(SEM VII) THEORY EXAMINATION 2019-20 GEOLOGY AND SOIL MECHANICS

SECTION A

(Attempt all questions)

(a) Rock forming minerals
Rock forming minerals are those minerals which commonly occur in the earth’s crust and combine together to form rocks. These minerals constitute almost the entire mass of igneous, sedimentary and metamorphic rocks. The most important rock forming minerals include quartz, feldspar, mica, amphiboles, pyroxenes, olivine, calcite and dolomite. Quartz is composed of silica and is highly resistant to weathering. Feldspar is the most abundant mineral in the earth’s crust and plays a major role in igneous rocks. Mica minerals show perfect cleavage and are common in metamorphic rocks. Calcite and dolomite are important constituents of sedimentary rocks such as limestone.

(b) Role of plate tectonics in the formation of earth’s crust

Plate tectonics is a scientific theory which explains the movement of large lithospheric plates over the semi-molten asthenosphere. These plates are continuously moving due to convection currents in the mantle. When plates collide, separate, or slide past each other, various geological features are formed. Mountain ranges like the Himalayas were formed due to the collision of the Indian plate with the Eurasian plate. Volcanic activity occurs at divergent boundaries, while earthquakes are common along transform boundaries. Hence, plate tectonics is responsible for the formation, deformation and continuous modification of the earth’s crust.

(c) Tsunami

A tsunami is a series of very large ocean waves generated due to sudden displacement of a huge volume of water. This displacement usually occurs because of undersea earthquakes, volcanic eruptions or submarine landslides. In deep oceans, tsunami waves travel at very high speeds with low wave height, but as they approach shallow coastal waters, their height increases drastically, causing massive destruction to coastal regions.

(d) Difference between geology and petrology

Geology is the branch of science that deals with the study of the earth as a whole, including its origin, structure, materials, and processes. It includes the study of rocks, minerals, fossils, geological structures and natural hazards. Petrology, on the other hand, is a specialized branch of geology that deals only with the study of rocks. It focuses on the origin, classification, composition and texture of rocks. Thus, geology is a broader subject, while petrology is a part of geology.

(e) Dip slope

A dip slope is a surface of land that slopes in the same direction as the dip of rock strata. It is commonly observed in areas consisting of inclined sedimentary rocks. Dip slopes are generally gentle and smooth, as they follow the natural inclination of the rock layers. Such slopes are important in engineering works because they may become unstable when water lubricates the bedding planes.

(f) Metamorphism

Metamorphism is the process by which pre-existing rocks undergo changes in mineral composition, texture and structure due to the action of heat, pressure and chemically active fluids, without melting. This process results in the formation of metamorphic rocks such as marble from limestone and slate from shale. Metamorphism usually occurs deep inside the earth’s crust where temperature and pressure conditions are high.

(g) Earthquake zones of India

India has been divided into four seismic zones based on the intensity and frequency of earthquakes. Zone II represents areas with low seismic activity, while Zone III includes regions of moderate seismic risk. Zone IV covers areas with high earthquake risk, and Zone V represents regions with very high seismic activity such as parts of the Himalayas and North-Eastern India.

SECTION B

(Attempt any three)

(a) Scope and application of geology in civil engineering

Geology plays a very important role in civil engineering projects. Engineering geology helps in selecting suitable sites for dams, bridges, tunnels, buildings and highways. It provides knowledge about the nature and behavior of soil and rocks, groundwater conditions, seismic activity and landslide-prone areas. Geological studies help engineers to avoid weak foundations, water seepage problems and structural failures. Therefore, geology ensures safety, economy and long-term stability of civil engineering structures.

(b) Folds and folding

Folding is the process by which rock layers bend due to compressional forces acting within the earth’s crust. When rocks are subjected to slow and continuous pressure, they do not break but bend to form folds. An upward arch-like fold is called an anticline, while a downward trough-like fold is known as a syncline. Depending on the nature of pressure and rock properties, folds may be symmetrical, asymmetrical, overturned or isoclinal. Folding plays an important role in mountain building and influences the distribution of mineral resources.

(c) Causes of mass movement and angle of repose

Mass movement refers to the downward movement of soil and rock materials under the influence of gravity. It is commonly caused by factors such as heavy rainfall, earthquakes, steep slopes, removal of vegetation and human activities like mining and road construction. The angle of repose is defined as the maximum angle at which loose granular material can remain stable without sliding. It depends on the size, shape and moisture content of the particles.

SECTION C

3(a) Internal structure of the earth based on seismological evidence

The internal structure of the earth has been understood mainly through the study of seismic waves generated during earthquakes. These waves travel at different speeds through different materials. Based on changes in wave velocity and behavior, the earth is divided into three main layers: crust, mantle and core. The crust is the outermost thin layer. Below the crust lies the mantle, which extends up to about 2900 km depth. The core lies at the center of the earth and is divided into a liquid outer core and a solid inner core. The absence of S-waves in the outer core confirms its liquid nature.