Geologist’s Exam 2010
Geology Paper – 1
Part – 1
1. 1. Define any ten in 3 to 4 sentences each: 5×10=50
(a) Tool marks
A tool mark is a mark produced by the impact against a muddy bottom of a solid object driven by a current moving over the bed. It is generally preserved as a cast, seen on the base of a sand or silt bed deposited on the muddy bottom soon after the marks have been formed. Tool marks are also found in other environments, however, notably in fluvial deposits and marine storm deposits. They are valuable indicators of paleocurrent direction and also give some information about the nature of the clay bottom and the sediment transport mechanism operating in the flow that deposited the overlying sand.
An outlier is an area of younger rock completely surrounded by older rocks. An outlier is typically formed when sufficient erosion of surrounding rocks has taken place to sever the younger rock’s original continuity with a larger mass of the same younger rocks nearby.
(c) Composition of the Earth’s core
The centre of the Earth is the core, which has two parts. The solid, inner core composed of iron. It is surrounded by a liquid, outer core composed of a nickel-iron alloy. The outer core is about 1,355 miles (2,180 km) thick.
(d) Stable Isotopes
The nucleus of each atom contains protons and neutrons. While the number of protons defines the element (e.g., hydrogen, carbon, etc.) and the sum of the protons and neutrons gives the atomic mass, the number of neutrons defines the isotope of that element. For example, most carbon (≈ 99 %) has 6 protons and 6 neutrons and is written as 12C to reflect its atomic mass. However, about 1 % of the carbon in the Earth’s biosphere has 6 protons and 7 neutrons (13C) forming the heavy stable isotope of this important element. Stable isotopes do not decay into other elements. In contrast, radioactive isotopes (e.g., 14C) are unstable and will decay into other elements.
The less abundant stable isotope(s) of an element have one or two additional neutrons than protons, and thus are heavier than the more common stable isotope for those elements. Both heavy and light stable isotopes participate freely in chemical reactions and in biological and geochemical processes, but the rate at which heavy and light stable isotopes react during physical or chemical reactions differ.
(e) Refraction cleavages
Cleavage refraction occurs in layered rocks, when there is appreciable rheological difference in response to stress. The smaller cleavage-bedding angle is always characteristic of the less competent layers, the greater angle is found in the more competent layers. The numerical difference in value of the angle is a function of competence contrast and of the location in the fold. If the geometry of the fold is associated with higher strains on the overturned limb than on the normal limb (as is frequent), the cleavage refraction is more marked on the normal limb than on the overturned limb. Generally the axial surface of the fold lies with an orientation between the two directions of the cleavage developed in competent and incompetent layers.
Cleavage refraction refers to a change in orientation as cleavage passes from one lithology to another. Cleavage in the gray beds is dipping to the left and is closely spaced. Cleavage in the lower buff-coloured beds is sub vertical and more widely spaced. The lower beds are more competent.
(f) Magnetic anomaly
In geophysics, a magnetic anomaly is a local variation in the Earth’s magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material. Magnetic anomalies are generally a small fraction of the magnetic field.
A magnetic anomaly is the change in magnitude of the earth’s magnetic field with respect to the expected value for that location. Large volumes of magnetic materials will change the intensity of the earth’s field.
The units of magnetic anomalies are nanoTesla (nT) or the equivalent gamma.
The magnitude of the earth’s magnetic field varies dramatically with latitude, from 25,000 gammas at the equator to 70,000 gammas at the magnetic poles. Variations due to geology typically have a magnitude of a few hundred gammas, which is a very small fraction of the total field.
(g) Trace Fossils
A trace fossil, also called an ichnofossil, is a geological record of biological activity. Trace fossils may consist of impressions made on the substrate by an organism.
TTrace fossils provide us with indirect evidence of life in the past, such as the footprints, tracks, burrows, borings, and feces left behind by animals, rather than the preserved remains of the body of the actual animal itself.
Trace fossils are generally difficult or impossible to assign to a specific maker. Only in very rare occasions are the makers found in association with their tracks. Further, entirely different organisms may produce identical tracks. Therefore, conventional taxonomy is not applicable, and a comprehensive form of taxonomy has been erected. At the highest level of the classification, five behavioural modes are recognized:
- Domichnia, dwelling structures reflecting the life position of the organism that created it.
- Fodinichnia, three-dimensional structures left by animals which eat their way through sediment, such as deposit feeders;
- Pascichnia, feeding traces left by grazers on the surface of a soft sediment or a mineral substrate;
- Cubichnia, resting traces, in the form of an impression left by an organism on a soft sediment;
- Repichnia, surface traces of creeping and crawling.
A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. Disconformities are marked by features of sub aerial erosion. This type of erosion can leave channels and paleosols in the rock record. A paraconformity is a type of disconformity in which the separation is a simple bedding plane with no obvious buried erosional surface.
Disconformities, however, can be more of a challenge to recognize. If strata in the sequence are fossiliferous, and you can recognize the fossil species and know their age, then you can recognize a gap in the fossil succession.
(i) Virtual Fixation Point
A seismograph, or seismometer, is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass do not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage. The electrical voltage is recorded on paper, magnetic tape, or another recording medium. This record is proportional to the motion of the seismometer mass relative to the earth, but it can be mathematically converted to a record of the absolute motion of the ground. Seismograph generally refers to the seismometer and its recording device as a single unit.
A simple seismometer, sensitive to up-down motions of the Earth, is like a weight hanging on a spring, both suspended from a frame that moves along with any motion detected. The relative motion between the weight (called the mass) and the frame provides a measurement of the vertical ground motion. A rotating drum is attached to the frame and a pen is attached to the weight, thus recording any ground motion in a seismogram.
Any movement of the ground moves the frame. The mass tends not to move because of its inertia, and by measuring the movement between the frame and the mass, the motion of the ground can be determined.
Nekton or necton refers to the aggregate of actively swimming aquatic organisms in a body of water. Nektonic organisms have a high Reynolds number (greater than 1000) and planktonic organisms a low one (less than 10). However, some organisms can begin life as plankton and transition to nekton later on in life, sometimes making distinction difficult when attempting to classify certain plankton-to-nekton species as one or the other.
Oceanic nekton comprised animals largely from these three classes:
- Vertebrates formed the largest contribution; these animals are supported by either bones or cartilage.
- Molluscs are animals such as squids and scallops.
- Crustaceans are animals such as lobsters and crabs.
There are organisms whose initial part of their lives was identified as being planktonic but when they grew and increased in body size they become nektonic. A typical example was the medusa of the jellyfish.
(l) Equal Area Projection
An equal-area map projection showing parallels and the equator as straight lines and other meridians as curved; used to map tropical latitudes.
A map projection in which areas on a sphere, and the areas of any features contained on it, are mapped to the plane in such a way that two are related by a constant scaling factor. No projection can be both equal-area and conformal, and projections which are neither equal-area nor conformal are sometimes called aphylactic. Equal-area projections are also called equivalent or homolographic.