When rainfall occurs, it is often in the form of torrential downpours from thunderstorms. The more prominent continuous joints are often called the master joints. Such granite projections are often found surrounded by ancient metamorphic rocks into which the granite was intruded. Geological Work of Atmosphere (Rock Weathering) 3. But in some regions, rock layers may be warped upward or downward. The work of natural agencies may be either destructive or constructive in nature in relation to the existing landform of an area at any given point of time. Geologists find it convenient to divide them into 'primary' and 'secondary' structures. In other words, there is no angular variation in the deposition of the rocks of the entire sequence. There may be or may not be an opening up of blocks perpendicular to the joint planes. Structural Features 4. Vast areas of the ancient continental shields are covered by thick sequences of horizontal sedimentary rock layers. Stratification and lamination are the most common primary structures of sedimentary rocks; crystalline structure is typical of igneous rocks and foliation is a typical primary structure of metamorphic rocks. A joint system is a group of two or more joint sets. For example, formed as magma crystallizes or as sediment accumulates. It indicates a gap or an interval of time or a hiatus in the geological history of the area during which the normal process of deposition was interrupted. The rivers deposit their load at appropriate places on the land, along sea-shore and even at the sea bottom, building up such features as alluvial plains, flood plains and deltas etc. Joints are of great practical importance for all those dealing with the rocks as sites, materials of construction, in prospecting for minerals, groundwater and oil and gas reservoirs. All the modifications of the original (primary) structures and development of new forms, shapes and rearrangement of the component grains, crystals or mineral constituents that are induced in the rocks after their formation are grouped as Secondary Structures. In arid climates, vegetation is sparse, and weathered rock and soil cover much of the land surface. A Joint set is a group of two or more joint surfaces trending in the same direction with almost the same dip. The intense runoff rapidly carves small stream basins that rapidly dissect the landscape into distinctive landforms. Very vast and extensive sand made regions, the deserts, are very important example of work of wind. At the center of the structure is a mountainous core of upwarped intrusive and metamorphic rocks revealed by erosion. These may be classified and discussed under two main classes: Those structural features that are developed in the body of a rock during its formation stage are termed as Primary Structures. Sugar Loaf Mountain, which rises above the city of Rio de Janeiro, Brazil, is a small granite dome-like projection of a batholith that underlies the area. Batholiths are huge bodies of intrusive igneous rock, formed deep below the Earth's surface. Weathering is a natural process of in-situ mechanical disintegration and/or chemical decomposition of the rocks of the crust of the Earth by certain physical and chemical agencies of the atmosphere. Those structural features that are developed in the body of a rock during its formation stage are termed as Primary Structures. After crustal uplift, these areas became continental surfaces. Structural geology deals with a variety of structural features that can range in size from microscopic (such as traces of earlier folds after multiple events of deformation have occurred) to large enough to span the globe (such as midoceanic ridges). After crustal uplift, these areas became continental surfaces. Figure 12.36 shows how domes are eroded over time to create hogback ridges and circular valleys. The post – Vindhyan unconformity is traceable in both Peninsular and extra peninsular India. are formed by magma from the molten interior of the Earth. Copyright 9. The three major classes of rock are igneous, sedimentary, and metamorphic rock. The forces most commonly responsible for the development of secondary structures are tectonic in nature, i.e. Unconformity 5. Unconformities of all types indicate a break or discontinuity in the sequence of the rocks. These are simply called veins when thin or in strands and fissure veins when their thickness is greater than 20 cm. An example of a large and rather complex dome is the Black Hills dome of western South Dakota and eastern Wyoming (Figure 12.36). Introduction to Rocks 2. When an unconformity is traceable over a large area, extending for hundreds of kilometers, it is conveniently called a regional unconformity. An attempt has been made in the following pages to give an outline of the principles, the processes and methods by which the natural agencies operate on the land and along seashore and oceans. Terms of Service 7. New deposition in basins is generally indicated by the presence of a layer of conglomerates made up of rounded, semi-rounded pebbles and gravels. As the belts of less resistant strata erode, subsequent streams develop and connect to the consequent streams. Stratification and lamination are the most common primary structures of sedimentary rocks; crystalline structure is typical of igneous rocks and foliation is a typical primary structure of metamorphic rocks. For modifying the physical features existing on the surface of unconformity represents younger! How domes are eroded over time to create hogback ridges and circular valleys be steeply inclined folded... 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