ROCKS AND THE ROCK CYCLE

Rocks are the basic foundation in the study of Geology. When Earth was in the process of formation it was mere fluid but as the time passed it turned out into the solid form. The solid form of the Earth was constituted in the formation of rocks. When we study rocks we come to know that there is an unending cycle going on for the formation of the rock known as “Rock Cycle.”

ROCK CYCLE:

The rock cycle is a combination of processes by which the rocks are formed on the Earth. Rocks are not in a stable form. They are continuously changing their form, from one mineral form to the other. But it all has to start from some point. The rocks formed are of three types:

@  Igneous rocks

@  Sedimentary rocks

@  Metamorphic rocks

The starting point of the rock cycle is the magma. The scorching hot molten material present in the region of mantle and outer core (inner core is in solid form). The magma is in molten form due to the residual heat and heat produced by the radioactive decay. This heat leads to the formation of heat diapers and these heat diapers extend to form Mantle plumes. When the mantle plumes increase in size they are known as Super plumes. However, these super plumes make up magma chambers and these magma chambers due to pressure and temperature lead to venting out of magma and thus this magma is erupted onto the land releasing lave, ash and numerous gases.

BASICS OF START OF ROCK CYCLE

Formation of Igneous Rocks:

When the lava erupted from volcano cools down it forms Igneous Rocks. Igneous is a Greek word meaning Fire. The igneous rocks formed are of three types formed on the basis of the depth they are formed:

@  Intrusive/ plutonic igneous rocks:

The rocks formed well below surface of the Earth and appear to be crystallized from magma emplaced or intruded in surrounding rock. Example granite, gabbro and sodalite etc.

@  Intermediate/ hypabasal igneous rocks:

The rocks that are neither plutonic nor volcanic but intermediate between them, found at the shallow depths. Example Dolorite etc.

@  Extrusive/volcanic igneous rocks:

The rocks that are exposed on the surface of Earth due to eruption of lava and solidify are known as extrusive rocks. Example rhyolite, basalt, trachyte etc.

Formation of Sedimentary Rocks:

Sediment is a collective name for loose, solid particles that originate from:

                                                                    i.            Weathering and erosion of pre-existing rocks.

                                                                  ii.            Chemical precipitation from solutions.

The processes that involve formation of sedimentary rocks are:

A.    Weathering/ Erosion:

The physical removal of the some part of the rock takes place with the help of some agent such as water; wind etc. or the other method of distortion of the rock is by the help of chemical or mechanical weathering.

B.     Transportation:

The material that is weathered and eroded is then transported from one place to another by the help of winds, water, tectonic activity etc.

C.     Deposition:

The deposition of the unconsolidated material that is transported takes place and is settled down at some particular area and it is known as environment of deposition.

D.    Lithification:

The combined processes by which the loose eroded sediments undergo for its conversion to as sedimentary rock is called Lithification. The tight packing up of sediments with a resulting decrease in the pore spaces is known as Compaction. When the remaining pore spaces are invaded by the underground water, they can be filled up with the loose particles called as cement and the process is known as Cementation.

Formation of Metamorphic Rocks:

The rocks that are produced by the action of metamorphism on the preexisting rocks i.e. action of high temperature or pressure or may be both and converting them into texturally or mineralogically distinct new rock but not melting the pre-existing rocks are known as metamorphic rocks and the phenomenon is called as Metamorphism.

But yes if the introduction of ions from external source brings about chemical change in the newly formed rocks it is known as Metasomatism.

 Temperature, pressure, fluids and time acting upon the rocks are the top factors of metamorphism. When the effect of these factors are increased from the safe level metamorphic rocks are tend to change to magma and the cycle starts again. However, some of the metamorphic rocks if exposed on the surface are likely to change back to sedimentary rocks.

The rock cycle take hundreds of millions of years to reach to completion. It is very slow process but resulting in the lack of equilibrium in the nature.

SEISMIC WAVES AND EARTH'S INTERIOR STRUCTURE

The basic use since seismic waves are known due to immense depth to the Earth core we are relied upon the indirect study of Earth’s interior. For this purpose we mainly use body waves i.e. S-waves and P-waves as they go deeper into the earth than those of surface waves.

The change in the velocity of the body waves guide us that how the Earth’s interior may look like. We use two processes to proceed towards our knowing:

@  Seismic reflection:

The return of part of the energy of seismic waves to Earth’s surface after the waves bounces off a rock boundary. This process helps geologists know the depth of the boundary.

@  Seismic refraction:

The bending of seismic waves as they pass from one material to another. The seismic refraction happens when the velocity of the waves differ in different layers depending upon their density or strength. Thus this process helps us in knowing of boundaries between different layers.

Now we will study layer to layer property earth’s interior show according to seismic waves.

