Rocks

Rock

Reason — A rock is a natural aggregate of one or more minerals with no fixed chemical composition that forms a definite part of the Earth's crust.

Dykes

Reason — Dykes form when magma fills vertical cracks and solidifies. Necks fill volcanic vents, sills form between rock layers, and laccoliths are dome-shaped magma intrusions beneath the crust.

Dykes — large deposits of cooled magma on the earth's surface.

Reason — The statement is incorrect because dykes are formed when magma solidifies in vertical cracks below the Earth's surface, not as surface deposits.

Lithification

Reason — Lithification means turning the loose sediments into hard rock. Compaction and cementation are only individual processes that take place within lithification, and not the complete process itself.

Cementation

Reason — Cementation is the bonding together of compacted sediments by natural materials such as calcium compounds, silica and iron, all of which have the properties of bonding the loose materials.

Evaporation — Compaction — Cementation.

Reason — Loose sediments become sedimentary rocks through evaporation, compaction, and cementation. First, the water evaporates, sediments are then compressed, and natural minerals bind them together through cementation process.

Sedimentary rocks have strata showing layers.

Reason — Sedimentary rocks are softer and have distinct layers, unlike the hard, crystalline igneous rocks. Their layered structure is the easiest way to identify them.

Both A and R are true and R is the correct explanation of A.

Explanation — Since the igneous rocks were the first to be formed, they are called primary rocks. They form the basis of formation of other types of rocks.

Carbonaceous rocks

Reason — The rocks like peat, lignite, bituminous and anthracite are termed as carbonaceous rocks because they contain a high proportion of carbon and are formed from the remains of dead plants that got buried and decomposed over long periods.

Glacial : ocean floor

Reason — The pair is incorrect because glacial rocks are formed from glacial deposits (till), not on the ocean floor. Marine rocks are formed by deposits on the ocean floor.

Metamorphism

Reason — Metamorphism is the alteration of rocks by heat, pressure, or other natural forces. Hypabyssal is a type of igneous rock, while compaction and cementation are processes that form sedimentary rocks.

Both A and R are true and R is the correct explanation of A.

Explanation — Igneous rocks are usually compact and are of interest to builders and sculptors.

Contact Metamorphism : high pressure below ocean bed.

Reason — The pair is incorrect because contact metamorphism is caused by the heat of magma, not by high pressure beneath the ocean bed.

A: action of agents of weathering B: deposition-sedimentation C: action of heat and pressure

Reason — In the rock cycle, igneous rocks are broken down by weathering (A) into sediments. These form sedimentary rocks through deposition (B), which are then changed into metamorphic rocks by heat and pressure (C).

Rock cycle

Reason — The continuous process of transformation of old rocks into new ones is known as the rock cycle. This cycle may pass through the process of formation of sedimentary, metamorphic and igneous rocks.

Batholiths

Reason — Batholiths are deep-seated, dome-shaped igneous intrusions that form the cores of mountain ranges, while sills, dykes, and laccoliths are minor intrusive forms.

Metamorphic rocks

Reason — The smooth, banded appearance is due to foliation during metamorphism. Metamorphic rocks are also harder and more compact than the original rocks.

Gypsum

Reason — Gypsum is a chemically formed rock produced by the evaporation of seawater, whereas peat, chalk, and limestone are organically formed from the remains of plants and animals.

Water

Reason — Glacier ice is solidified or frozen water, so water is the single mineral of which it is composed.

Three types of rocks are-

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks

Two characteristics of Igneous Rocks are-

1. They are hard and compact.
2. They are formed by solidification of molten magma.

Two characteristics of Basic Igneous Rocks are-

1. They have higher percentage of oxides of denser elements and silica content varies between 40 to 60 percent.
2. It is heavy and dark in colour.

Two important rock forms made by Igneous Rocks are lava sheets and lava plateaus.

Sometimes magma flows between layers of rocks horizontally. It then hardens there. This layer of intrusive rock is called sill. For example- Archuleta Mesa in southern Colorado.

Igneous rocks are associated with ores of metals and sedimentary rocks are associated with fossil fuels.

Two characteristics of Sedimentary Rocks are-

1. Sedimentary rocks are formed from materials derived from other rocks. They may also have plant and animal remains. These rocks may thus contain fossils.
2. Sedimentary rocks are generally soft and layered as they are formed by deposition of sediments.

Sedimentary rocks are most widespread on the earth. Examples of sedimentary rocks are Gypsum and sand stone.

The three stages of lithification of Sedimentary rocks are-

1. Evaporation
2. Compaction
3. Cementation

The types of Sedimentary rocks based on agents of formation are-

1. Riverine rocks
2. Lacustrine rocks
3. Glacial rocks
4. Aeolian rocks
5. Marine rocks

The main agents responsible for deposition of sediments are water, wind and ice.

Metamorphic rocks are formed by complete change of texture and composition of pre-existing rocks through volcanism and diastrophism. For example, slate is formed from clay and graphite from coal.

Due to mountain building process, igneous and sedimentary rocks are buried deep inside the crust. The pressure of overlying rocks and the intense heat caused by large-scale earth movements change such rocks. Such a process is known as regional metamorphism.

Igneous rocks are formed due to cooling, solidification and crystallisation of magma found at great depths in the interior of the earth.

The main characteristics of igneous rocks are-

1. They are formed by the solidification of magma.
2. Hard, compact, granular, and crystalline.
3. May be fine-grained or coarse-grained, depending on the cooling rate.
4. Rich in silicate minerals.
5. Non-layered and resistant to chemical weathering.
6. Commonly associated with volcanic regions.

