided into two layers as upper mantle and lower mantle. The upper mantle is about 670 kilometers in depth. It is brittle and less dense. It is made up of peridotites. These are rocks made up of olivine and pyroxene minerals. These are largely silicate minerals and the rocks are basic in character. These rocks are highly enriched with iron and magnesium, and hence they are called as “ultramafic” rocks. These ultramafic rocks are dark in color due to the presence of iron and magnesium. These rocks are extremely heavy and dense compared with the typical surface rocks. The rocks in the upper mantle are more rigid and brittle because of cooler temperatures and lower pressures. The Lower Mantle is much thicker and denser. It is 670 to 2900 kilomteres below the Earth’s surface. This layer is hot and plastic. The higher pressure existing in this layer causes the formation of minerals that are different from those of the upper mantle. The mantle varies in its state of matter. It is soft and in nearly liquid condition near its inner boundary with the liquid outer core and again near the top, a few kilometers under the earth’s crust. In other areas, it may show nearly brittle condition and solidity.

The Earth’s Core:

The Earth’s central Core contains two different layers as Outer Core and Inner Core.

The Outer Core is a hot liquid layer and the Inner Core is a hot and solid layer.

Beno Gutenberg discovered the boundary as a discontinuity between the mantle and the outer core. This boundary was named after him, as Gutenberg discontinuity. The outer core is at 2890-5150 km below the earth’s surface.

The temperature in the outer core is about 4000-50000C. The molten, liquid iron in the outer core is important because it helps to create the Earth’s magnetic field. The outer core is about 2250 km thick. The outer core is known to exist in a liquid state because of the behavior of earthquake waves, particularly shear body waves or secondary waves. Liquid cannot respond to shear forces, so it can’t transmit shear waves. As a result, there is a seismic shadow on the side of the earth antipodal to an earthquake’s epicenter.

Solid Inner Core

The inner core is 5150-6370 km below the earth’s surface. It mainly consists of iron, nickel and some lighter elements , probably sulphur, carbon, oxygen, silicon and potassium.

The temperature in the inner core is about 5000-60000C. Because of the high pressure, the inner core is solid. The solidity of the inner core is due to the presence of iron and nickel. The core is incredibly hot in the centre and the pressure is so great that the melting point of iron and nickel is elevated far beyond those high temperatures (6,500 K), leaving the nickel-iron as solid.

2. THE LITHOSPHERIC PLATES

The lithosphere is broken up into tectonic plates. There are currently seven or eight major and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries as convergent( or collisional )boundaries, divergent boundaries(also called as spreading centers); and conservative transform boundaries.

Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest. Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere.

Tectonic plates probably got developed in the earlier period of the Earth's 4.6-billion-year old history. They have been drifting on the surface ever since-like slow-moving bumper cars repeatedly clustering together and then separating each other.

The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents.

The supercontinent Columbia or Nuna was formed during a period of 2.0–1.8 billion years. It got broken up about 1.5–1.3 billion years ago.

The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents. It also got broken up into eight continents around 600 million years ago.

The eight continents later re-assembled into another as a supercontinent called as Pangaea. This Pangaea also got broken into two units as 1) Laurasia (which became North America and Eurasia) and 2) Gondwana (which became the remaining continents).

The Major plates of the earth are called as primary plates:

 African Plate  Antarctic Plate  Indo-Australian Plate, sometimes subdivided into: o Indian Plate o Australian Plate  Eurasian Plate  North American Plate  South American Plate and  Pacific Plate.

The notable Minor plates are called as secondary plates:

 Arabian Plate  Caribbean Plate  Juan de Fuca Plate  Cocos Plate  Nazca Plate  Philippine Sea Plate and  Scotia Plate.

There are also some tertiary plates which are included with the major plate families.

