STRUCTURE OF THE EARTH The Ancient Greeks knew that the Earth was about 12,750 kilometres in diameter, but it was only in the 20 th century that geophysical studies showed that the Earth has several distinct layers, i.e. the crust, the mantle and the core, as shown in the diagram below. Structure of the Earth Wikimedia Creative Commons Attribution Share-Alike: CCASA Kelvinsong The Earth s Crust This is the outermost layer of Earth and is made up of the continents and the ocean basins. The crust varies in thickness from 35 to 75 km on the continents and 5 to 10 km on the ocean basins. Version 1: January 2014 Copyright My Cyberwall 2014 1
Under certain mountain ranges, such as the Alps, the crust is as thick as 100 km. The crust that forms the continents is made up of rock called granite and the crust under the oceans is made mostly of a rock called basalt. Granite Basalt The Earth s structure can be compared to a boiled egg, with the crust being the rigid and thinnest layer, like the shell of the egg. Like the eggshell, the crust is brittle and the layers can break. Diagram of the Structure of the Earth Version 1: January 2014 Copyright My Cyberwall 2014 2
The Mantle The mantle is the 2 nd layer and is a hot layer of semi-solid rock. It is made up of ferro (iron) magnesium silicates. The mantle is about 2 900 km thick and is separated into the upper and the lower mantle. Pressure increases as one travels into the mantle, towards the centre of the Earth. The mantle is where most of the internal heat of the Earth is located. Large convection cells in the mantle circulate heat and may cause tectonic plates to move. Comparing again to an egg, the mantle would be the egg white. The Core This is the last and inner layer and unlike the egg yolk, is divided into two parts, the liquid outer core and the solid inner core. The outer core is 2 300 km thick and the inner core 1 200 km thick. The outer core is made up of a nickel-iron alloy and the inner core is mainly iron. The Earth s magnetic field is controlled by the liquid outer core, which spins as the Earth rotates. The core has extreme temperatures and pressures, which keep the iron and metals liquid and flowing. The core is twice as dense as the mantle. Version 1: January 2014 Copyright My Cyberwall 2014 3
In addition to the layers of the Earth, based on its composition, the structure of the Earth is also divided into layers based on its mechanical properties, as shown in the diagram below: Diagram of Asthenosphere and Lithosphere Lithosphere This is the top layer which consists of the crust and the upper mantle. Lithos in Greek means stone. This layer has many plates which contain the Earth s continents and oceans and which move around due to tectonic forces. The upper part of the mantle is cooler and more rigid than the deep mantle. The lithosphere is thinnest under oceans and volcanically active areas. Asthenosphere This is a semi-liquid hot layer in the lower mantle, which is weaker than the lithosphere and therefore allows the lithosphere to move around. Asthenes in Greek means weak. Version 1: January 2014 Copyright My Cyberwall 2014 4
The Tectonic Plates The tectonic plates are huge masses of land on Earth which float across the surface of the mantle. A plate is a large, rigid slab of solid rock and the word tectonics comes from the Greek to build. If you put these 2 words together, plate tectonics refers to the how the surface of the Earth is built of plates. Plate tectonics is the term that is used for the theory and study of the structure of the tectonics plates and was developed in the 1960s. Plate tectonics helps us to understand why earthquakes, volcanoes and tsunamis occur, as well as how great mountain ranges like the Alps, Himalayas and the Andes were formed. Alps Mountain Range Wikimedia Creative Commons Attribution Share-Alike: Stevie-Ray78 Andes Mountain Range Wikimedia Creative Commons Attribution Share-Alike: Penarc Version 1: January 2014 Copyright My Cyberwall 2014 5
Himalayas Mountain Range Courtesy of NASA The Earth s lithosphere (outermost layer) is broken into the following 7 large plates: - Eurasian Plate - Indo-Australian Plate - Pacific Plate - North American Plate - South American Plate - African Plate - Antarctic Plate Version 1: January 2014 Copyright My Cyberwall 2014 6
There are also smaller plates, such as: Arabian Plate, Filipino Plate, Caribbean Plate, Philippine Plate, Juan de Puca Plate, Nazca Plate, Cocos Plate and Scotia Plate. The plates float on the magma and move up and over each other. This movement of the plates means that continents move. This is known as Continental Drift. The plates move both horizontally (sideways), and vertically (up and down). These plates are all moving in different directions and at different speeds, from 1 cm to 10 cm per year. Over long periods of time they also change in size. They are anything from 80 to 400 kilometres thick. Version 1: January 2014 Copyright My Cyberwall 2014 7
