1.0: STRUCTURE OF THE EARTH

1.1: The earth’s crust, the mantle, the core and their respective characteristics

·         Identify the concentric zone of the earth

·         Account for variation in density and thickness of the concentric zones of the earth’s crust.

·         Describe characteristics of the earth’s crust, the mantle and the core

Concept of the earth’s structure

The earth’s structure is composed of two parts

      (i)            Internal structure

    (ii)            External structure

Internal structure is composed of three concentric layers

      (i)            Crust (lithosphere)

    (ii)            Mantle (mesosphere)

   (iii)            Core ( barysphere)

These layers are classified according to the basis of density of their rock.

The less dense rock float top of denser rock. For example, the heavier element like nickel and iron are settled down the core.

(i) The crust

-it is outermost and the thinnest zone of all zones.

- It has thickness of about 8 kilometres to 50 kilometres

- The average density is about 2.7gm/cc.

- it consists two sub layers which are SIMA and SIAL.

SIAL

-    continental part of the crust which is composed of granite.

-    The rock is composed of silica and aluminum.

SIMA

-          This layer is made up of basaltic rock rich in silica and magnesium

-          This layer is beneath with SIAL

The SIMA and SIAL is separated by layer called Conrad discontinuity.

(ii) The mantle

-          It lies between crust and core

-          It is made of dense igneous rocks of iron, silicon, oxygen, aluminum ad magnesium

-          It extends downward about 2900km

-          It consists of rock density about 3.0gm/cc to 3.4gm/cc

-          The mantle is divided into two part; upper mantle and lower mantle

-          The upper mantle combines with crust to form large layer called lithosphere.

-          Below of upper mantle form asthenosphere layer.

-          A layer separating mantle from the crust is called mohorovic discontinuity.

The core

-          This is inner part of the earth

-          It consists of nickel and iron

-          The core is under great pressure and and heat with average density about 10.5gm/cc

-          The core is divided into two parts, namely outer core and inner core.

-          A layer that separate between mantle and core is called Gutenberg discontinuity.

The outer core

-          It is about 2300 kilometres thick.

-          The temperature increases along the boarder of mantle and core

-          The outer core is liquid in nature.

-          The temperature is about 3700⁰C

The inner core

-          It is solid due to great pressure exerted by other layers towards the centre.

-          It has temperature about 5500⁰C and density about 16 -  17 gm/cc

-          The inner core has a diameter of about 2600 – 2700 kms

The external structure of the earth

- It is composed of atmosphere, hydrosphere and biosphere.

The atmosphere is mixture of gases which forms an envelope like cover which surround the earth.

Hydrosphere is water masses part of the external structure of the earth

Biosphere is composed of all living things like human being, plants, animals and micro organisms like bacteria.

The earth's atmosphere

The earth's atmosphere refers to the thin layer of gases held by gravitational force around the earth.

Gases percentage volume includes nitrogen 78.09%, Oxygen 20.95%, carbon dioxide 0.03% and argon and water vapour occupy 0.93%.

Layers of atmosphere

(i) The troposphere

-It is lower part that contains air mass about 75%

- The height ranges from 0 to 10 kilometres

- It is layer for rainfall formation

- Temperature decrease due to increase of height about 0.6°C at every 100m

- This layer is zone called tropopause

(ii) The stratosphere

-This is second layer of the atmosphere

-It extends between 10 to 50 kilometres from the earth surface

- There is ozone layer O₃ which absorbs utra violet radiation.

- The layer separate stratosphere is called stratopause

(iii)The mesosphere

This layer is above the stratosphere

-The height ranges between 50 to 85 kilometres from the Earth's surface.

- It is coldest layer. Temperature decreases to -90°C as altitude increases

- The temperature decrease because of strongest wind about 3000km/hr and absence of water vapour, clouds and dust which absorbs solar radiation.

- It is separated from the thermosphere by the layer called mesopause.

(iv) The thermosphere

-It is highest layer of atmosphere

- Temperature increases to 1500°C due to absorption of utra violet and x- rays radiation by atoms and molecules.

-The height ranges between 85 to 1000 kilometres

In this layer radio waves are reflected so it is very important for communication

1.2: Types of rock of the earth’s crust

-Rocks are natural solid materials made up of various minerals in different ratios.

Rocks differ in colour, texture, density, mode of formation and ability to resist erosion.

-They differ also in chemical composition, age, permeability and hardness.

Types of rock according to the mode of formation

         i.            Igneous rocks.

       ii.            sedimentary rocks.

     iii.            metamorphic rocks.

i. Igneous rocks

-The term ‘igneous’ comes from the Latin word “ignis” which means fire.

-Igneous rocks are rocks formed by the process of cooling and solidification of molten materials (magma or lava).

-Igneous rock can be formed either intrusive or extrusive.

i. Intrusive igneous rocks These are rocks formed when magma flows through vents or cracks then cools very slowly and solidifies before reaching the earth’s surface. Examples of rocks are granite, gabbros and diorite.

The intrusive rocks are divided in two types, namely hypabyssal intrusive igneous rocks and plutonic igneous rocks.

Hypabyssal intrusive igneous rocks

-These are types of igneous rocks formed when magma cools and solidifies within but near the earth’s surface.

