ROCKS
Rocks are hard substances consisting of an aggregate of minerals which undergo physical, chemical, and biological weathering to give rise to soil.
TYPES OF ROCKS
1. Igneous rocks 2. Sedimentary rocks 3. Metamorphic rocks
IGNEOUS ROCKS
Igneous rocks are derived from molten lava which has cooled down and solidified. Igneous rocks are called fire rocks and are formed either underground or above ground.
TYPES OF IGNEOUS ROCKS
There are two types of igneous rocks:
1. Plutonic or intrusive igneous rocks: These rocks are formed when molten magma or lava cools and solidifies underground. Examples are granite, gabbro, diorite, peridotite, syenite, dolerite, granophyre, pegmatite, dolerite and tuff.
2. Volcanic or Extrusive igneous rocks: These rocks are formed when molten magma or lava cools and solidifies above the ground as a result of volcanic eruptions. Examples are basalt, pumice, felsite, scoria, obsidian, rhyolite, and andesite.
CHARACTERISTICS OF IGNEOUS ROCKS
1. They are heavy
2. They do not allow fluids to pass through (impervious)
3. They are massive
4. They are crystalline in nature
5. They are hard
6. They contain no fossil (remains of plants or animals).
7. Resistant to weathering or erosion
8. They are not arranged in layers (stratified)
SEDIMENTARY ROCKS
These rocks are generated from the settling of broken-down particles of other weathered rocks and organic materials. As these particles are deposited on each other, they are squeezed up and cemented to form rocks.
TYPES OF SEDIMENTARY ROCKS
There are three types of sedimentary rocks:
1. Mechanically-formed (clastic) sedimentary rocks: These rocks are formed when particles of other weathered rocks accumulate and cement together over a long period of time. Examples are conglomerate, sandstone, grindstone, shale, mudstone, siltstone and clay.
2. Organically - formed sedimentary rocks: These rocks are formed when plants and animal parts accumulate and are compressed to formed rocks. Examples coal, peat, lignite petroleum, limestone, chalk, coral, dolomite, loess, diatomite, anthracite, natural gas etc.
3. Chemically - formed sedimentary rocks: These rocks are formed when dissolve minerals precipitate from solution. Examples rock salt (sodium chloride), iron ore, gypsum, dolomite, limestone, nitrate, potash, flint, chert etc.
CHARACTERISTICS OF SEDIMENTARY ROCKS
1.They are relatively soft
2. They may contain fossil
3. They are non-crystalline
4. They are arranged in layers
5. They are dull and rough in appearance
6. They allow fluids to pass through them
7. They contain fragments of preexisting rocks.
8. They generally not resistant to weathering.
DIFFERENCES BETWEEN IGNEOUS AND SEDIMENTARY ROCKS
IGNEOUS ROCKS SEDIMENTARY ROCKS
1. They are not arranged in layers 1. They arranged in layers
2. They are crystalline in nature 2. They are not crystalline in nature
3. They are hard 3. They are relatively soft
4. They don’t allow fluids to pass through them 4. They allow fluids to pass through them
5. They are formed from hot magma 5. They are formed from existing rocks and fossil
6. contain many minerals 6. contain few minerals
7. They have no bedding planes 7. They have bedding planes
METAMORPHIC ROCKS
Metamorphic rocks are rocks formed when igneous and sedimentary rocks change due to high pressures and temperatures.
TYPES OF METAMORPHIC ROCKS
There are two types of metamorphic rocks:
1. Foliated metamorphic rocks: These rocks are formed when heat and pressure within the earth’s crust squeeze the flat or elongated minerals within a rock such that they become layered or branded. The individual mineral grains are oriented parallel to one another. They tend to break along the flat planar surface.
Examples are Gneiss (from granite), serpentine (from gabbro), slate (from shale), schist (from conglomerate), phyllite (from shale), biotite (from gabbro) etc.
2. Non – foliated metamorphic rocks: These types of rocks are made from minerals that are not flat or elongated. No matter how much pressure you apply the grains will not align to form layers. They massive and do not possess cleavage (do not break along flat planar surface.
Examples: quartzite (from sandstone), marble (from limestone), anthracite (from bituminous coal) , graphite (from coal), hornfels, novaculite.
CHARACTERISTICS OF METAMORPHIC ROCKS
1. They are very heavy
2. They are massive and hard
3. They are crystalline and shiny
4. They may contain fossil
5. Some may occur in layers
6. They break more easily in one direction.
7. Foliated ones have grains arranged parallel to one another.
IMPORTANCE OF ROCKS
1. Source of fuel example petroleum and coal
2. Source of food example sodium chloride
3. source of minerals example gold, diamond, limestone
4. Rocks are used to control soil erosion
5. sources of plant nutrients like iron, calcium, potassium
6. Construction purpose example sandstone.
7. Formation of soil through weathering
8. For decoration example marble stones.
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