Unit 12: Rocks
An introduction to the building blocks of our planet: their formation, types, and transformation.
12.45 Minerals & Rocks
To understand geology, we must first distinguish between minerals and rocks.
- A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a specific crystal structure. Examples include quartz ($SiO_2$), calcite ($CaCO_3$), and haematite ($Fe_2O_3$).
- A rock is a natural, solid aggregate of one or more minerals. For example, the rock granite is composed of the minerals quartz, feldspar, and mica.
In simple terms, minerals are the fundamental building blocks, and rocks are the structures built from them.
Solved Examples:
-
Is haematite ($Fe_2O_3$) a rock or a mineral? Explain.
Solution: Haematite is a mineral because it is a single compound with a definite chemical formula and crystal structure. A rock containing a high concentration of haematite would be called iron ore. -
What is the difference between a rock and a mineral?
Solution: A mineral is a pure substance with a fixed chemical composition, while a rock is a mixture of different minerals.
12.46 Types of Rock (Igneous, Sedimentary, Metamorphic)
Rocks are classified into three main types based on how they are formed.
- Igneous Rocks: Formed from the cooling and solidification of molten
rock (magma or lava).
- Intrusive Igneous Rocks (e.g., granite) form when magma cools slowly beneath the Earth's surface, resulting in large, visible crystals.
- Extrusive Igneous Rocks (e.g., basalt, pumice) form when lava cools quickly on the Earth's surface, resulting in very small crystals.
- Sedimentary Rocks: Formed from the accumulation, compaction, and cementation of sediments over millions of years. The sediments can be fragments of other rocks, organic matter, or chemical precipitates. They often form in layers. Examples include limestone, sandstone, and shale.
- Metamorphic Rocks: Formed when existing igneous or sedimentary rocks are changed by intense heat and pressure deep within the Earth's crust. This process, called metamorphism, changes the rock's mineral composition and texture. Examples include marble (from limestone) and slate (from shale).
Solved Examples:
-
A rock sample has large, interlocking crystals. What type of rock is it
likely to be?
Solution: The large crystals suggest it cooled very slowly, which is characteristic of an intrusive igneous rock like granite. -
How is marble formed?
Solution: Marble is a metamorphic rock. It is formed when sedimentary limestone is subjected to intense heat and pressure, causing the calcite crystals to recrystallize and grow larger. -
What is the key feature of sedimentary rocks?
Solution: They are formed from cemented sediments and are often found in distinct layers or strata.
12.47 Weathering of Rocks (Physical, Biological, Chemical)
Weathering is the process of breaking down rocks into smaller particles (sediments). It is a key part of the rock cycle. There are three main types:
- Physical (Mechanical) Weathering: The breakdown of rocks without changing their chemical composition. The most common form is freeze-thaw weathering, where water gets into cracks, freezes and expands, forcing the rock apart.
- Biological Weathering: The breakdown of rocks by living organisms. A common example is a plant's roots growing into cracks and forcing them wider.
- Chemical Weathering: The breakdown of rocks through chemical reactions,
which change the composition of the minerals. Key types include:
- Carbonation: Carbon dioxide in the air dissolves in rainwater to form weak carbonic acid ($H_2CO_3$). This acid rain reacts with and dissolves carbonate rocks like limestone.
- Oxidation: Oxygen from the air reacts with iron-containing minerals in rocks, causing them to "rust" and become weaker.
- Hydrolysis: The chemical breakdown of a rock due to reaction with water.
Solved Examples:
-
What is the difference between weathering and erosion?
Solution: Weathering is the process of breaking down rocks in place. Erosion is the process that involves the transport of those broken-down rock fragments by wind, water, or ice. -
Explain how acid rain contributes to the weathering of a limestone
statue.
Solution: This is an example of chemical weathering by carbonation. The acidic rainwater (containing carbonic acid) reacts with the calcium carbonate in the limestone, converting it into soluble calcium hydrogencarbonate, which then washes away, slowly dissolving the statue. -
A rock is broken apart by an expanding ice wedge. What type of weathering is
this?
Solution: This is physical (or mechanical) weathering. -
What is the name for the type of rock formed from cooled lava?
Solution: Extrusive igneous rock. -
What is the parent rock of slate?
Solution: Shale. Slate is the metamorphic rock formed from the sedimentary rock shale.
🧠Quiz
Answer: A naturally occurring solid
with a definite chemical composition and crystal structure.
Answer: Igneous rock.
Answer: The breaking down of rocks into
smaller particles.
Answer: Sedimentary rock.
Answer: The expansion of water when it
freezes in rock cracks.
Answer: A rock is an aggregate of one
or more minerals.
Answer: Metamorphic rock.
Answer: Chemical weathering
(specifically carbonation).
Answer: Large crystals, because it
cooled slowly.
Answer: Biological weathering.
Answer: Lava.
Answer: Limestone.
Answer: Silicon dioxide ($SiO_2$, e.g.,
quartz).
Answer: They are formed from compacted
sediments and often show layers.
Answer: Oxidation.
Answer: Intrusive rocks cool slowly
below the surface (large crystals), while extrusive rocks cool quickly on the
surface (small crystals).
Answer: Metamorphism.
Answer: Sediments.
Answer: A rock.
Answer: Calcium carbonate ($CaCO_3$).