ICSE CLASS 9 GEOGRAPHY//ROCKS

Question 1.

State two points of distinction between rocks and minerals.

Answer:

Rocks :

·         Rocks contain minerals.

·         Rocks are the aggregates of minerals and other rock material.

Minerals :

 

·         Minerals do not contain rocks.

·         Mineral are the compounds of pure elements with a definite chemical Composition.

Question 2.

Name any three elements of the earth’s crust.

Answer:

These are silicon, aluminum, iron, magnesium, calcium, potassium, sodium and nickel.

 

Question 3.

Name three types of rocks.

Answer:

Three types of rocks are sedimentary, igneous and metamorphic rocks.

 

 Question 4.

Why are the igneous rocks also called the primary rocks?

Answer:

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

 

Question 5.

Give one difference between Extrusive igneous and Intrusive igneous rocks.

Answer:

Extrusive igneous rocks are formed by the cooling of molten magma on the earth’s surface.

Intrusive igneous rocks are formed when the magma solidifies within the earth’s crust forming coarse texture.

Question 6.

Name any two chief characteristics of Igneous Rocks.

Answer:

Igneous rocks are formed by cooling of magma and these are crystalline rocks and full of mineral compounds.

 

Question 7.

What is the main characteristics of Basic Igneous Rocks?

Answer:

Basic igneous rocks have higher percentage of oxides of denser elements and dark in colour, e.g. Basalt, dolerite etc.

Question 8.

Name two important landforms made by Igneous Rocks.

Answer:

Conical mountains and extensive lava plateaus.

 

Question 9.

What are Sills? Give an example.

Answer:

Sill, also known as a sheet, is a flat igneous rock intrusion that occurs between layers of rock. Sills run parallel to the bedding of the surrounding rocks, and although they can be vertical or horizontal, nearly horizontal sills are the most common.

Sills of varying sizes and ages have intruded into the surrounding rocks in an impressive range of ways. Archuleta Mesa, near the New Mexico border, is one of the highest.

Question 10.

Which rocks are associated with ores of metals ?

Answer:

Igneous rocks are associated with ores of metals due to their origin by cooling magma resulting in crystalline rocks rich in mineral content

Question 11.

Which rocks are associated with fossil fuels ?

Answer:

Sedimentary rocks are associated with fossil fuels due to fossils trapped in between layers of these rocks.

 

Question 12.

Mention any two chief characteristics of Sedimentary Rocks.

Answer: Two characteristics of Sedimentary Rocks are-

 

1.   Sedimentary rocks, also known as secondary rocks, are produced when sediments from other rocks are sorted.

2.   The majority of sedimentary rocks are located on the surface of the Earth. They take up 75% of the planet's surface.

3.   In most cases, these rocks are not crystalline. Since they are formed by the deposition of sediments, they are soft and have multiple layers.

4.   In between the layers of these rocks, there may be the remains of plants and animals.

5.   Sedimentary rocks are categorized according to the type of sediment, their origin, and their composition.

Question 13.

Give two examples of Sedimentary Rocks.

Answer:

Sandstone

Limestone

Question 14.

Name the rocks which are most widespread on the earth.

Answer:

Sedimentary rocks are most widespread on the earth.

Question 15.

Name the three stages of lithification of Sedimentary rocks.

Answer:

Lithification is the process of turning loose rock material into hard rock through evaporation, compaction and cementation.

 

Question 16.

Name the types Sedimentary rocks based on agents of formation.

Answer:

There are five main types of 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 sand accumulation by the wind.

5.   Marine Rocks These are formed by the deposition of sediments at the ocean floor. These types are Calcarious and Carbonaceous sedimentary Rocks.

Question 17.

Which agents are responsible for deposition of sediments?

Answer:

There are three main agents of deposition i.e., water, wind and ice, which mainly act as agents of change on the surface of earth for deposition of sediments.

 

Question 18.

What are known as metamorphic rocks ? Give two examples.

Answer:

Metamorphic rocks are formed from other types of rocks that have undergone drastic changes from their initial igneous, sedimentary, or metamorphic states.

When rocks are exposed to high heat, high pressure, hot mineral-rich fluids, or a combination of these factors, metamorphic rocks form.

These kinds of conditions can be found deep inside the Earth or where tectonic plates collide.

Example

Anthracite, quartzite, granite, slate, granulite, gneiss, and schist are examples of metamorphic rocks.Question 19.

What is Mechanical Metamorphism ?

Answer:

Three kinds of metamorphism may happen contingent upon the general impact of mechanical and chemical changes. Dynamic transformation, or cataclasis, results for the most part from mechanical deformity with minimal long haul temperature change. Surfaces delivered by such changes range from breccias made out of precise, broke stone sections to exceptionally fine-grained, granulated or powdered rocks with clear foliation and lineation.

