Textbook Solutions: The Interior of the Earth | Window on the Word Class 7: Solutions, Notes & Worksheets PDF Download

A. Choose the correct answer

1.
Ans: (a)

2.
Ans: (c)

3.
Ans: (d)

4.
Ans: (c)

B. Fill in the blanks

C. Answer in one or two sentences.

D. Answer in a paragraph.

1.
Ans: The oceanic crust differs from the continental crust in several ways. Firstly, the oceanic crust is thinner and denser than the continental crust, extending to a depth of 5 km to 10 km compared to the 35 to 70 km depth of the continental crust. Secondly, the oceanic crust is composed mainly of silica and magnesium, and it forms the ocean floor, whereas the continental crust is composed mainly of silica and aluminium and forms the continents.

2.
Ans: The three types of rocks are igneous, sedimentary and metamorphic rocks. Igneous rocks are formed when magma rises to the crust, cools down and solidifies. Sedimentary rocks are formed when rocks break down into small fragments known as sediment, which is then transported and deposited in different places by wind and water, and over time, gets compressed and hardened under pressure and varying temperatures. Metamorphic rocks are formed when igneous and sedimentary rocks transform or metamorphose under heat and pressure to form new kinds of rocks.

3.
Ans: Extrusive rocks are formed when magma, known as lava when it reaches the surface, cools and solidifies quickly on the Earth's surface after a volcanic eruption, resulting in very finely grained or even glassy rocks. Conversely, intrusive rocks are formed when magma gets trapped under the crust and cools down and solidifies very slowly, forming large crystals and a coarse structure. Examples of extrusive rocks include basalt and pumice, while granite and gabbro are examples of intrusive rocks.

4.
Ans: The rock cycle starts with magma cooling down to form igneous rocks. These rocks are brought to the Earth's surface due to earth movements like folding and faulting. Over millions of years, the rocks break down due to weathering and erosion to form sediments. These sediments are transported by water, wind or ice to different places, where they are deposited in layers. Over time, the sediments build up, until they are eventually compressed and hardened to form sedimentary rocks. Sometimes, when they are subject to conditions of high temperature and pressure, igneous and sedimentary rocks are transformed into metamorphic rocks. Movements of the Earth can push metamorphic rocks deep into the mantle, where they change back to molten magma, which can again solidify to form igneous rocks. This continuous cycle, through which one form of rock is changed into another, is called the rock cycle.

5.
Ans: Minerals are different naturally occurring substances that compose rocks, each with a unique chemical composition and definite physical properties. Examples of minerals include iron ore, copper, bauxite, limestone, mica, and kaolinite. Minerals are useful as they are the source of several important metals, such as iron, gold, aluminium, copper, and silver, which are used in various industries. Non-metallic minerals are used in various industries, for instance, limestone is used to make cement.

6.
Ans: Fossil fuels are important sources of energy found in sedimentary rocks, formed as a result of the decomposition of fossils, which are plants and animals that died millions of years ago and got buried deep in the Earth's crust. Fossil fuels are used for various activities, including cooking, driving vehicles, and running factories, and most of the energy we use comes from fossil fuels.