THE CRUST:

Seismic waves have clearly distinguished between the thicknesses of the crust as thinner beneath the oceans and thicker on the continental area. Moreover the point to ponder upon is that seismic waves travel faster in oceanic crust i.e. 7km/second as compared to in the continental crust i.e. 6km/second. This shows that the rocks present in both the crusts differ from each other. The velocity of waves is greater in the rocks with greater density such as basalt and gabbro and samples taken from the ocean floor also suggest likewise. The velocity of seismic waves on continental crust suggest that rocks present here should be Granitic but as for evidence it is not true as all rocks present on Earth surface are not granitic but also consists of plutonic rocks, gneisses and schist and capped up by sedimentary rocks.

 THE MANTLE:

The data collected through seismic processes shows that mantle is divided into concentric layers. Yet at the same time they show that the layers are in the solid form. The velocity of the P-waves in the upper mantle is 8 Km/second and the increased velocity of seismic waves shows that it is composed of dense igneous rocks known as ultramafic rocks for example peridotite. These rocks are made chiefly of ferromagnesian minerals such as olivine and pyroxene.

Below the uppermost mantle layer a curious boundary is present. Here, the velocity of seismic waves slow down which means that this area have low density and because of this reason it is said that this region is partially melted forming a crystal-liquid slush. This region is known as “low velocity zone” or asthenosphere.

From the data obtained by seismic reflection and seismic refractions clear boundaries at 400 and 670 Km below earth surface has been observed. It is suggested by geologists that if the boundaries between mantle layers represent pressure-caused transformations of minerals, the entire mineral may have the same chemical composition throughout, although not the same mineral composition. For example, at a pressure equivalent to a depth of about 670 Km, the mineral olivine should collapse into the denser structure of the mineral perovskite.

CORE MANTLE BOUNDARY:

At the core mantle boundary the P-wave velocity drops down dramatically and this region is up to 200Km thick and is known as Ultra-low velocity zone (ULVZ). Latest studies have likely tried to explain the reason of this low velocity. According to scientists liquid iron alloys from liquid outer core may react with silicates in the lower mantle to form iron silicates. And this layer is electrically conductive and thus explains decrease in the seismic velocities resulting in ULVZ.

THE CORE:

When we talk about the core of the earth then of course no geologist has ever seen the core but yes the studies done by different methods we have come up to the results about the core of the earth. Seismic data has given us the proof of the presence of the core. While studying seismic waves we observe following phenomena:

When seismic waves are produced during earthquake we notice that they do not reach certain areas on opposite side of the Earth.

@  P-Wave Shadow Zone:

The P-waves generated spread out from a quake until at 103° of arc from the epicenter (area of generation of quake) they suddenly disappear from seismograms. And then at 142° they reappear. The region between 103°-142° is called as P-wave shadow zone.

@  S-Wave Shadow zone:

Now as we know that S-waves can only travel through solids and when it disappears at 103° completely it means that the core of the earth is liquid or at least acts like a fluid. And the area beyond 103° is S-wave shadow zone.

The careful analysis of the seismograms show the refraction of P-waves within the Earth’s core suggesting that core has two parts, a liquid outer core and a solid inner core. Seismic and density data of the core suggests that the composition of core is of largely of iron mixed up with small amount of lighter element such as oxygen, sulfur or silicon.

SEISMIC WAVES

WHAT ARE SEISMIC WAVES?

Before we try to explore about seismic waves and its use in geology we should know that what does seismic means. So here is the thing that word seismic is originated from Greek word “seiein” which means “to shake”. Now when we study seismic waves it is the shaking caused by waves that helps us study the detail of Earth’s Interior.

The point from where the seismic waves are originated; i.e. the seismic waves can be produced by earthquake, explosion or volcanic eruptions that cause shaking of earth; it is called as the focus.

TYPES OF SEISMIC WAVES:

There are two types of seismic waves originated:

@  BODY WAVES

@  SURFACE WAVES

WHAT ARE SEISMIC WAVES?

Before we try to explore about seismic waves and its use in geology we should know that what does seismic means. So here is the thing that word seismic is originated from Greek word “seiein” which means “to shake”. Now when we study seismic waves it is the shaking caused by waves that helps us study the detail of Earth’s Interior. Now a day’s not just the Earth’s interior but also a lot of Earth’s information is explored by the use of these waves.

The point from where the seismic waves are originated; i.e. the seismic waves can be produced by earthquake, explosion or volcanic eruptions that cause shaking of earth; it is called as the focus.

TYPES OF SEISMIC WAVES:

There are two types of seismic waves originated:

@  BODY WAVES

@  SURFACE WAVES

BODY WAVES:

“Body waves are the seismic waves that travel through the Earth’s interior”

Body waves are less destructive as compared to surface waves as they are not near the earth’s

surface and moreover they are relatively fast than surface waves. However, these waves are

further categorized as:

@  P-Waves:	@  S-Waves:
The compressional or longitudinal waves	The transverse waves.
Rock particles vibrate back and forth i.e. parallel to the direction of wave propagation.	Rock particles vibrate by a shearing motion i.e.  Perpendicular to the direction of wave propagation.
These are the fast waves.	These waves are the slower waves.
P-waves travel at the speed of 4 to 7 kilometers per second.	S-waves travel at the speed of 2 to 5 kilometers per second.
Can travel through the liquids. Such as liquid asthenosphere.	S-waves are not able to travel through liquids.