Before plate tectonics, Continental Drift explained the break up of the supercontinent Pangaea (meaning all lands ), over 200 million years ago, eventually resulting in the continents as we know them today. Map of Pangaea Showing Modern Day Continents Wikimedia Creative Commons Attribution Share-Alike:Kieff Go to the module to see an animation of the break up of Pangea: http://mycyberwall.co.za/get-smart/geography/grade-7/structure-earth Version 1: January 2014 Copyright My Cyberwall 2014 8
Plate Boundaries At places on Earth where 2 or more plates meet, a plate boundary is formed. These are called different names depending on the movement they make, i.e.: Crashing/crunching convergent boundary Pulling apart divergent boundary Side sweeping transform boundary Version 1: January 2014 Copyright My Cyberwall 2014 9
Diagram Showing the Various Tectonic Plate Boundaries Convergent Boundary Places where the plates crash or crunch together are called convergent boundaries. These plates only move a few centimetres a year, so collisions are very slow and last millions of years. Sometimes they result in huge mountain ranges or deep trenches. Version 1: January 2014 Copyright My Cyberwall 2014 10
As the plate digs into the Earth s hot interior, some of the rock in it melts. This rock rises, causing earthquakes on the way up and volcanic eruptions when it reaches the surface. When a thin, oceanic plate collides with a lighter and thicker continental plate, the oceanic plate is forced under the continental plate. This is known as subduction. When two land masses meet, neither will slide under each other. Instead, they will crush together, crumple and fold and this results in folded mountain ranges. An example of a convergent boundary is in South America, where the oceanic plate is crashing into the South American continent. This is formed in the Andes mountains in South America, the Himalayas in Asia and a deep trench in the Pacific Ocean near Japan called the Mariannas Trench. Aerial View of Andes Mountains Wikimedia Creative Commons Attribution Share-Alike: Howcheng Himalaya Mountains, Asia Map of Mariannas Trench Near Japan Wikimedia Creative Commons Attribution Share-Alike: Kmusser Version 1: January 2014 Copyright My Cyberwall 2014 11
Divergent Boundary These boundaries are found in places where the plates are pulling apart or moving away from each other. This creates earthquakes and volcanoes. When the Earth s surface area (lithosphere) is pulled apart, it breaks down parallel faults that tilt slightly apart from each other. As the plates separate, the block between the fault cracks and drops down into the soft interior, the asthenosphere. This causes a rift valley. Magma (liquid rock) comes up to fill the cracks and forms a new crust. Earthquakes occur along these boundaries and volcanoes form where the magma reaches the surface. When this happens on land, large rift valleys, between 30 and 50 km wide are formed, for example, the East African Rift Valley in Kenya and Ethiopia and the Rio Grande Rift in New Mexico. Version 1: January 2014 Copyright My Cyberwall 2014 12
The map below shows the location of historical volcanoes, as well as the Afar Triangle (shaded at the center) Thich is where three plates are pulling away from one another; the Arabian Plate and two parts of the African Plate (the Nubian and Somalian ), splitting along the East African Rift Zone. Map Showing Location of East African Rift Valley Rift Valley in Ecuador Wikimedia Creative Commons Attribution Share-Alike: Creationlaw Version 1: January 2014 Copyright My Cyberwall 2014 13
When two oceanic plates move away from each other, this is known as seafloor spreading. The rift valley that is formed will be much narrower than the one created by two land masses pulling apart, usually about 1 km wide. Examples of this are the Mid Atlantic Ridge and the East Pacific Rise. Plate separation on divergent boundaries is slow. For example on the Mid Atlantic Ridge, it is about 2 cm a year. Map Showing Mid-Atlantic Ridge Mid-Atlantic Ridge through Iceland Version 1: January 2014 Copyright My Cyberwall 2014 14
Transform Boundary In this type of plate boundary, the plates slide together in a sideways movement. They push against each other causing tension and then release this tension in a sudden, violent jerk, which causes an earthquake. These boundaries are marked in some places by features like stream beds that have been split in half and the two halves have been moved in opposite directions. The most famous transform boundary is the San Andreas Fault in California, USA. To the west of the fault is the Pacific Plate which is moving north west. To the east of the fault is the South American Plate, which is moving south east. Although transform boundaries do not have spectacular features, their sliding motion causes lots of earthquakes. The most famous earthquake on the San Andreas Fault was in 1906. It destroyed much of San Francisco and killed more than 600 people. Aerial View of San Andreas Fault Version 1: January 2014 Copyright My Cyberwall 2014 15
View of Destruction Caused by 1906 Earthquake in San Francisco Version 1: January 2014 Copyright My Cyberwall 2014 16