Civil Engineering ► Engineering Geology ► The interior structure and composition of the Earth

Interior Structure of the Earth

Layers of the Earth

The earth is divided into three main layers: Inner core, outer core, mantle and crust.

The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulphur (S). The inner core is under such extreme pressure that it remains solid.

Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner.

Earth Crust:

THE CRUST

1. Continental crust (25-40 km)
2. Oceanic crust (~6 km)
3. Mantle
   1. Upper mantle (650 km)
   2. Lower mantle (2235 km)
4. Core
   1. Outer core: liquid (2270 km)
   2. Inner core: solid (1216 km)

The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle , so it can fracture in earthquakes .

The shell of the earth, the crust, can be said to have two different thicknesses.

Under the oceans, it is relatively thin. It varies in thickness from 5 to 8 km. Under the land masses, it is relatively thick. The thickness of the continental crust varies from 10 to 65 km.

The eggshell analogy for the crust is not an exaggeration. It is paper thin compared with the radius of the earth which is approximately 6400 km.

The total weight of the continental crust is less than 0.3% of the weight of the earth.

Variations in the crust thickness are compensated by the weight of the water and the differences in the specific gravities of the crust under the oceans (3.0 to 3.1) and under the continents(2.7 to 2.8).

If one thinks of the crust as virtually floating on the mantle, one is less likely to wonder why the earth does not wobble as it rotates about its axis.

The weight of the crust plus the mantle has a reasonably uniform distribution over the globe.

THE MOHO

The Moho, or the Mohorovicic Discontinuity, refers to a zone or a thin shell below the crust of the earth that varies in thickness from 1 to 3 km.

In seismology, the term "discontinuity" is used in its general sense. It refers to a change over a short distance of a material property. In this case, the "short distance" may be as long as 3 km, a trifle compared with the radius of the earth.

In that zone, the P-wave velocity has been observed to increase from approximately 6 to approximately 8 km/sec.

The Moho is considered to be the boundary between the crust and the mantle.

The increase in P-wave velocity is ascribed to change in composition of the medium. Rocks of the mantle are poorer in silicon but richer in iron and magnesium

THE MANTLE

The mantle can be thought of having three different layers. The separation is made because of different deformational properties in the mantle inferred from seismic wave measurements.

(1) The upper layer is stiff.

It is presumed that if the entire mantle had been as stiff, the outer shell of the earth would stay put. This stiff layer of the mantle and the overlying crust are referred to as the lithosphere. The lithosphere is approximately 80-km thick

(2) Beneath the lithosphere is a soft layer of mantle called the asthenosphere.

Its thickness is inferred to be several times that of the lithosphere.

One may think of this as a film of lubricant although film is not exactly the word for something so thick. It is assumed that the lithosphere, protruding (meaning: extending beyond) parts and all, can glide over the asthenosphere with little distortion of the lithosphere

(3) The mesosphere is the lowest layer of the mantle.

Considering the vagueness in defining the lower boundary of the asthenosphere it would be expected that the thickness and material properties of the mesosphere are not well known.

It is expected to have a stiffness somewhere between those of the lithosphere and the asthenosphere.

THE CORE

It is known that the pressure increases toward the center of the earth. So does the temperature. The liquid outer layer versus the solid inner layer is rationalized by recognizing that the melting point of the material increases (with pressure) at a faster rate than the temperature as the center of the earth is approached.

Minerals and Rock

Minerals are naturally occurring inorganic substances of more or less definite chemical composition, displaying more or less definite physical properties.

Rocks:

Geologist define rock as aggregates or mass composed of one or more commonly, several of minerals. There are few exceptions to this rule: not all rocks are composed of minerals-for example, coal.

Engineers (or contractor) define rock to be a ‘hard, durable material that can’t be excavated without blasting’. The definition is based on strength and durability.

Minerals are naturally occurring inorganic substances of more or less definite chemical composition, displaying more or less definite physical properties.

As the basic constituent of rock, minerals control much of rock behavior. Some minerals are very strong and resistant to deterioration and produce rock with similar properties, while others are much softer and produce weaker rock.

More than different 2000 minerals are present in the earth’s crust. They can be identified by their physical and chemical properties; by standard tests; or by examination under microscope.

1. Colour
2. Streak
3. Hardness: Mohs scale of hardness
4. Cleavage
5. Fracture
6. Luster

Color:

1. Some minerals have characteristics color due to composition of the minerals and the arrangement of the constituent atoms: for example black color of magnetite, green of chlorite and brassy yellow of pyrite
2. Minerals like quartz and calcite have variable color
3. Color can’t be sole identification property

Streak:

1. Color of mineral in powder form is called streak
2. Powder is obtained by crushing the mineral.
3. Color of the streak differs from color of mineral: for example the color of pyrite is brass yellow and its streak is dark green.

