What do Geologic Samples Reveal?

By Samuel

Over decades to centuries, scientists as well as some philosophers have been studying our planet earth as well as our solar system. Many discoveries and inventions in natural sciences as well as in life sciences were made possible with the help of geologic samples. Geologic samples have been widely and are still widely used in our world today. Without these samples, the simplest and common postulated theories about our planet Earth and its origin would not have been made known to human kind.

In childhood, children use clay, sand or mud around the beach molding bricks up, building some sort of dirty stuff to make their day but never ask what those materials are. Jewelries, the so call trophies given to athletes, coal, gas, crude oil, some drugs, lotions, perfumes, cement and are either geologic samples, made by the use of geologic samples or produced by exploiting or researching geologic samples.

But what do geologic samples reveal?

All geologic samples are varied. It could be a piece of rock material collected from the ground mass, core drilled from the ground or a piece of glacial deposit or some kind of ancient organism (fossil) as well as meteoric and asteroid materials. Usually before any geologic sampling is carried out, a field visit is usually planned base on the locality to get samples from. There are two types of sampling methods namely subsurface sampling (drilling out cores) and surface sampling.

In order to sample any surface area to get your geologic samples, here are some procedures to take note of (emphasis is laid on surface sampling here);

  • Working in the field of outcrops, observing and describing them as they appear in situ.
  • Use of certain equipment like GPS to get the longitude as well as the latitude of the outcrop and clinometer for dip and strike measurements if required, measure every aspect you find to be important in the outcrop or what so ever you are conducting your sampling on like measuring the length, width, thickness, layers, colours and so on.
  • Recording of every information in the field must be done.
  • Taking of pictures is very important
  • Sample papers and sample bags required to store and label collected samples of the overall outcrop or sampling space.
  • In some field visits, some chemicals such as dilute hydrochloric acid are usually required. For example in determining whether a rock is of carbonate origin.

Generally speaking, after sampling any area, the samples collected are then taken to the laboratory to perform some experimental procedures, followed by computer based works on the samples if necessary then a report is written about what the sample reveals.

Let’s get to know actually what geologic samples reveal.

Geologic samples carry a lot of information in them, reason why nowadays they are called the key to the pass. Geologic samples from the surface reveal exactly how the rocks were formed, or where they originated from.

  • Geologic samples tell us about how our planet earth was formed as well as its changing geologic processes.

During the formation of our planet earth, many of such rocks found on earth today were also formed. This rocks formed at the time of the earth’s formation now serve as geologic clocks to mankind. This means that dating rock samples (mostly radiometric dating) collected from such ancient outcrops  serve the purpose for which the age as well as the way the formation of our planet took place. On the other hand, dating some meteoric as well as naturally formed rocks can reveal to us the climate of the pass.

  • Geologic samples also help to tell us about the forces interacting in our planet.

For example, sampling of the core through seismic or geophysical analysis reveals that the core (preferably the outer core) is responsible for the earth’s magnetic field. This can be clarified through the compositional content of the outer core said to be made up of Iron and some other magnetic minerals.

  • Geologic samples also give us a glimpse of what could be found underground, like the oil and gas reservoirs as well as minerals.

This can be done during oil and gas exploration where many activities are usually performed in the locality where a geologic sample may have been collected dated to know the era in which they were formed. For example many carbonate rock samples were formed mostly during the carboniferous era which signifies that they are made of ancient organisms and hence may contain petroleum or natural gas. The samples usually collected for such investigation are those that are highly rich in fossils, because mankind already knows that oil and gas are formed from fossil fuels. So studying such fossilized rock samples reveal a lot about the reservoir.

In the same way rocks that are found very deep in the earth surface like most metamorphic rocks and some igneous rocks are highly rich in minerals, this means researching on such rock samples collected from already exposed outcrops to the surface can reveal to us what type of minerals may be present beneath that same rock outcrop deep underground.

  • They also help keep us up to date with the current or future changes in our earth and its processes.

You can always see that many researches now predict what could happen in future or what is about to happen in a short while from then. In the same way, geologist use geologic samples to predict the future of our earth and its processes. As earlier mentioned, geologic samples are varied; some may not think that a piece of ice block collected from a glacial environment is a geologic sample. That is absolutely wrong, even water in, unconsolidated mud, sand, as well as ice from falling rain is a geologic sample. In case of ice geologist and in many cases climatologist use it to predict the future changes in climate as well as what might happen then.

  • Geologic samples helps geologist to predict what could be impending such as an earthquake or volcanic eruption.

This is usually done scientifically through seismic studies where a rock sample is observed in the laboratory and it’s properties studied detail because rocks generally experience changes in properties especially before such catastrophic events due to stress strain relationships, migration of fluids as well as minerals.

  • Geologic samples are very important in human life as it reveals the best type of materials suitable for human or civil works such as building of houses and bridges and so on.

When humans first discovered that not all rock types were suitable for civil works, they had to dive into studying various rock samples in order to know the most suitable type to perform civil works as well as in manufacturing some civil materials like cement.

  • Geologic samples are also used in forensic sciences especially when a crime is committed in a geologic-like environment.

A forensic scientist would use such samples date them with forensic chemicals to get a detail hand, foot, shoe or body bring of the guilty person.

  • Geologic samples of ancient remains of plants and animals tells us much about ancient organisms as well as those that involved in the formation of valuable resources such as crude oil and gas.
  • Surface geologic samples are also eye catching and are used in decorations, making of status and graves, kitchen utensils and so on.

Understanding the structure of the earth from geologic samples

Because almost all of the geologic samples used by man today are gotten within the earth, it is important to study the structure of the earth by knowing what these geologic samples reveal. Looking at a giant object like the earth, in order to accurately or maybe make things clear to humans, it is important to lay out some important facts about the earth.

