STRUCTURE OF THE EARTH

structure of the earth

The Earth consists of four concentric layers: inner core, outer core, mantle and crust. The crust is made up of tectonic plates, which are in constant motion. Earthquakes and volcanoes are most likely to occur at plate boundaries.

The structure of the Earth

 The Earth is made up of four distinct layers:

•          The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C. With its immense heat energy, the inner core is like the engine room of the Earth.
•       The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel. It is still extremely hot, with temperatures similar to the inner core.
•          The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt.
•          The crust is the outer layer of the earth. It is a thin layer between 0-60 km thick. The crust is the solid rock layer upon which we live.
•    There are two different types of crust: continental crust, which carries land, and oceanic crust, which carries water.
•    The diagram below shows the structure of the earth. In geography, taking a slice through a structure to see inside is called a cross section

Distribution

•       The Earth's crust is broken up into pieces called plates. Heat rising and falling inside the mantle creates convection currents generated by radioactive decay in the core. The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart. Where convection currents converge, plates move towards each other. The movement of the plates, and the activity inside the Earth, is called plate tectonics.
•     Plate tectonics cause earthquakes and volcanoes. The point where two plates meet is called a plate boundary. Earthquakes and volcanoes are most likely to occur either on or near plate boundaries. 

Different plate boundaries

•          At a tensional, constructive or divergent boundary the plates move apart.
•          At a compressional, destructive or convergent boundary the plates move towards each other.
•          At a conservative or transform boundary the plates slide past each other.
• Different exam boards and textbooks may use different names for each of the boundary types. For example, a destructive boundary may also be called a collision boundary. Use any term so long as you use it correctly, but it is best to stick to the terms you have been taught.

The diagram above shows the zone of subduction, oceanic ridge and an oceanic trench.

rocks

Ø  There are three main classes of Rocks. They are classified according to how they originated.

Ø   Igneous rocks form from cooling bodies of magma.

Ø  Over time, various weathering processes erode these rocks and the resulting particles or chemicals settle into beds and are compressed and cemented into sedimentary rocks.

Ø  If these rocks are buried, heated and highly compressed they will be made into metamorphic rocks.

Ø  If these rocks continue to be heated and compressed to the point that they melt, then the molten rock might eventually form another igneous rock.  This is called the rock cycle.  It forms a complete circle as one rock can be turned into another.  They can even form different rocks of their own class.  A sedimentary rock such as a sandstone can be weathered and eroded and those fragments might eventually end up as part of a shale, a different sedimentary rock.

Ø  Understanding rocks, their origins and classifications, is important to understanding the minerals that are found in them, found on them or are a part of them. Below are the major types of rocks: Rocks are made of minerals. They can be made of a single mineral or a combination of several minerals.

There are three main types of rocks:

·         Igneous (IG-nee-us) rocks are formed from hot, molten rock, called magma. Granite and basalt are examples of igneous rocks. These are first stage rocks or primary rocks because they were the first to be formed. These rocks are divided into plutonic and volcanic also known as intrusive and extrusive.

Ø  Plutonic/ Intrusive rocks are formed through the solidification of magma inside the earth’s crust. These rocks have large crystals and take time to solidify.

Ø  Volcanic/ Extrusive rocks are formed due to the solidification of magma outside of the earth’s crust. These have small crystals and quick to solidify.

·         Sedimentary rocks are formed from bits and pieces of rock which have broken off. These bits and pieces are called sediments. The sediments were carried by water and wind and dropped to a new location. The sediments are piled layer upon layer and pressed or cemented together to form a new kind of rock. The most common sedimentary rocks are sandstone, limestone, shale, and conglomerates. These are secondary rocks because they were formed from the sediments rom weathered material from other rocks.

Ø  These rocks are divided into mechanically, organically and chemically formed types.

Ø  Mechanically formed

Ø  These have been formed through the sedimentation of fine silt (sand) to formed layered rocks which are hard examples include sand stone.

Ø  Organically formed these were formed from the sedimentation of plant and animal remains that have been cemented by weather elements.

Ø  Rocks that originate from plants include coal and those that originate from animal include chalk.

Ø  Chemically formed rocks are as a result of chemical reaction of minerals that have been deposited by water which were dissolved from other rocks to form other rocks these include potash and gypsum.     

·         Metamorphic rocks are igneous or sedimentary rocks which have been changed by heat, pressure or chemical action. Examples of metamorphic rocks are slate which is formed from shale, and marble which is formed from limestone.

FOLDING AND FAULTING

• v  Folding is the bending of the earth’s crust due to compressional forces to form up-folds and down-folds called anticlines and synclines respectively
• v  Faulting is the cracking or breaking of the earth’s crust or sedimentary rocks due to compressional and tensional forces.

A geological fold occurs when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Syn-sedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales