ROCKS AND MINERALS AND THEIR EXTRACTION , IGCSE Environmental Management 0680

CHAPTER 1

 

ROCKS AND MINERALS AND THEIR EXTRACTION

 

The Earth is a rocky planet. It is made from rocks and metal ores, which makes it the densest planet in the solar system. This is mainly because the core consists of iron surrounded by a mantle of rock.

What is a rock?

A rock is a solid mass of geological materials.

A rock is a solid collection of mineral grains that grow or become cemented together.

 

Formation of rocks:

The planet Earth has formed about 4.5 billion years ago. The force of gravity pulled the heavier elements together first, forming the core. The lighter elements then formed the Earth’s crust about 3–4 billion years ago. The mantle developed as a layer between the dense core and the light crust. This structure still exists today.

 The structure of the Earth: 

1.  Igneous rocks:

 

Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies.

 

·        When molten rock from the crust and upper mantle cools, igneous rocks are formed. The molten rock is called magma when it is still below the surface and lava when it reaches the surface.

·        Magma is found in the outer mantle; it is a hot, liquid rock that is under pressure from the rocks above it. When it cools it turns to solid rock. When liquid magma rises to the surface from volcanoes the cooling occurs quickly and forms lava.

 

·        Igneous rocks are made of material that was once molten; they usually contain crystals that are formed as the molten material cools.

 

·        The crystals found in rocks are formed when solutions of minerals cannot absorb any more dissolved minerals.

 

·        Some of each mineral type precipitates out of solution to form the center of a crystal. This then provides a surface for more mineral ions to precipitate onto. The crystal becomes larger until the solution disappears.

 

·        Extrusive igneous rock: If the rock cools quickly, only very small crystals can form before the rock becomes solid e.g basalt.

 

·        Intrusive igneous rock: If magma rises from the mantle into the crust without reaching the Earth’s surface, then the magma cools more slowly, allowing the formation of larger crystals. Many of these crystals contain valuable minerals that are used for a wide range of industrial processes e.g granite.

2. Sedimentary Rocks

 

 

·        Are formed by the weathering of existing rocks at the Earth’s surface, the accumulation and fossilization of living material, or the precipitation of dissolved materials out of solution in water.

 

·        Weathering processes release small mineral particles that accumulate to form sediment (small particles of rocks). Over time, layers of sediment build-up to form sedimentary rock.

 

·        The sediments include different-sized mineral particles. The smallest particles are clays, followed by silts and then sands. These particles are important in the formation of soils. Larger particles of gravels and small boulders can also be found in sediments.

 

·        The particles are transported by streams and rivers and then deposited as sediment. Each layer of sediment becomes more compact and harder because of the pressure created by the newer deposits above them. Examples of sedimentary rock are limestone, sandstone, and shale.

 

3.  Metamorphic rocks:

 

·        Are created from existing rocks when the heat or pressure or both heat and pressure, causes changes in the rock crystals without melting the existing rock. The existing rock, therefore, changes in structure, becoming a metamorphic rock. The changes in structure can be chemical or physical or both.

 

·        Sedimentary and igneous rocks can become metamorphic rocks, and metamorphic rock can become another metamorphic rock.

 

·        Metamorphic rocks are usually harder than sedimentary rocks.

 

·        Examples of metamorphic rocks are marble and slate.

 

 

The Rock Cycle:

 

·        When the Earth’s crust first formed, all the rocks were igneous.

 

These rocks were slowly eroded, releasing small particles that formed sediment, and these sediments built up over time to form sedimentary rocks. The rocks that make up the Earth’s crust are always moving, which creates the heat and pressure needed to

form metamorphic rock.

All rock types are constantly eroded and formed in the rock cycle.

 

·        The rock cycle is a representation of the changes between the three rock types and processes causing them.

Characteristics of the different rock types.

Igneous	Sedimentary	Metamorphic
Made from liquid magma	Made from other rock fragments	Made from existing rock
Magma cools to form solid rock	Rock fragments become buried and increased pressure forms a rock	The original rock is changed in form by heat and pressure
Mineral crystals sometimes present; the size of the crystals depend on the speed of cooling	Crystals absent	Mineral crystals present
No fossils present	Fossils maybe present	No fossils present

  

 

 

Key Terms

 

Rock: a combination of one or more minerals

 

Mineral: a naturally occurring inorganic substance with a specific chemical composition

 

Igneous rock: rock made during a volcanic process

Magma: molten rock below the surface of the Earth

 

Solution: formed when a solid is dissolved in a liquid

precipitates: when a substance comes out of solution

 

Ion: An atom in which the number of positively charged protons is not equal to the number of negatively charged electrons.

 

Sedimentary rock: a rock formed from material derived from the weathering of other rocks or the accumulation of dead plants and animals

 

Metamorphic rock: a rock formed from existing rocks by a combination of heat and pressure