SURFACE WAVES:

“Seismic waves that travel on earth’s surface.”

Surface waves are the slowest waves that are set off during earthquake. As these waves are

nearer to the earth’s surface and have very slow speed thus these waves are extremely

destructive. Surface waves are of following two types.

@  Love waves:

Love waves are just like S-waves they move the ground perpendicular to the direction of

propagation of the waves and this result in the knocking off of the buildings from their

foundations and such destructions. Love waves cannot travel through water body i.e. liquids just

like S-waves.

@  Raleigh waves:

The most destructive seismic waves that cause major damage to the buildings are Raleigh waves.

These waves are very slow and take very long to pass through the land surface. The movement

of these waves is like rolling oceans waves they cause ground to move in an elliptical path

opposite to the direction the wave passes.

BASICS OF INTERNAL STRUCTURE OF EARTH

To understand geology first of all we have to understand the structure and composition of Earth itself as it is the key role playing for the further processes of understanding.

 EARTH STRUCTURE:

Internal structure of Earth resembles as a layered cake. 

Layered internal structure of Earth

The basic three lyers that are present in Erath are:

@  Crust

@  Mantle

@  Core

With the time as advancement of technology took place we were able to understand these layers in further detail.

DETAILED INTERNAL STRUCTURE OF EARTH

DETAILED EXPLANATION:

Now we will learn about the earth’s internal structure in detail:

A.    CRUST:

Crust is the top most layer of the planet earth, the layer that accommodates all the living beings on it. Now the crust is divided into two parts:

@  Oceanic crust

@  Continental crust

Oceanic crust:

Oceanic crust is the part of crust that is submerged under water that is oceans and seas. It’s thickness as compared to that of continental crust is very less that is just 7km.The density of the oceanic crust is more than continental crust i.e. 3.0gm/cm³. The type of material found in oceanic crust is basaltic which is underlain by gabbro. Basalt and gabbro they both are igneous rocks and as compared to metamorphic and sedimentary they have high density causing density of oceanic crust more than continental crust.

Continental crust:

Continental crust is the exposed part of the crust on which mostly human activities are taking place. This part of crust is less dense and has density of 2.7gm/cm³. The thickness of continental crust extends up to 30-50km. The reason of more thickness of continental crust as compared to that of oceanic is due to exposed portions of sedimentary and metamorphic rocks in the form of high rise mountains etc. in further discussions we will study about the processes going on the planet causing formation of different features of earth. The probable composition of continental crust is granite, other plutonic rocks, schist, gneiss (with sedimentary rock cover).

B.     MOHOROVICIC DISCONTINUTY:

Mohorovicic discontinuity is the boundary between the crust and the mantle.

C.     THE MANTLE:

Mantle is the 2^nd basic division of the earth’s internal structure. There are visible further divisions of mantle on the basis of difference of composition of mantle rock. The chemical composition of the mantle rocks is continuous throughout the mantle layer but the mineral formed and the physical changes in the minerals form further layers of the mantle. The layers formed are:

@  70-125 or 200 km- Asthenosphere:

According to some scientists after the Mohorovicic discontinuity the asthenosphere which is also known as low velocity zone is present which is extended from 70-125km or in different theories from 70-200km. the asthenosphere is considered to be formed by the partially molten rocks or we can say crystal and liquid slush. The upper layer of mantle and crust forming the tectonic plates of earth move over asthenosphere as it provides lubrication to their movement.

@  400-670km:

In the mantle there is prominent division at 400km and 670 km. the reason of the division is based upon the difference of minerals formed with the change of pressure and temperature while moving deep inside towards the center of the earth.

While at 670 km the prominent change is observed that separates upper mantle from the lower mantle and the difference between the two is based upon the chemical and physical change of the material.

D.    THE CORE:

Core is the 3^rd and last layer of the Earth. Its temperature is merely very high as compared to that of the mantle and the core. The core is basically divided in to two further layers on basis of their physical and chemical combination:

@  Outer core:

The outer core I the hot liquid layer composed largely of melted iron metal and the Earth’s magnetic field is also explained by this liquid art of the earth. As iron is a very good conductor and does possess high electric and magnetic properties. The density of outer core is less i.e. 10-12gm/cm³ as compared to that of the inner core.

@  Inner core:

Inner core is the solid layer which consists of iron-nickel alloy. Nickel metal has high density than iron so when iron and nickel combine together this tends to high density of inner core that is 12-13gm/cm³.