The question now is what really is under the earth that makes it as massive as predicted. Early in the twentieth century, many geologists as well as philosophers dived into the geologic world as a result of seismic waves from beneath the earth’s surface. Taking the example of eggs it is only upon breaking it that one knows what exactly it looks like within. This is the same with the earth, the only difference being that you cannot cut what is Trillion times heavier and bigger.

Take a look again at an orange; identifying if there are any bad spots on the surface of the orange generally gives you a glimpse of what it will look like inside. The same is true with the earth. Geologist use topographic as well as bathymetric maps to observe rocks (outcrop) brought to the surface either through volcanic eruptions, earthquakes due to the high temperatures and pressures existing at depth. Initially you can only know what is beneath the earth through the exposed rocks to the surface. Geologists therefore use gravitational as well as magnetic readings of any locality before and after or generally after such catastrophes to know about the structure of our earth and the processes responsible.

The earth can be structurally classified into physical and chemical; physically the earth can be classified as being lithospheric, mesospheric or asthenosphere. On the other hand, chemically the structure of the earth consists of three major layers namely; the crust, mantle and core. The mantle of the earth consists of the upper and lower sections and the core consists of the inner and outer sections.

According to geophysical studies of seismic waves from earthquakes, the structure of the earth has been inferred indirectly using the travel time reflection principles. Because shear waves cannot penetrate the core, a better source of wave traverse was thought of being seismic velocity which generally changes as we go deeper into other layers of the earth. Owing to one of our prestigious physical laws- Snell’s law, it is said that as the seismic velocity keep changing between layers of the earth, it is usually refracted back upward to any device that could be used for analyzing what is in the ground. With regard to all the studies carried out about the structure of the earth, it has been revealed that the earth from where we stand till the end of the core is up to 6360 km.

The layers of the earth based on the physical and chemical classification.

  1. A.    Physical Classification of the layers of the earth

As mentioned earlier, the earth is layered physically into three layers namely lithospheric, mesospheric or asthenosphere.

  • The Lithosphere

The place on which we humans as well as other animals and plants stand or work and walk on is the lithosphere. Yes you can feel its solid like nature and by geological researches; its estimated thickness is as much as 100 km. It embodies the crust but also the upper part of the mantle of the earth. The crust is subdivided into oceanic and continental crust. By thickness, the continental crust is about 20 to 60km thick and it has a density of 2.9g/cm3. On the other hand, the oceanic crust which carries the waters is around 10 to 11km with a density of 3.3g/cm3. It is because of this high density of the crust that makes it hard to sink into any liquid medium like water in seas and oceans. Typically speaking, the upper part of the mantle which is also part of the crust is about 50km thick. Between the crust and the upper mantle is the mohorovicic discontinuity which is an abrupt contact between the two layers.

  • Mesosphere.

The mesosphere is a very thick structural layer of the earth which include the lower mantle and a portion of the upper mantle. As we descent below the surface of the earth, it has been estimated that the mesosphere is around 350km below the ground which we stand on. The mesosphere is a layer of great heat as well as high pressures. Even though the mesosphere is very hot, the rocks and other materials in the layers are able to act against or over power the high temperatures because of the high pressures at depth.

  • The Asthenosphere

Have you ever imagined how hot a furnace is?  Did you ever thought of what the temperature of magma at the surface is during an eruption? If yes, then you are definitely in the right place. Talking of temperatures and pressure of magma to be greater than 7000C and 700Pa is dead. The asthenosphere is a hot gel-like layer which is part of the upper mantle extending from 100km to 350km from beneath the surface of the earth. The truth be told that the asthenosphere reacts as a solid but in the presence of stress or pressure, it reacts like a plastic or gel-like substance.

It is very important to note that magma is usually generated in the asthenosphere when the down going oceanic or continental plate may have reached a depth of at least 100km (beginning of the layer asthenosphere) beneath the earth’s surface usually as a result of dewatering of the down going oceanic or continental crust triggered by the earth’s geothermal energy.

B.     Chemical Classification of the layers of the Earth.

Compositionally, the earth is classified into being the core, mantle and the crust.

  • The Core

Compositionally, the core is mainly composed of iron (Fe), nickel (Ni), Sulphur (S) and silicon (Si). Typically speaking the core has very high density compare to our surface water in oceans and seas which by scientific researches has been estimated to have a density of 10.8g/cm3. The core is approximately 16.2% by volume and 31.5% by mass of our planet earth. Due to the high pressures existing at depth, is not surprising to say that the inner core, is solid –like and the outer core composition is liquid-like. Therefore, the outer core undergoes buoyancy. Some scientist and geophysicists have proclaimed that the outer core composition is responsible for the earth’s magnetic field.

  • The Mantle

Abruptly separated from the core by the Mohorovicic and Gutenburg discontinuities, the composition most also be widely different from that of the core. Chemically, the mantle is predominantly of peridotite which consists mostly of olivine, pyroxene and garnet minerals. The mantle occupies the highest of both the volume and the mass of the earth with the volume being 82.3% and the mass of 67.8%. On the other hand, the density of the mantle is far less than that of the core being 5g/cm3. Even though the mantle is very hot, the rocks and other materials in the layer are able to act against or over power the high temperatures because of the high pressures at depth.

  • The Crust

The crust ifs composed of two main layers being the continental and oceanic crust. The continental crust has a density of less that 3g/cm3 while the oceanic crust is just above 3g/cm3. With an average thickness of around 33km, the continental crust is composed of granitic minerals such as feldspar, mica and quartz while the oceanic crust with an average thickness of 10km is made of minerals plagioclase, pyroxene. The crust occupies just around 0.7% by mass and 1.6 % by volume of the earth.

What geologic samples could reveal is endless as more and more researches and discoveries are made and published every day.

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