There
are three basic types of rock:
IGNEOUS rocks are formed when molten rock cools, it
is found in two major forms
-
Basalt
is lava from a volcano that has been spewed out and cooled
on the surface.
-
Granite
is formed when magma, found in the earth's mantle, rises
closer to the surface then cools slowly inside the earth's
crust.
SEDIMENTARY
rocks are formed when weathered or eroded material is deposited
on the bottom of rivers, lakes, seas and oceans. Over long
periods of time this sediment is buried and compressed. Often
plant and animal material is buried along with it and is found
as fossils. Coal, limestone and sandstone are sedimentary
rocks.
METAMORPHIC rocks are formed when rock is structurally
altered through intense heat and pressure. Marble is produced
when limestone is subjected to these stresses.
Trapped in each type of rock are different minerals vital
for survival. All rocks undergo weathering that continuously
breaks them down into smaller particles. Natural weathering
happens in three different ways:
-
Physical
weathering causes rocks to break up through the action
of winds and rain. In addition water in cracks expands
when freezing breaking rocks apart and thawing carries
particles away.
-
Chemical
weathering slowly decomposes rocks by chemical reactions
altering the minerals reducing the way they are bonded
together. Oxygen weakens rocks by reacting with the minerals
to form oxides.
-
Biological
weathering is the action of plants and animals upon rock.
For example roots force rocks apart as they grow, animals
scrape and dig whilst looking for food.
Soil
is made up of rock particles, plant and animal material, nutrients,
air and water. Dissolved in the water are the minerals from
rocks essential for plant growth. The mineral elements needed
by plants are absorbed and a cycle takes place - weathering
from rock - uptake (absorption) by plants into the plant's
structure -death and decomposition of the plant releases the
minerals back into the soil for reuse by other plants. Potassium
and phosphorous are examples of minerals that follow these
cycles.
Some soil is naturally washed into rivers and lakes and eventually
to seas where it remains on the ocean beds to become the sedimentary
rocks of the very distant future so continuing the cycle with
new rock formation.
Human activities such as intensive farming, deforestation
and agrochemicals cause
soil to become unstable and more easily washed away and deposited at river estuaries
and eventually swept out to sea. It is estimated that the earth is losing 30,000
million tonnes of soil per year in this way, which is not recoverable. Soils are
being washed away faster than new rocks form. Once the soil structure is weakened
desertification occurs
as soils are rapidly removed through the actions of wind and rain.
In addition, mining removes rocks from the natural cycle and
dramatically alters ecosystems.
Coal is mined for fuel, limestone for road chippings and cement,
sandstone and marble for buildings are a few examples. Minerals
and elements are also mined and extracted from rock structures.
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