1) The African Plate encompasses Madagascar Plate, Nubian Plate, Seychelles Plate, and Somali Plate. 2) The Antarctic Plate including Kerguelen microcontinent, Shetland Plate and South Sandwich Plate. 3) The Caribbean Plate including Panama Plate and Gonâve Microplate. 4) The Cocos Plate including Rivera Plate 5) The Eurasian Plate including Adriatic or Apulian Plate, Aegean Sea Plate (or Hellenic Plate), Amurian Plate, Anatolian Plate, Banda Sea Plate, Burma Plate, Iberian Plate, Iranian Plate, Molucca Sea Plate , Halmahera Plate, Sangihe Plate, Okinawa Plate, Pelso Plate, Sunda Plate, Timor Plate, Tisza Plate and Yangtze Plate. 6) The Indo-Australian Plate including Australian Plate, Capricorn Plate, Futuna Plate, Indian Plate, Kermadec Plate, Maoke Plate, Niuafo'ou Plate, Sri Lanka Plate, Tonga Plate and Woodlark Plate. 7) The Juan de Fuca Plate including Explorer Plate and Gorda Plate. 8) The North American Plate including Greenland Plate and Okhotsk Plate. 9) The Pacific Plate including Balmoral Reef Plate, Bird's Head Plate, Caroline Plate, Conway Reef Plate, Easter Plate, Galapagos Plate, Juan Fernandez Plate, Kula Plate, Manus Plate, New Hebrides Plate, North Bismarck Plate, North Galapagos MicroPlate, Solomon Sea Plate and South Bismarck Plate. 10) The Philippine Sea Plate includes Mariana Plate and Philippine Microplate. 11) The South American Plate including Altiplano Plate, Falklands Microplate, and North Andes Plate.

3. PLATE TECTONICS

The theory of “plate tectonics” is the most important advancement in earth sciences in the 20th century. It provides the framework for all understanding of the earth’s processes that are happening today and also which have happened in the past.

In 1915, a Bavarian scientist named Alfred Wegener noticed, while working near the North Pole, that his compass needle did not point to where north "should" have been. In other words, true north and magnetic north were in two separate localities.

Wegener conceptualized that the poles of the earth (both North and South) were "wandering" with time. He called this as "Polar Wandering".

Subsequent to his first observation, he began to also notice how continents fit together like a jigsaw puzzle…most fittingly the western coast of Africa and the eastern coast of South America.

In addition, rocks from these localities were also of the same type, same age, and type of fossils. He revised his theory and called it as "Continental Drift", due to the idea, that it was not the poles that got shifted, but the continents themselves have moved apart.

Wegener died of a heart attack on a voyage studying glaciers near the North Pole in the early 1930's and his work was virtually forgotten.

After the World War II, Echo Sounding technology was developed. It provided the opportunities to carry out stunning discoveries by a geologist and seaboat commander, Harry Hess.

He noticed that rocks on either side of a prominent geologic feature, in the middle of the Atlantic Ocean were a perfect mirror image of each other on either side of the rift zone.

This was called as the mid-oceanic rift zone. He found that the rift zone was oozing out magmatic material from submarine volcanoes and that the material got spread laterally across either sides of the rift.

As time progressed, Hess took more and more samples to back up his findings. He did it as part of a series of drill voyages aboard using the research vessel, Glomar Challenger.

Later in the 1960’s, Frederick Vine and Drummond Matthews, a professor/student team, discovered the principles of magnetic patterns on the ocean floor.

They published their findings in journal first and then delivered lectures at Scripps Oceanographic Institute.

Further findings revealed that not only did the stripes have the same age of rocks, but a magnetic polarity image resulted in addition to it.

This has shown that during the earth history, there have been several "magnetic reversals" occurred in which the compass needle have pointed towards south instead of north, in several places. In the late 1960's and early 1970's, two scientists, revisited Wegener's findings.

They combined the observations along with Hess' discoveries to formulate a new package called as "Plate Tectonics".

Robert Palmer and Donald Mackenzie are credited with naming and synthesizing them into one common theory of “plate tectonics”. Alfred Wegener was referred to as the "Father of Plate Tectonics".

The plate motion is driven by one or more of the following mechanisms: 1. Convection -- heat transferred by movement of a fluid (magma) 2. Conduction -- heat transfer by touching plates 3. Push-Pull Slab -- heavy slabs pull plates downward and magma forced upward pushes plates to the surface (upwelling.)

There are several geological processes that occur where plates meet each other. They are called as plate boundaries or margins. The notable processes are: 1. Volcanoes tend to erupt at plate margins as a result of a process called subduction 2. Earthquakes occur where plates grind against or over one other 3. Mountain building occurs as one plate is pushed over another 4. Seafloor spreading occurs where two oceanic plates pull apart.

4. EARTH’S INTERNAL PROCESSES

Earth’s internal processes are controlled by the geothermal energy of the earth’s interior and the plate movements. Innumerable number of other tectonic processes happen in association with the plate movements.