Question 20.

What is meant by Rock Cycle ?

Answer:

The earth is said to be 4700 million years old and the rocks came into existence 3400 years ago. Since then these rocks have undergone various changes by which multiple transformation took place within the rocks. This continuous process of transformation of old rocks into new rocks is known as rock cycle for figure See Page no. of this book.

Question 21.

What processes are involved in the formation of Igneous Rocks ?

Answer:

Igneous rocks are associated with volcanic activity, cooling of magma and mechanical weathering etc.

Question 22.

How are sedimentary rocks classified on the basis of their formation ?

Answer:

There are three types of this type of rocks :

 

Mechanically formed rocks : Denudation of rocks by agents of change results in layers of sedimentary rocks by mechanism.

Chemically formed rocks : These are formed by evaporation of calcarious water and accumulated along lake and lagoon beds, e.g. gypsum and rock salt.

Organically formed rocks : These are formed by deposition of dead plants and animals e.g. limestone rock (or Calcareous rock) lignite, bituminous and anthracite coal are carbonaceous rocks formed by compression of vegetation under the layers of sedimentary rocks. In the same way petroleum in between the sedimentary rocks is the result of zurasic age dinosaurs e.g. shale rock, containing oil.

 

Question 23.

Extrusive Igneous Rocks.

Answer

Extrusive rock is a type of igneous volcanic rock produced when hot magma from deep inside the Earth flows out (extrudes) as lava or bursts violently into the atmosphere and falls back as pyroclastics or tuff. In comparison, the magma does not hit the surface of intrusive rock formations.

Extrusion has the primary effect of causing the magma to cool much faster in the open air or underwater, leaving little time for crystal formation. A small portion of the matrix can never crystallize, turning into natural glass or obsidian instead.

If the magma contains a lot of volatiles that are released as free gas, it can cool with large or small vesicles (bubble-shaped cavities), like in pumice, scoria, or vesicular basalt. Basalt, rhyolite, andesite, obsidian, and pumice are some examples of extrusive rocks.

Question 24.

Laccoliths and Batholiths.

Answer

Laccoliths are formed when magma seeps into the erupting channel underneath the crust. Batholiths, such as the Ranchi Batholiths, are deep-seated dome-shaped magma intrusion formations that typically form the base of mountain ranges.

An igneous intrusion (also known as a concordant pluton) inserted between two layers of sedimentary rock is known as a laccolith. The magma pressure is high enough to push the overlying strata upward, giving the laccolith a dome or mushroom-like shape with a typically planar base.

Batholiths are enormous volcanic nosy stone emplacements framed profound inside the Earth's covering by cooled magma. Batholiths are almost often made up of felsic or intermediate rock forms including granite, quartz monzonite, or diorite.

Question 25.

Fossil fuels.

Answer

The word "fossil fuel" refers to a fuel that is made by natural processes, such as the anaerobic decomposition of buried dead animals, and contains organic molecules from ancient photosynthesis that release energy when burned.

 Question 26.

Distinguish between Plutonic and Volcanic rocks.

Answer

Plutonic rocks :

These are intrusive igneous rocks.

These cool down very slowly due to heat in the interior

Volcanic rocks :

These are extrusive igneous rocks.

These cool down abruptly due to less temperature on the surface region.

Question 27.

Distinguish between Thermal and Dynamic Metamorphism.

Thermal metamorphism:

Thermal metamorphism is the alteration of the shape and texture of rocks as a result of extreme temperatures.

Hot magma or rock friction may be causing the heat.

Dynamic Metamorphism:

Excessive compression caused by tectonic forces within the earth allows Dynamic Metamorphism to occur.

It happens as a result of deep pressure-induced physical or chemical changes.

Question 28.

Distinguish between Sills and Dykes.

Sills

Sometimes magma flows between layers of rocks horizontally and hardens there. This layer of intrusive rock is called sills.  

Dykes

The magma when forced upwards fills vertical cracks or fissures in existing rocks and then hardens there to form dykes.

 

Question 29.

Calcarious and Carbonacious rocks.

Answer:

Calcarious rocks : The sedimentary rocks formed by the deposition of shells, skeletons of sea organism as corals, clams and oysters etc. are called calcarious rocks due to excess of calcium.

Carbonacious rocks : The sedimentary rocks formed by the deposition of large scale vegetation and animals etc. are called carbonacious rocks due to the excess of carbon.

Question 30.

Acid Igneous Rocks and Basic Igneous Rocks.

Answer:

Acid Igneous rocks : These are lighter rocks with silica content 65 to 85 percent with density less than 2 g/Cm3, e.g. Granite.

Basic igneous rocks : These are denser rocks with silica content 40 to 60 percent with average density between 2.8 g/Cm3 and 3.0 g/Cm3 e.g. Basalt.