-These rocks can be exposed by prolonged erosion on the earth’s surface. They are medium in size including rocks like granophyre, polyphones and dolerite.

These are rocks formed when magma cools and solidifies deep in the earth’s crust.

The rock has large particles due to slow rate of cooling. Examples of these rocks are granite, diorite, gabbro and peridotite 

ii. Extrusive igneous rocks

-These are rocks formed on the surface of the earth when magma erupts and reaches the earth’s surface as lava or fragmental ejecta then cools and solidifies relatively quickly. Examples of these rocks are basalt, pumice and rhyolite. In Tanzania, basalt, pumice and rhyolite are found in Kilimanjaro and Rungwe.

Characteristics of igneous rocks

(a) They are hard rocks

(b) They are crystalline as they do not occur in layers

(c) They do not contain fossils

(d) They can undergo metamorphism to form metamorphic rock or weathered to form sedimentary rocks

(e) They contain minerals like iron and magnesium

ii. Sedimentary rocks

-These rocks are formed by the deposition of eroded materials transported by moving water, wind, ice, ocean currents, waves and drifts.

-They are a result of continued weathering and erosion, transportation, deposition and lithification (compaction and cementation) of sediments.

Types of sedimentary rocks according to their mode of formation.

         i.            (i) mechanically formed sedimentary rocks

       ii.           (ii) organically formed sedimentary rocks

     iii.           (iii)  Chemically formed sedimentary rocks

i. Mechanically formed sedimentary rocks are formed by deposition of sediments from eroded materials of existing rocks. Examples sandstone, mudstone, shale, and boulder, clays.

ii. Organically formed sedimentary rocks

These are rocks formed from deposition of plants and animals remains. Examples of such rocks are limestone, chalk and coral from animals and peat; and coal and lignite from plants.

iii. Chemically formed sedimentary rocks are formed by deposition of solid substances from solution. for example, rock salt, gypsum dolomite, potash and nitrate gypsum.

-They also form part of the ingredients in inorganic fertilizer manufacturing as well as in building material industries.

Characteristics of sedimentary rocks

(a) They are stratified

(b) They contain fossils

(c) They can undergo metamorphism to form metamorphic rocks

(d) They are soft and well-jointed

(e) They are non-crystalline

iii. Metamorphic rocks is a type of rock formed when igneous and sedimentary rocks are subjected to great heat and pressure, they change their physical and chemical properties.

- The word “metamorphic” is derived from two Greek words ‘meta’ and ‘morph’. Meta means change and morph means form.

-So, metamorphic means to change form. Examples of metamorphic rocks include marble, which is derived from limestone

-Slate derived from clay

-Graphite from coal

-Quartz from sandstone

-Gneiss from granite

Types of metamorphic rock

         i.            (i) Thermal metamorphic rock

       ii.            (ii) Dynamic metamorphic rock

     iii.            (iii) Thermal-dynamic metamorphic rock

i. Thermal metamorphic rocks These are formed by intense heat. Formation of these types of rocks occurs when the existing rocks come into contact with magma or lava. These rocks change their form and character. Examples are limestone to marble and sandstone to quartzite.

ii. Dynamic metamorphic rocks These are formed by the influence of pressure exerted by the earth’s horizontal and vertical movements which also cause mountain formation. Examples are shale to schist and clay to slate.

iii. Thermal-dynamic metamorphic rocks These rocks are formed by the process that takes place as a result of the combination of intensive temperature and pressure. An example is when coal changes to graphite.

Characteristics of metamorphic rocks

(a) They are hard compared to all other types of rocks

(b) These rocks are sometimes found in strata, but in a crystalline form

(c) They are formed from other types of rocks due to the influence of intense heat and pressure

(d) The rocks can change into other types of rocks through melting, weathering and deposition.

A rock cycle

This is the process where a rock changes from one type of rock to another. This process is endless as no rock can remain unchanged forever.

For examples as described below in diagram

a)       (i) Igneous rock formed by melting of molten materials from the ground

b)   (ii) Igneous rock changes to sedimentary rock

c)   (iii) Igneous rock changes to metamorphic rock

d)  (iv) Metamorphic rock changes to sedimentary rock

e)  (v) Metamorphic rock melts to form igneous rock

f)  (vi) Metamorphic rock undergoes further metamorphism to form Metamorphic rock

g) (vii) Sedimentary rock undergo metamorphism to form metamorphic rock.

h)(viii) Sedimentary rock melts to form igneous rock.

Simplified geological time scale

A geological time scale is a chart used for dating the history of the earth, including its rocks. It attempts to show the age of rocks as far back as 600 million years before the present. The scale is divided into eras, periods and years.

Four main types of eras

(a) The ancient or Pre-Cambrian era

(b) The Paleozoic era

(c) The middle or Mesozoic era

(d) The recent or Cenozoic era

Importance of the geological time scale

                     i.            It reflects the ages of rocks by showing the time of their formation. It

                   ii.            helps in understanding how different features were formed

                 iii.            It enables prediction of crustal changes over time by observing the evolvement of old and young rocks

 Activity

1. With examples, discuss the major eras of the geological time scale

2. Why is the geological time scale important in the study of rocks development?

Economic importance of rocks