Cleavage:

1. The cleavage of the minerals is its capacity to split more readily in certain directions than in others, due to the arrangement of the atoms.
2. Minerals break with ease producing smooth surfaces is called perfect cleavage. It can be either good, distinct, indistinct and imperfect.
3. Some minerals such as mica have perfect cleavage in one direction. The feldspars, which is the most abundant of all minerals, have two cleavages.

Luster:

1. Appearance of mineral in ordinary light (that is the appearance due to reflected light). Luster may be metallic, glassy, earthy, pearly or silky
2. If the minerals looks metal as do galena and pyrite, its luster is said to be metallic. If the minerals looks glassy, like quartz, its luster is glassy.

Hardness:

1. The hardness of a mineral, as commonly determined on fresh material, is measured by its ability to resist scratching. If a mineral is scratched by a knife, it is softer than the knife. If it cannot be scratched by a knife, the two are equal hardness or the mineral is the harder.
2. In order to have a standard method of expressing hardness of minerals, a simple scale, known as the Mohs scale, has been universally adopted.
3. In sequence of increasing hardness from 1 to 10, the following minerals are used as standard of comparison:
4. Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase (feldspar), Quartz, Topaz, Corundum and Diamond

Other Characteristics:

Crystal Form: Internal atomic arrangement in definite geometric patterns is sometimes outwardly expressed in crystal form.

Specific Gravity is meant the weight of a substance compared with the weight of an equal volume of water. The specific gravity of quartz is 2.65. Some minerals are heavy than the others. The specific gravity of majority minerals range from 2.55 to 3.2.

Magnetism: A few minerals are attracted by a magnet. Of these minerals, magnetite, and pyrrhotite are the most common examples.

 

Rock Forming Minerals

 

1. Feldpars:

Feldspar is the most abundant minerals. There are two types. Orthoclase feldspars contain potassim (KalSi3O8) and usually range from white to pink. Plagioclase feldspars contain sodium (NaAlSi3O8), calcium (CaAl2Si2O8) or both, and range from white to gray to black,. Feldspars have moderate hardness.

2. Quartz is also very common ingredient in many kinds of rock. It is silicate (SiO2), and usually has a translucent to milky white color. The luster is vitreous. Quartz is harder than most minerals (hardness 7), and thus is very resistant to weathering. Chert is a type of quartz sometimes found in sedimentary rocks. It can cause problem when used as concrete aggregate.

3. Mica: Translucent thin sheets or flakes. There are two common varieties. Muscovite is potassium aluminium silicate of colorless or silvery tint, pearly luster and especially one very perfect cleavage which permits the mineral to be split into thin elastic sheets that when bent spring back to shape. Biotite, the other common variety, is a complex silicate of potassium, magnesium and iron and aluminum.

Mica: Biotite and muscovite are similar in physical properties. Both are soft, 2.5-3, with one perfect cleavage. The sheets of mica have very low coefficient of friction, which can produce shear failure in certain rocks, such as schist.

4. Ferromagnesian minerals: A class of minerals, all of which contain both iron and magnesium. This class includes pyroxene, amphibole, hornblende and olivine. These minerals are dark color and a moderate hardness.

5. Calcite: A mineral made of calcium carbonate (CaCO3). It is usually white, pink or gray. It is soluable in water, and thus can be transported by ground water into cracks in rock where it precipitate out of solution. It also can precipitate in soil, becoming a cementing agent. Calcite is much softer then quartz or feldspar. The hardness is 3. Have vigorous reaction to hydrochloric acid.

6. Dolomite: Similar to calcite with magnesium added. Less vigorous reaction to dilute hydrochloric acid.

7. Iron Oxides: Another class of minerals, all of which contain iron (FeO3). The most common iron oxides are hematite, Fe2O3 ; hydrous iron oxide that are often called limonite and magnetite. Although less common, these minerals give a distinctive rusty color to some rocks and soils and can act as cementing agents. The compact varieties have a hardness of 5.5-6, but earthy form are soft. The luster is sub-metallic.

8. Gypsum: A soft minerals often occuring as a precipitate in sedimentary rocks. It is colorless to white and has economic value when found in thick deposits. For example, it is used to make drywall. Gypsum is water soluble and thus can dissolve under the action of ground water, which can lead to other problems.

Related Pages: Origin of the Earth | Interior of the Earth | The Geological Cycle | Rock Forming Minerals