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UNIT TWO: Structure and Composition of the Earth
2.1. Structure of the earth
.
We now know that Earth is comprised of layers.
–The Crust.
–The Mantle.
–The Core.
– Crust
Continental
Oceanic
–Mantle
Upper
Lower
–Core
Outer – Liquid
Inner – Solid
2.1.1. Crust, mantle and core
The Crust
 The outermost “skin” of Earth with variable thickness.
 Thickest under mountain ranges (70 km – 40 miles).
 Thinnest under mid-ocean ridges (3 km – 2 miles).
 Covers 1% of the thickness of the earth
 The only portion of the earth that man has direct
knowledge and accessibility.
 The crust is made up of silicate minerals, which is composed of
silicon and oxygen.
 The components are made from lighter substances compared to
mantle and core.
 Therefore, Crustal material contains lighter elements like Si, O,
Al, Ca, K, Na, etc...
 Feldspars (Anorthite, Albite, and Orthoclase) are common
minerals in the crust (CaAL2Si2O8, NaALSi3O8, and KALSi3O8).
Generally Crustal Composition is that:
 98.5% of the crust is comprised of just 8 elements.
 Oxygen is (by far!) the most abundant element in the crust.
 This reflects the importance of silicate (SiO2-based) minerals.
 As a large atom, oxygen occupies ~93% of crustal volume.
Two Types of Crust
Continental crust
 Underlies the continents.
 Covers 45% of the crust
 Avg. rock density about 2.7 g/cm3.
 Avg. thickness 35-40 km.
 Made from Sial (silica- alumina). Sial makes the rock granite
 Felsic composition. Avg. rock type = Granite
Oceanic crust
 Underlies the ocean basins.
 Covers 55% of the crust
 Density about 3.0 g/cm3.
 Avg. thickness 7-10 km.
 Made from Sima (silica- magnesium). Sima makes the
rock basalt
 Mafic composition
 Avg. rock type = Basalt/Gabbro
The transition b/n crust and upper mantle is called
Mohorovičić discontinuity or “Moho”. It is named
after a Yugoslavian seismologist known as Andrija
Mohorovicic. Who first identified it in 1909.
Mantle
A compositional layer between the crust and the core.
It is made from ultramafic minerals (magnesium and ferrous)
minerals particularly olivine.
The mantle is made up of Si and O, like the crust, but it
contains more Fe and Mg.
Thus,
Olivine (Fe2SiO4-Mg2SiO4) and
pyroxene (MgSiO3-FeSiO3) are abundant in the mantle.
The thickness of the mantle is about 2,885 km; the mantle is
82% of Earth’s volume.
Below ~100-150 km, the rock is hot enough to flow.
It convects: hot mantle rises, cold mantle sinks.
Three subdivisions: upper, transitional, and lower.
The upper mantle’s high temperatures of 2,800–3,200 °C can
melt rocks.
The semi-solid layer in the upper part of the mantle is called
the asthenosphere.
The asthenosphere is a solid that flows like a liquid.
This physical property is called plasticity. Scientists believe
that the lithosphere and the crust are able to move slowly over
the top of the mantle and asthenosphere.
Lower mantle (also called mesosphere): 660 km - 2900 km,
strong ultramafic rock
 Based on composition and physical properties Mantle
categorized in to two. Viz:
Upper Mantle (lithosphere)
Lithosphere provides plates that cause continental drift
 Upper mantle begins at the base of the crust
 Contains part of the lithosphere (or rigid sphere), the
asthenosphere (or weak sphere, soft, easily
deformable)
 Gradual increase in pressure and temperature
Lower Mantle (asthenosphere)
 Asthenosphere = weak sphere
– Near to molten rock
– It is a source of energy for most of the tectonic processes.
– Begins at 670 km discontinuity, ends at the core‐mantle
–
–


boundary
Gradual increase in pressure and temperature and seismic
velocities
Different from upper mantle due to an increase in temperature
and in seismic velocities
At the bottom of asthenosphere low velocity zone. P waves
reduce velocity S waves cease to exist.
Gutenberg discontinuity separates mantle from the core
The Core
A compositional layer found at the central position of the
earth.
What the core made of mostly is iron; it is high density and
common in the solar system (one of the most abundant after H
& He)
An iron-rich sphere with a radius of 3,471 km.