The Convergent plate movement is associated with the following: a. Compression b. Reverse faulting c. Creation of a subduction zone d. Mountain building processes e. Three modes of Collisions of plates as: 1. Continent vs. continent 2. Continent vs. oceanic 3. Oceanic vs. oceanic.

The Divergent plate boundaries are associated with the following: a. Tension or extension (pulling apart) b. Normal faulting c. Rifting (as in the mid-oceanic rift zone) d. Creation of magma material inside the rift zone

The Transform plate boundaries are associated with the following: a. Horizontal grinding motion b. Strike-slip faulting c. Lateral offset of rock units.

Both continental and oceanic origin Volcanic Zones are associated with Plate Tectonics.

The Seismic (Earthquake) Zones are also associated with Plate Tectonics. They exist in three zones as: 1. Subducting oceanic plate; shallow focus as plate subducts. 2. Intermediate focus earthquakes; partial melting and rising of magma; "Benioff Zone". 3. Deep focus as slab of crust is pulled by sheer gravity.

The following are the features or landforms associated with Plate Tectonics Activities: 1. Subduction zone: this is the continent vs. oceanic collision zone containing a. Deep sea trenches b. Volcanic arcs c. Andesitic volcanic rocks 2. Divergent zone: mainly oceanic a. Basaltic magma b. Spreading centers 3. Ocean vs. ocean collision zone: a. Deep sea trenches b. Volcanic island arcs c. Basaltic volcanic rocks 4. Continent vs. Continent collision lead to a. Granitic rocks b. Mountain building processes c. No volcanism or d. No magmatic activities.

5. THE MAGMA AND ITS EMPLACEMENTS

Emplacement of magma from the earth’s interior is another effect. Magma is a fully or partially molten rock mass of the earth’s interior.

It is usually consisting of silicate liquid. Magma migrates either at depth or to the Earth's surface, where it is ejected as a lava. A magma is characterized by the interactions of several physical properties.

These properties include: a) chemical composition, b) viscosity, c) content of dissolved gases, and d) temperature of the molten mass.

The properties of magma are: 1) Temperature 2) Density 3) Viscosity 4) Gas content 5) Abundance. 6) Chemical composition- major and minor elements.

Magma gets solidified and form the igneous rocks. This process is called as crystallization process.

Because oxygen and silicon are the two most abundant elements in magma, it is convenient to describe the different magma types in terms of their silica content (SiO2). There are three types of magma as recognized as: 1. Mafic Magma 2. Felsic Magma 3. Intermediate magma.

The mafic magma have relatively low silica and high Fe and Mg contents. Mafic magma will cool and crystallize to produce the volcanic rock basalt.

The felsic magma are characterized by relatively high silica and low Fe and Mg contents. The felsic magma will crystallize to produce dacite and rhyolite.

The Intermediate-composition magmas will crystallize to produce the rock andesite. Because the mafic rocks are enriched in Fe and Mg, they tend to be darker in color than the felsic rock types which are lighter in color.

If we look at the evolution of magma, there are two stages of development as 1) Primary melts and 2) Parental melts. Primary melts are derived from the mantle. Others are derived from the bodies developed from primary melts. It is very difficult to fully understand the scientific mysteries behind many of these internal processes.

Magma is the source of all igneous rocks.

Emplacement of magma could be seen from the present day volcanic eruptions. A magma has enormous thermal and chemical energy.

The magmatic melt is less dense than its source rock and hence it is propelled upward through the lithospheric layers. Not all the magma reaches the surface. Some may intrude and gets solidified well below the surface.

On an average, 60 of the earth’s 550 historically active volcanoes are in the eruption process every year.

The rate of increase in temperature of the earth’s interior is knows as geothermal gradient.

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. It varies considerably from place to place. Away from tectonic plate boundaries, it is 25–30°C per km of depth. Away from active volcanic centres, the average gradient is nearly 300C per km.

If this downward rate of increase continued uniformly, the temperature at which basaltic rocks would melt at 1050oC at a depth of about 35 km.

It is also controlled by the thermal conductivity of the rock masses. There is also a heat flow generated by the radioactive elements in various rock types.

The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. Heat flows constantly from its sources within the Earth to the surface. Heat from Earth's interior can be used as an energy source, known as geothermal energy.

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