THE INTERNAL STRUCTURE OF THE EARTH

THE CRUST

The crust makes up only 0.5 % of the Earth's total mass and can be subdivided into two main parts: (i) continental/ upper crust/ sial (ii) oceanic/ lower crust/ sima The crust average between 5 km to 70 km in thickness. Both differ in thickness, density and composition. The continental crust is much thicker than the oceanic crust. Continental crust: It varies between 10 km - 70 km thick. The continental crust is less dense than oceanic crust. The continental consist chiefly of the minerals silicon and alumina, hence the continental crust is sometimes called SIAL. It is largely composed of volcanic, sedimentary and granitic rocks.,

Oceanic crust: It varies between 5 km - 10 km thick. Oceanic crust is primarily composed silica, iron and magnesium, hence it is sometime called SIMA. It is thinner than continental crust, or sial. It is generally less than 10 kilometers thick. The Oceanic crust is denser than the continental crust. The Moho The Mohorovicic discontinuity, usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. The Moho is a part of the lithosphere. The Mohorovicic discontinuity was first identified in 1909 by Andrija Mohorovicic, a Croatian seismologist. The lithosphere The lithosphere is a layer that includes the crust and the upper most portion of the mantle. This layer is about 100 kilometers thick. The lithosphere is also the zone where earthquakes, mountain building, volcanoes, and continental drift activity takes place THE MANTLE The mantle is almost 2900 kilometers thick and comprises about 83 % of the Earth's volume. The mantle is subdivide it into an upper mantle and lower mantle. There is also a zone in the upper mantle that we call the asthenosphere. It is located at depths of 70 km to 250 km. Most magmas (molten rock) are generated in this zone. The Upper mantle The outer mantle is a lot thinner than the inner mantle. It can be found between 10 km and 400 km below the surface of the earth. It makes up 10 % of the Earth's total mass. The bottom layer is tough liquid rock and is called in plate tectonics the asthenosphere, The rocks in this zone is close to their melting point. Lower Mantle: The lower mantle can be found between 400 km and 2,900 km below the earth’s surface. The lower mantle is 41 % of the Earth's total mass. The rock is solid because of the high pressures. THE CORE The inner part of the earth is the core. The core is a dense ball of the elements iron and nickel. It is divided into two layers, the inner core and the outer core. The outer core is so hot that the metal is always molten, The outer core is at 2,900 - 5,150 km below the earth's surface. The outer core is liquid and mainly consists of iron, nickel, sulphur and oxygen. Because the earth rotates, the outer core spins around the inner core and that causes the earth's magnetism. The inner core - the center of earth - 5,150-6,370 km below the earth's surface and mainly consists of iron, nickel and some lighter elements probably sulphur, carbon, oxygen, silicon and potassium. Because of the high pressure, the core is solid.

Plate Tectonics Plate tectonic is the theory that the Earth's lithosphere (the crust and upper portion of the mantle) is divided into about 7 large plates and several small ones that float on or travel over the asthenosphere. Each plate consists of rigid rocks. Where two plates converge (meet), a subduction zone forms. In these areas one plate is forced under another and into the Earth's mantle. N.B. The majority of the earthquakes and volcanoes on the Earth's surface occur along the margins of tectonic plates. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. Each plate is made up of two layers:

i. the crust ii. the rigid uppermost part of the mantle.

Types of plate boundaries Three types of plate boundaries exist: The different types of plate boundaries are:

1. Transform boundaries/margins: These occur where plates slide or grind past each other along faults. The motion is horizontal

2. Divergent boundaries/margins : occur where two plates slide apart from each other 3. Convergent boundaries/margins : occur where two plates slide towards each other.

Transform boundary (also known as transform fault boundary, transform plate boundary, transform plate margin, strike-slip boundary, sliding boundary, or conservative plate boundary)

A transform boundary occur when tectonic plates slide and grind against each other along a fault. The plate motion of is horizontal. Since the plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other, transform boundaries has no spectacu-lar features. Lots of earthquakes are generated at transform boundaries. Most transform boundaries are found on the ocean floor. However, the most famous transform boundaries are found on land. Many transform boundaries are locked in

tension before suddenly releasing, and causing earthquakes. In a transform boundary, neither plate is added to at the boundary, nor destroyed. Examples of transform boundary/margins include: 1. The San Andreas Fault in California, where the Pacific Plate meets the North American Plate and they grind past each other 2. The Alpine Fault in New Zealand It forms a transform boundary between the Pacific Plate and the Indo-Australian Plate. 3. the northern of the Caribbean Plate and the Southern edge of the North American Plate. Divergent boundaries

Places where plates are coming apart are called divergent boundaries. Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. As the plates separate along the boundary, the block between the spreading center cracks and drops down into the asthenosphere. The sinking of the block forms a valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface. Where a divergent boundary crosses the ocean floor, the rift valley is much narrower,

only a kilometer or less across, and it runs along the top of an oceanic ridge. Oceanic ridges rise a kilometer or so above the ocean floor and form a global network tens of thousands of miles long.

