Rocks
ICSE Class 9 · Geography · Chapter 6
What this chapter covers
- Minerals and rocks
- Types of rocks and how they are classified
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
- Comparing the three types of rocks
- The rock cycle
Key points
- A mineral has a definite chemical composition and internal structure; a rock is a natural solid aggregate of one or more minerals with no fixed composition.
- Feldspar, quartz, mica, pyroxene, amphibole and calcite are the main rock-forming minerals.
- Rocks are classified by their mode of formation into igneous, sedimentary and metamorphic rocks.
- Igneous rocks form when magma or lava cools and solidifies; they are the primary or parent rocks.
- Intrusive rocks such as granite cool slowly underground and are coarse-grained; extrusive rocks such as basalt cool quickly on the surface and are fine-grained.
- Acid igneous rocks are rich in silica, light in colour and hard; basic igneous rocks are poorer in silica, dark and heavy.
- Igneous rocks are crystalline, hard, unlayered and never contain fossils.
- Sedimentary rocks form by weathering, erosion, transportation, deposition, compaction and cementation of sediment.
- Sedimentary rocks are mechanically formed (sandstone, shale), organically formed (limestone, coal) or chemically formed (rock salt, gypsum).
- Sedimentary rocks are layered, often contain fossils, are generally soft and porous, and hold coal, oil and gas.
- Metamorphic rocks form when existing rocks are changed by heat and pressure without melting, by contact or regional metamorphism.
- Limestone becomes marble, sandstone becomes quartzite, shale becomes slate and granite becomes gneiss.
- Metamorphic rocks are hard, crystalline, often banded, and rarely contain fossils.
- The rock cycle is the continuous change of rocks from one type to another, driven by the earth's internal heat and the sun's energy, with no beginning and no end.
Questions answered
Define a mineral. Name two rock-forming minerals.
A mineral is a naturally occurring inorganic substance with a definite chemical composition and internal atomic structure, usually with a regular crystal form. Quartz and feldspar are rock-forming minerals.
Give three differences between a mineral and a rock.
A mineral has a definite chemical composition, a rock does not; a mineral is a single substance, a rock is usually an aggregate of several minerals; a mineral has a definite crystal structure and fixed properties, while the properties of a rock vary from sample to sample.
On what basis are rocks classified into three types? Name the types.
Rocks are classified by their mode of formation into igneous, sedimentary and metamorphic rocks.
Why are igneous rocks called primary rocks?
They were the first rocks to form as the molten earth cooled, and all other rocks are ultimately formed from them.
Distinguish between intrusive and extrusive igneous rocks, with one example of each.
Intrusive rocks form when magma solidifies below the surface; they cool slowly and are coarse-grained, for example granite. Extrusive rocks form when lava solidifies on the surface; they cool quickly and are fine-grained, for example basalt.
Give a reason why the Deccan Plateau has black soil.
The plateau is built of basalt, a basic igneous rock formed from lava flows; basalt is rich in iron and magnesium and weathers into fertile black soil.
Explain the terms compaction and cementation.
Compaction is the squeezing of sediment by the weight of the layers above, which pushes out water and packs the grains together. Cementation is the gluing of the grains by minerals such as silica or calcite deposited from water moving through the sediment.
Name one mechanically, one organically and one chemically formed sedimentary rock.
Sandstone (mechanically formed), coal or limestone (organically formed), rock salt or gypsum (chemically formed).
Past board exam questions
ICSE 2019 sample paper · 2 marks
State two differences between rocks and minerals.
Show answer
1. A mineral is a naturally occurring inorganic substance with a definite chemical composition and physical properties (e.g. quartz, feldspar); a rock is an aggregate of one or more minerals and has no definite chemical composition (e.g. granite).
2. Minerals are the building blocks of rocks; rocks are the materials that make up the Earth's crust. A mineral often has a definite crystalline form, colour and hardness, whereas a rock's properties vary with the minerals it contains.
ICSE 2019 sample paper · 3 marks
Draw a neat well labelled diagram of the Rock Cycle.
Show answer
Diagram (cycle with arrows): Magma -> (cooling and solidification) -> Igneous rocks -> (weathering, erosion, transportation, deposition, compaction and cementation) -> Sedimentary rocks -> (heat and pressure) -> Metamorphic rocks -> (melting) -> Magma. Also arrows: igneous rocks -> (heat and pressure) -> metamorphic rocks; metamorphic and sedimentary rocks -> (weathering and erosion) -> sediments; igneous rocks -> (melting) -> magma.
ICSE 2019 sample paper · 3 marks
Give a reason for each of the following: (i) Igneous rocks are also called Primary rocks. (ii) Fossils are present in Sedimentary rocks. (iii) The core of the Earth is in a semi-solid state.
Show answer
(i) Igneous rocks were the first rocks to be formed, by the cooling and solidification of molten magma; all other rocks are formed directly or indirectly from them, so they are called primary (parent) rocks.
(ii) Sedimentary rocks are formed by the deposition of layers of sediments, usually in water; remains of plants and animals get buried in these layers and are preserved as fossils because the rocks are not subjected to great heat.
(iii) The core has extremely high temperature, which would melt the iron and nickel, but the enormous pressure of the overlying layers prevents it from becoming fully liquid; so the outer core is molten/semi-solid while the inner core is solid.