It accounts 17% of the earth’s volume.
Mainly, it is composed of Nife (Nickel and Ferrous)(dense
minerals)
Little knowledge due to its remoteness
Generates earth’s magnetism
Extreme pressure due to compression
Based on composition core categorized in to two. Viz:
Outer core
Liquid iron-nickel-sulfur
Density – 10-12 g/cm3
The outer core is less dense than the inner core and, therefore,
is located around the inner core.
The temperatures range from 4,000 °C to 5,000 °C.
The outer core is approximately 2,200 kilometers thick and is a
combination of mostly molten Iron (Fe) and Nickel (Ni).
Molten describes materials that change to liquid form when
exposed to extreme amounts of thermal energy.
Inner core
Solid iron-nickel alloy
Density – 13 g/cm3
Very extreme pressure
The deepest layer in Earth is the inner core.
It is located at the center of Earth because it contains the
densest material of all of Earth’s layers.
The inner core is solid and mostly composed of the element
iron (Fe). It’s extremely hot temperature is estimated at 6,000
°C.
This layer is approximately 1,250 km thick.
2.2. Composition of the earth
2.2.1. Rock forming minerals
Meaning of rock forming minerals
How many minerals do you think exist in the crust of the Earth?
Geologists have discovered more than 3000 mineral species been
in the Earth, but all of them are not of common occurrence.
In fact more than 99% of rocks of the crust are made up from
only 20 minerals and each rock being composed of two or three
minerals or so.
Many rocks contain only one mineral in abundance, and few
others being present only in small accessory amounts.
Such common minerals, which constitute the bulk of the crust of
the Earth, are called Rock forming minerals.
How do we classify minerals????
In most case, however minerals are classified by the dominant
element they contain.
For example,
minerals that contain silicon-oxygen complexes are called
silicates,
those which contain sulfur ions, called sulfides, and
those which contain carbonate are called carbonate minerals.
Thus, rock- forming minerals may be broadly classified as
silicate and non silicate minerals.
Silicate minerals are those minerals whose dominant
constituents are silicon with different proportion of
oxygen and other elements.
 Most of rock forming minerals are composed largely
of oxygen and silicon with Aluminium and at least one
or more of the abundant eight elements.
A.
 Silicates are the most abundant mineral groups, and
because of their structure, the varied and the complexes of
the mineral groups.
 These silicate minerals are again subdivided into groups
on the base of the arrangement of the elements that are
bounded together.
1. Feldspars are the most abundant rock forming minerals
contain almost half of the Earth’s crust.
 It is found in more kinds of rocks than other minerals.
 They are major constituents of igneous rocks.
 It may present in various colours such as light, white, pink, gray etc.
2. Flow over the asthenosphere over geologic time. The lithosphere is
also the zone of Mica Groups
Mica:
are again complex
containing K, Mg, Fe and etc.
hydrous
silicates,
They have sheet layers; thus, cleaves into their flexible
layers.
They occur in rocks in the form of thin, shiny, and flat
plates.
3. Pyroxene Group: A complex group of silicate minerals
containing different proportion of Ca, Ma, Fe, etc.
It is known by dark colors, and hardness.
It has many verities of minerals the commonest
variety being Augite.
It is found in many igneous and metamorphic rocks. It has poor luster.
B. Non- silicate Minerals: are minerals without silica elements.
They are classified on the base of their chemical
composition as.
a)
Oxides: This group of minerals is composed of one or more
metallic elements combined with oxygen, water, or hydroxyl
(OH).
The minerals in this mineral group show the greatest variations
of physical properties.

Some are hard, others soft.

Some have metallic luster; some are clear and transparent.
Some representative oxide minerals include the following ones.
I. Magnetite: magnetite iron oxide (Fe3O4)
 Both its color and streaks are black
 Its hardness ranges from 5.5-6.5
 Commonly occurs in igneous rocks in the form of small
crystals.
 When in sufficient concentration, it is an important source of
iron ore.
II. Hematite: iron oxide (Fe2O3)
 Reddish-brown to black or gray in color; generally present in
association to metamorphic rock and igneous rocks and chiefly
used as source for extracting iron.