(i) Igneous rocks also called the primary rocks as they form the basis of formation of other types of rocks.

(ii) Igneous rocks are used by builders and sculptors because these rocks are easily available, economical, and feasible in nature. These rocks are very strong, which is considered the best option for the construction of strong structured statues and buildings.

(iii) Sedimentary rocks are called secondary rocks because they are formed by the solidification of sediments of original igneous, sedimentary or metamorphic parent rocks.

1. A — Rock A is Extrusive Igneous rock. This is because it is located on the surface of the Earth, where lava has flowed out through fissure and solidified.

2. B — Rock B is a Dyke. This is because magma when forced upwards fills vertical cracks or fissures in existing rocks. It then hardens there to form dykes.

3. C — Rock C is a Sill. This is because magma flows between layers of rocks horizontally and then hardens there. This layer of intrusive rock is called Sill.

When igneous rocks are exposed on the surface of the earth, they are broken down by weathering and carried away by forces of gradation. They are then deposited as sediments over long periods in seas, lakes, streams etc. Subsequent layers are added and due to their own weight from above, the sediments get cemented together to form sedimentary rocks.

There are five main types of sedimentary rocks on the basis of agents of formation:

1. Riverine Rocks — These are formed by the sediments deposited by running water or river.
2. Lacustrine Rocks — These are formed by the deposition of silt etc on the bed of lakes.
3. Glacial Rocks — These are formed by the debris carried on and deposited by glaciers.
4. Aeolian Rocks — These are formed by the deposition of sand particles accumulated by wind.
5. Marine Rocks — These are formed by the deposition of sediments at the ocean floor. These are of two types- Calcarious sedimentary rocks and Carbonaceous sedimentary rocks.

(i) Mechanically formed sedimentary rocks are also termed as stratified rocks because such rocks are formed in layers.

(ii) Rocks like peat, lignite and anthracite are mainly made out of carbon composition (almost 60-90%). Due to this majority of carbon composition in these rocks, these rocks are known as carbonaceous rocks.

(iii) Fossils are found in sedimentary rocks, where gentler pressure and lower temperature allows preservation of past life-forms. Fossils become a part of sedimentary rocks when sediments such as mud, sand, shells and pebbles cover plant and animal organisms and preserve their characteristics through time.

Metamorphism refers to the alteration of the composition or structure of a rock by heat, pressure or other natural agency. Metamorphism is caused by heat and pressure.
For example- graphite.

The chief characteristics of metamorphic rocks are-

1. They are harder and more compact than their original form.
2. Most of them are impermeable.
3. They do not have fossils.
4. They are formed due to change in texture and composition of the pre-existing rocks.
5. New minerals are formed during the process of metamorphosis.

The economic significance of rocks are-

1. Rocks are of great resource value, some directly and some as constituents of minerals.
2. Soils are derived from weathering of rocks.
3. Almost all types of building materials used for paving roads, floors or building walls of houses or various other structures including bridges come from rocks.
4. Rocks are a source of precious metals like gold, silver, platinum etc.
5. Apart from minerals and fossil fuels, even rock wastes have now been used in manufacturing various articles.

Rocks undergo cyclic transformation. The continuous process of transformation of old rocks into new ones is known as rock cycle. The cycle may also pass through the process of formation of sedimentary, metamorphic and igneous rocks.

To keep the earth young, rocks melt again resulting in the formation of igneous rock. This disintegrated material again forms sedimentary rock, which takes hundreds and thousands of years.

Yes, rocks are still important to humans today for several reasons:

1. Rocks (such as granite, sandstone, limestone, marble) are still used in construction to build houses, roads, bridges, and other structures.
2. Rocks and minerals are important for industrial manufacturing, such as for making glass, cement, ceramics, and various types of metals.
3. Some rocks like peat, lignite, coal etc., are used as fuel for power generation.
4. Rocks have practical applications in technology, such as for semiconductors and batteries used in electronic devices.
5. Rocks are a source of precious metals like gold, silver, platinum, etc. They are used in making jewellery and art, and some are used in medicine and cosmetics.
6. Soils are derived from weathering of rocks.
7. Apart from minerals and fossil fuels, even rock wastes have now been used in manufacturing various articles.

Yes, the minerals in a rock provide important clues about how it was formed. Different minerals form under different conditions, so their composition helps identify the rock type and its origin.

1. Igneous rocks contain interlocking crystals formed by the cooling and solidification of magma.
2. Sedimentary rocks consist of cemented sediments, often bound by minerals such as quartz, calcite, or clay.
3. Metamorphic rocks form when existing rocks are altered by heat and pressure, causing minerals to recrystallize and rearrange.

Thus, the minerals present in a rock help determine its origin, formation process, and geological history.

The rock cycle is the continuous process by which rocks are transformed from one type to another. It has no beginning and no end because any type of rock can change into another over time.

1. Igneous rocks are formed when magma or lava cools and solidifies.
2. These rocks are broken down by weathering and erosion into sediments, which are deposited and undergo compaction and cementation to form sedimentary rocks.
3. Under heat and pressure, sedimentary or igneous rocks change into metamorphic rocks.
4. If metamorphic or any other rocks melt into magma and cool again, they form igneous rocks. Similarly, any rock exposed at the surface can weather into sediments and become sedimentary rock again.

Thus, rocks are continuously transformed from one type to another over millions of years, making the rock cycle a never-ending process with no definite beginning or end.