STRUCTURE OF THE EARTH
The structure of the earth can be categorized into two:
1. The internal structure (inner layers)zones
2. The external structure (outer layers)zones
INTERNAL STRUCTURE OF THE EARTH (inner zones)
The earth consists of three concentric layers
1. The crust
2. The mantle
3. The core
The density of the materials in these layers increases towards the centre. This is because of the differences in pressure and temperature.
1. THE CRUST (LITHOSPHERE)
It is the solid outer and thinnest layer of the earth. It is between 8 to 50 km thick and largely composed of igneous rocks
It consists of silica and aluminium (sial). It forms the upper layer of the continents and is mostly composed of granite rock.
The Crust consists of two layers namely
(a) Sial
(b) Sima
Sial
Is the outermost layer which forms the continent. It is made of granite rocks with a density of about 2.65 to 2.70gms/cc
These rocks contain a large proportion of silica and aluminium hence they are collectively known as SIAL.
Sima
Is the layer beneath Sial which forms oceanic floor.It is made of heavy density balsatic rocks with about 2.9gms/cc
These rocks contain a large proportional of silica and magnesium, also contain iron(mafic crust).Sima has the temperature of about 1200⁰.
Sima is separated from Sial by the zone called Conrad discontinuity.The crust is separated by Mantle by the layer known as Mohorovicic discontinuity.
2. THE MANTLE (MESOSPHERE)
Is the second layer which is found below the crust.
It is composed of iron and manganese. It lies between the crust and the core. The mantle is made up of very dense rocks.
It’s thickness is 2900km. The temperature ranges between 5,000C and 7,0000C. The density of the mantle is 3 -3.5g/cm3.
The mantle is divided into two parts namely
(a) The Upper Mantle
(b) The Lower Mantle.
The Upper Mantle
is a solid layer which consists part of lithosphere. Below the Upper Mantle, there is a layer called asthenosphere, this layer is in semi solid state with the thickness between 100 to 200kms.Asthenosphere is the layer which separate between Upper and Lower Mantle. Mantle is separated from the core by a layer called Gutenberg discontinuity.
The Lower Mantle
Is the layer below upper Mantle mainly composed of silicon and magnesium,it contains of about 50% of the total mass of the planet.Below Lower Mantle is Core which is separated by a layer known as Guternburg Discontinuity.
THE CORE (BARYSPHERE) OR CENTROSPHERES
Is the inner most layer of the earth. It is composed of nickel and iron. Its diameter is about 6900km. The average density is about 5.2 g/cm3. The core is divided into the outer core and inner core. In the outer core rocks exist in molten state while in the inner core, rocks are in solid state.
Exercise
Draw a well labeled diagram of the concentric zones of the earth.
TYPES OF ROCKS OF THE EARTH’S CRUST
Meaning of “rock”
A rock is an aggregate of mineral particles in a solid state. The earth’s crust consists of rocks; and rocks consists of a combination of different minerals. All minerals are formed from one or more eight elements. These are oxygen, silicon, potassium, sodium, calcium, magnesium, iron and aluminium.
Types of rocks
Rocks are classified according to their mode of formation and their chemical composition. According to their mode of formation, rocks can be classified as follows
1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks
IGNEOUS ROCKS (ignis = fire( fire made rocks)
Are rocks formed when molten rocks cools and solidifies either within or outside the earth’s surface. This process is referred to as crystallization.
Crystals form when the rock is cooling and the rocks are called crystalline rocks, some igneous rocks cool quickly and contain small crystals eg basalt while others cool slowly and contain large crystals eg granite.
All igneous rocks originate inside the earth, where they are under great pressure. If a crack develops in the earth’s surface, the pressure forces some of it’s rocks materials into the cracks.
Two main types of igneous rocks
1. Plutonic or intrusive Igneous Rocks
Are formed when molten materials cool and solidify inside the earth’s crust .Sometimes they can be seen after been exposed by erosion. Intrusive rocks cool slowly and form large crystals and are characterized by coarse texture e.g. granite
2. Volcanic or extrusive Igneous Rocks
Are formed when molten materials cool and solidify outside the earth’s crust. Intrusive rocks cool quickly and form small crystals with fined texture (fine grained) e.g. basalt, pumice, tuff, rhyolite, andesite, phonolite.
Characteristics of igneous rocks
1. Igneous rocks reflect light
2. They are hard non-stratified (no layers)
3. They do not contain fossils
4. They are formed through cooling and solidification of magma.(crystalline rocks)
5. They can undergo metamorphism to form metamorphic rocks and weathering to form sedimentary rocks.
6. They contain different minerals like iron, magnesium etc.
7. They can be basic, acidic or intermediate depending on the amount of silica.
In Tanzania many igneous rocks are found in Dodoma, Iringa, in the shores of Lake Victoria (Mwanza). The main examples are granite, gabbro, basalt and diorite. Some are found in Kilimanjaro and Rungwe such as basalt, pumice, diorite, gabbro, syrenite and peridotite rocks.
SEDIMENTARY ROCKS (stratified rocks)
Are rocks formed due to erosion, transportation and deposition of sediments or from the remains of once living organisms such as animals and plants. The rocks are formed when the sediments or the remains of once living organism are accumulated, compacted and cemented together. The sediments are compacted by compression to form sedimentary rocks.