III. Chromites: oxide of iron and chromium (Fe2Cr2O4)
 Occurs in ultra-basic igneous rocks
 Chief source of chromium used in the making of stainless
steel
IV. Bauxite: hydrated and aluminium oxide (Al2O.2H2O)
 Found as residual deposits and it is formed by the
weathering of aluminium--bearing igneous rocks under
humid, tropical conditions.
b. Carbonate minerals: They are mainly found in
sedimentary rocks.
 The carbonates group consist of minerals, which contain one
or more metallic elements chemically associated with the
compound CO4.
 Most carbonates are lightly coloured and transparent when
relatively pure.
 All carbonates are soft and brittle.
Some common carbonate minerals include:
I. Calcite (calcium carbonate-CaCO3)-the most abundance
mineral in the crust other than silicate mineral and is chief
constituent of limestone.
II. Dolomite (carbonate of ca and mg)-[CaMg(CO3)2]: Easily
dissolved in water, but relatively resistant in dry climates.
III. Sulphides:
notable
example,
being
(CaSO4.2H2O) hydrate sulphate of calcium.
gypsum
Most ores of important metals such as mercury
(cinnabar-HgS) and lead (galena-PbS) are extracted
from sulphides.
2.2.2. Physical properties of minerals
Physical properties refer to the characteristics of a certain minerals
that can be seen and measured.
Specific gravity: the ratio of substances weight to
the weight of an equal volume of pure water.
Metals have high Specific gravity b/c atoms are
closely arranged.
But non-metals with loose atmospheric structure
have smaller Specific gravity.
2. Cleavage: it is a line along which minerals break.
Such a break forms smooth, flat surface is called
cleavage plane.
1.
3. Fracture: minerals that do not cleave b/c their bonds are
equally strong in all directions and are distributed
uniformly throughout the crystal will breakdown or
fracture.
4. Colour: show the external or partial expression of minerals.
It is the weakest characteristics of minerals
5. Streak: a colour of mineral in its powdered form.
Metallic minerals have a dark streak where as
Non-metallic minerals yield a white powder.
6. Cluster: It describes how a mineral surface reflects light.
Metals display a shiny metallic luster.
Non metallic clusters are more poorly earthly, pearly,
silky, and glassy.
7. Crystal form: Crystal form is the external expression of
minerals that reflects the orderly internal arrangement of
atoms. E.g. Quartz
8. Smell and taste: e.g. Sulphur → rotten egg
Taste →rock salt
9. Hardness: refers to a measure of resistance from being
scratched. E.g. diamond is the most hardest.
Mohos scale of hardness
 The higher the value the hardest the element/ mineral.
Besides each reference mineral is able to scratch any mineral
with lower number.
 For instance,
 Talc will be scratched by any all known minerals.
 Conversely Diamond can scratch all known minerals.
The end
for unit
two
The Earth’s Layers Questions
Label the layers of the earth.
Read the questions below and circle the letter of the correct answer.
1. Which of the following is NOT one of the Earth’s layers?
a. Inner core
b. Crust
c. Outer mantle
d. Lower mantle
2. Starting with the outermost layer, what is the order of the Earth’s layers?
a. Core, mantle, crust
b. Crust, upper mantle, lower mantle, outer core, inner core
c. Mantle, inner crust, core
d. Crust, inner core, mantle
3. The core of the Earth is made up almost completely of which element?
a. Iron
b. Granite
c. Sulfur
d. Calcium
4. Which layer of the earth is the thickest?
a. Mantle
b. Core
c. Crust
d. All layers are the same thickness
5. Which of the earth’s layers is liquid?
a. Lower crust
b. Outer core
c. Inner core
d. mantle
Read each statement below. If it is true, write a “T” in the blank; if it is false, write an
“F”.
____1. The center of the Earth is made of molten rock.
____2. Most of the Earth’s heat is stored in the mantle.
____3. The outer core is liquid.
____4. Earthquakes occur when heat travels through the mantle and causes tectonic
plates to shift.
____5. The thinnest parts of the Earth’s crust are its continents.
____6. Extreme pressure causes the inner core of the Earth to remain solid.
____7. The crust of the Earth is much cooler than its other layers.
____8. The Earth’s mantle is flexible and shifts under heavy loads.
____9. The core is where geologists look for oil, gas, and other resources.
___10. The crust is broken down into two parts: the upper and lower crust.