Rocks

Rock

Dykes

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

Lithification

Cementation

Evaporation — Compaction — Cementation.

Sedimentary rocks have strata showing layers.

Reason — Igneous rocks are hard, compact, granular and crystalline, while sedimentary rocks are softer than igneous rocks and show layers. However, the best way to identify them would be by their layers which can be clearly seen due to sediments' deposition over one another.

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

Glacial : ocean floor

Metamorphism

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.

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

Rock cycle

Batholiths

Metamorphic rocks

Gypsum

Three elements of the earth's crust are iron, silicon and aluminium.

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 landforms 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.

Mechanical metamorphism is the mechanical transformation of pre-existing rocks, in which texture of rocks is changed by crushing and rubbing processes associated with heat and compression created by mechanical action.

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 hard and compact.
2. They are formed by solidification of molten magma.
3. They are granular and crystalline.
4. They are either fine grained, smooth and compact or may have large crystals with coarse texture.
5. The size of their crystals depends upon the rate of cooling of the molten material.
6. These rocks are less affected by chemical weathering because water does not percolate in these rocks.
7. Most of them consist of silicates.
8. They do not have layers. They are generally weathered by mechanical weathering.
9. They are associated with volcanic activity and are found mostly in 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. Aeolean 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.

Thus, rocks still play a crucial role in human civilization, both in our daily lives and in various industries.

Yes, the minerals in a rock provide an essential clue about how the rock was formed. Different minerals form under different conditions and the combination of minerals in a rock can help identify the type of rock and its formation process. Let us consider some examples:

1. Igneous rocks are formed from the solidification of molten material, and their minerals are usually interlocking crystals that form as the material cools.
2. Sedimentary rocks are formed from the accumulation and cementation of sediments, and their minerals are typically cemented together by minerals such as quartz, calcite, or clay.
3. Metamorphic rocks form from pre-existing rocks that are changed by heat and pressure, resulting in the rearrangement of minerals into new forms.

Thus, the presence, absence, or combination of minerals in a rock provides important clues about the rock's origin, formation process, and the geological history of the area where it was found.

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.

1. The igneous rocks are the primary source of all other rocks. They are formed on the outer surface of the earth due to volcanic activities. Rocks are disintegrated and eroded by climatic factors. The disintegrated material is removed and transported to lower levels by the forces of running water, winds, glaciers and ocean currents. In the basins, lakes or ocean beds the sediments accumulate.
2. The accumulations undergo lithification through the processes of evaporation, compaction and cementation.
3. Once the loose sediment is transformed into sedimentary rock, it is carried in depressions or on the sea floor further down under the earth. There may be further disintegration and recrystallisation into igneous rock and then it may be transformed into metamorphic rock.
4. In case the rock melts it may again result in formation of igneous rock. The disintegrated material may again form sedimentary rock. In completing any of these changes hundreds and thousands of years are involved.

In the operation of the rock cycle, the earth is kept young and movements of adjustments specially through horizontal flow or convection currents maintain the balance between various elements of the earth like oceans and continents.