Characteristics of sedimentary rocks
1, They are formed when particles or sediments accumulate, compact and cemented together
2. They do contain fossils
3. They are stratified ( in layer (strata)
4. They do not reflect light
5. They are non-crystalline rocks
6. They can undergo metamorphism to form metamorphic rocks; and can melt then cool and solidify to form igneous rocks.
TYPES OF SEDIMENTARY ROCKS
There are three(3) types of sedimentary rocks, these are
Mechanically formed sedimentary rock
Are formed due to breaking and disintegration of rocks. When weathering agents erode and deposit rock particles, they are accumulated, compacted and cemented together to form sedimentary rocks. Examples of chemically formed sedimentary rocks are clays, gravels and alluviums (all deposited by water), moraines, boulder clay and gravels (deposited by ice) and loess (deposited by wind)
Chemically formed sedimentary rocks
Are formed through various chemical process. They include carbonation (as it is in stalactite) and stalagmite), sulphate, chloride etc.
The main examples include gypsum, rock salt, lignite, dolomite, flint, borax, limonite, hematite etc.
Organic formed sedimentary rocks
Are formed from the remains of once living organisms either plants or animals. The remains of living organisms are accumulated, compacted and cemented together to form these sedimentary rocks. The main examples are coral,chalk and limestone(formed from animal shells), peat, coal and lignite (formed from plants).
Metamorphic rocks
Are rocks which have been changed after been subjected to great temperature, pressure or both. This process is referred to as metamorphism. Any rock can be changed into a metamorphic rock. Examples of metamorphic rocks are marble, slate, gneiss, quartzite and graphite.
.
Characteristics of metamorphic rocks
1. They are very hard due to prolonged action of heat and pressure.
2. Any type of rock can be subjected to metamorphism to form metamorphic rock.
3. They can undergo weathering to form sedimentary rock and can melt, cool and solidify to form igneous rocks.
Types of metamorphism
(i) Contact or thermal metamorphism
It is metamorphism caused by intense heat.
(ii) Dynamic metamorphism (kinetic metamorphism)
It is metamorphism caused by intense pressure.
(iii) Regional (thermal dynamic (metamorphism)s
It is metamorphism caused by both heat and pressure
Examples
of metamorphic rocks
|
Original
rock |
Metamorphic rock |
|
Sedimentary Sandstone Limestone Dolomite Coal Clay
Shale Mudstone
|
Quartzite
Marble Dolomite
marble Graphite Slate mica
schist Slate
|
|
Igneous granite Basalt |
Gneiss
amphibole (amphibolite) |
|
Metamorphic graphite |
Crystalline
diamond |
Rock cycle
This is the cycle in which rocks tend to change from one type to another. For instance igneous rocks may change to metamorphic rocks or sedimentary rocks. Sedimentary rocks to igneous or metamorphic rocks. It is a relationship in which rocks tend to change from one type of rock to another.
Necessary conditions for rock cycle to take place
1. First the molten rocks erupt from the interior of the earth, then cool and solidify to form igneous rocks.
2. Secondly, the igneous rocks are subjected to denudation process (weathering) to form sedimentary rocks.
3. Third either igneous or sedimentary rocks undergo metamorphism due to great heat and pressure to form metamorphic rocks.
4. Fourth, metamorphic or igneous rocks can undergo weathering process through erosion and transportation of sediments which are further deposited in layers in the ocean or lake floors where they are cemented and consolidated to form sedimentary rocks and vice versa.
5. Fifth, metamorphic or sedimentary rocks can be subjected to heat and pressure where melting take place and later cooling due to low temperature, to form igneous rocks.
ROCK CYCLE
SIMPLIFIED GEOLOGICAL TIME SCALE (G.T.S)
The geological time scale is a chart for dating the history of the earth including rock span. It tries to explain the age of rocks as far back as 600 million years ago.
It is divided into three main eras
1. The ancient or Paleozoic
2. The middle or Mesozoic
3. The recent or Cenozoic
The period before Paleozoic era is the Precambrian period.
A simple geological time scale (G.T.S
Economic importance of rocks
1. Rocks are very important in the formation of soils, which can be used for agricultural production
2. Rocks are used for building purposes. Some rocks such as limestone, sandstone, gravels and sand are used for building houses and construction of roads etc.
3. Some rocks are used as source of fuel or energy such as coal and petroleum (mineral oil)
4. Limestone is widely used for cement manufacturing. In Tanzania cement is produced in Tanga, Mbeya, Mtwara and Wazo Hill in Dar es salaam.
5. Salt extraction. Salt usually originate from rocks bearing strata. For instance in Tunisia and Morocco there are large deposit of salts.
6. Manufacture of chemicals. Some rocks contain nitrate or phosphate while others have potash. This kind of rocks can be used for making dyes, fertilizers and medicines.
7. Mineral deposits. Mineral ores occur in veins of some rocks such as igneous rocks. The minerals are formed when the magma cools down. Valuable minerals extracted from rocks includes gold, lead, tin, silver, diamond, copper, zinc, aluminium, calcium and manganese.
8. Some rocks are so impressive such that they attract tourists to come and view them. In so doing, the country earns a lot of foreign exchange.
9. Some rocks are used for decoration of houses as ornaments or they are grinded to produce powder which is used for decoration.
10. Store underground water. Some rocks are source of ground water which can be drilled and used for domestic and industrial purposes.