Examples include:

(a) the Mid-Atlantic ridge. is an underwater mountain range of the Atlantic Ocean and Arctic Ocean. The highest peaks of this mountain range form islands. The Mid-Atlantic Ridge forms part of the global mid-oceanic ridge system (b) The East Pacific Rise/ridge

(c) The Indian Ocean Ridge

Where a divergent boundary crosses the land, the rift valleys which form are typically 30 to 50 kilometers wide. Examples include: (a) the East Africa Rift Valley: is a vast geological feature, approximately, 6000 kilometres (3,700 mi) in length. It extends from Lebanon in the north to Mozambique in the south. The rift valley varies in width from thirty to one hundred kilometers, and in depth from a few hundred to several thousand meters. (b) The Rio Grande rift in New Mexico. (c) The boundary between the African Plate and Arabian Plate (the Red Sea) Plate separation is a slow process. For example, divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimeters per year. Sea-floor Spreading Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart (diverge). As the plates move apart, the rocks break and form a crack between the plates and magma rises through the cracks. As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor. This chain is called an oceanic ridge. The boundaries where the plates move apart are 'constructive' because new crust is being formed and added to the ocean. Earthquakes and volcanic activity occur along the plate boundary. There are two processes that cause sea –floor spreading. These include ridge-push and slab-pull. Ridge-push occurs when the weight of the ridge pushes the rest of the tectonic plate away from the ridge, often towards a subduction zone. At the subduction zone, "slab-pull" comes into effect. This is simply the weight of the tectonic plate being subducted (pulled) below the overlying plate dragging the rest of the plate along behind it. The other process contributing to the formation of new oceanic crust at mid-ocean ridges is "mantle convection ".

Convergent boundary/margin (destructive plate boundary) A convergent boundary is a region where two (or more) tectonic plates or fragments of lithosphere move towards one an-other. When two plates move toward one another, they either subduct or collide. In a subduction zone, the subducting plate (oceanic crust), moves beneath the other plate, (either oceanic or continental crust). At a collision zone two continental plates collide or converge. Oceanic-oceanic convergence - Subduction zone When two plates with oceanic crust converge they create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. As the subducting oceanic crust melts as it goes deeper into the Earth, the newly-created magma rises to the surface and forms volcanoes. If the activity continues, the volcano may grow tall enough to reach the surface of the ocean creating an island. A deep undersea trench is located in front of such arcs where the descending slab dips downward. Example The Marianas Trench, Good examples of this type of plate convergence would be Japan Islands, the Aleutian Islands in Alaska, the Lesser Antilles and the Solomon Islands

Oceanic-continental - Subduction zone

When one plate is composed of oceanic lithosphere and the other is composed of continental lithosphere, the oceanic plate is subducted. When an oceanic plate pushes into and subducts under a continental plate, the overriding continental plate is lifted up and a mountain range is created. Example the Andes Mountains ( South America) and the Rocky Mountains (North America). An oceanic trench is formed where the denser plate is subducted underneath the other plate. Volcanoes are formed fairly close to, but not right next to the trench. Example Peru-Chile Trench Continental - Continental - Collision Zone When two plates containing continental crust collide, both are too light to subduct. When two continents meet head-on, neither is subducted

because the continental rocks are relatively light and resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. A continent-continent collision creates large mountain ranges. This best example is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalayas. These areas are often called collision zones

Examples of convergent plate boundaries/margins (i) the collision between the Eurasian Plate and the Indian Plate which is forming the Himalayas. (ii) subduction of the northern part of the Pacific Plate and the NW North American Plate is forming the Aleutian Islands. (iii) subduction of the Nazca Plate beneath the South American Plate to form the Andes Mountains.

Major plates African Plate, - covering Africa - Continental plate Antarctic Plate, - covering Antarctica - Continental plate Eurasian Plate - covering Asia and Europe - Continental plate North American Plate - covering -North America - Continental plate South American Plate - covering South America - Continental plate Pacific Plate - covering the Pacific Ocean - Oceanic plate Indo-Australian Plate - Covering Australia and India - both Oceanic and Continental Minor plates include: Arabian Plate, Cocos Plate, Juan de Fuca Plate, Nazca Plate, Philippine Plate, Scotia Plate, Burma, Plate Caro-line Plate, Hellenic Plate, Iranian Plate, Mariana Plate, Sandwich Plate, Scotia Plate, Solomon Plate, Somali Plate, Bismarck Plate, Tonga Plate, Juan de Fuca Plate, Iraq Plate, Caribbean Plate

OCEANIC CRUST

OCEANIC CRUST

ASTHENOSPHERE

Ocean Trench

SOMALIA PLATE

ARABIA

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