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Rocks are a renewable solid that covers the crust of the earth. They are known to contain an
aggregate of minerals with combinations like chemicals compositions, texture, grain size and
many other unique characteristics. With the help of these combinations, rocks are mainly divided
into three major types; igneous, sedimentary and metamorphic. Additionally, rocks from these
types are classified differently according to an alternate categorized system.
Rock B is an extrusive igneous rock. It can be identified as this because due to the holes created
by the gas escape. They are dense and appear in dark-gray colour. Igneous rocks are featured in
different textures. The main textures are aphanitic, glassy, pegmatite, phaneritic, porphyritic and
pegmatitic. The vesicular rocks have an aphanitic texture which features a vesicular texture,
possessing holes and no minerals because the condensation is occurred from the rapid
crystallization of lava. From the Latin word ‘igneous’ meaning fire, igneous rocks are formed
through the cooling and solidification of magma and lava. It is the most common type of rock
found on the crust of the earth. Igneous rocks that are formed from lava are called extrusive
(volcanic) rocks. Extrusive rocks are formed when the hot molten rock is exposed to the earth's
surface due to volcanic eruption. They mainly consist of plagioclase and pyroxene. When
exposed on the surface, the lava tends to cool faster on the surface, causing to create finegrained rocks with a vesicular texture, no or small crystals formed. Examples of such rocks are
basalt,tuff and obsidian. Throughout New Zealand, igneous rocks are most likely to be found in
the areas of volcanoes like in the grand volcanic chain of Ruapehu and Tongariro. If igneous rocks
are found, it could be understood that the site were possessed with many active volcanoes years
Rock A is a foliated, regional metamorphosed rock. This can be identified from the visible lines
formed by the foliation. The rock’s colour is mixed with khaki and grey, composed with light
orange coloured lines. Metamorphic rocks are classified by different textures and mineral
assemblage. The most common texture that can be found are gneissic, tabular and flaky. By the
intense pressure it is forced, the rocks commonly tend to possess a squashed structure featuring
grain lines, foliated or banded. Regional metamorphic rocks are formed from the alteration of
other igneous and sedimentary rocks. When the contact metamorphic rocks are mainly alterrated
from heat, regional rocks are formed mostly from pressure. From the direct pressure and heat, the
rocks’ mineralogy and texture are commonly changed. Varieties of physical condition changes are
also found like temperature, lithostatic pressure and shear stress. Interestingly, regional
metamorphic rocks possess a very similar chemical composition to their sedimentary or igneous
precursors. This is because the composition was stable during the metamorphism. Also,
metamorphosed rocks could be in many different steps. For example, a sedimentary rock shale
could be metamorphosed to slate, then to phyllite, schist, gneiss. This is possible because the
metamorphism of a rock is dependant on the amount of pressure and heat forced on the rock. If
the provided heat on the rock is too strong, the rock could melt and condensed into an igneous
rock. Currently in New Zealand, many active volcanoes are visible on the North Island. However,
volcanic metamorphosed rocks are commonly found in the South Island such as Dunedin.
Furthermore if a metamorphosed rock is found, it can be predicted the rocks that form
metamorphic rocks are from nearby volcanoes and watery areas.
Rock C is a clastic sedimentary rock. Sedimentary rocks are made out of sediments throughout
thousands of years; variety of loose soil and rock pieces. They are normally well exposed on
surface and water. The sedimentary rocks are mainly formed by weathering, erosion and
deposition of the materials within water and the surface. The sedimentary rocks’ textures differ
regarding the compositions. As sedimentary rocks are formed from tiny pieces of other rocks that
went through sedimentation, the size of the grains control the texture of a rock. Size of a grain is
differed from 2mm to less than 63µm. Rock C’s texture is fine-grained, which classifies as a
clastic sedimentary rock. Sedimentary rocks are classified into three main types; chemical, clastic
and organic. Clastic sedimentary rocks occur when there is a buildup of tiny pieces of broken
rocks (clastics). These are deposited due to mechanical weathering and lithification. Breccias,
shale, sandstone and siltstone are all examples of this type of sedimentary rocks. Sedimentary
rocks are formed below water. Therefore, if these are found it could be said that the region is or
used to be a watery area. In New Zealand sedimentary rocks are commonly found on beaches or
large rivers like Waikato, Greymouth and Hawke’s Bay.
Between the three main types of rocks, the biggest difference is the time taken to become a rock.
An igneous rock is formed by the condensation of lava or magma. Compared to other rock types,
extrusive igneous rocks form and cool down faster, even in a few days, weeks. Also when igneous
and metamorphic rocks are found on the earth’s crust, sedimentary rocks are mostly found in
water bodies. This is because sedimentary rocks are in need of the pressure from other soils on
top of them.
When geologists study rocks, many significant information could be found between the scientific
research. For example in the form of sedimentary rocks, weathering, erosion and deposition is
essential. Studying weathering could provide information where the rain and wind is strong in the
country. This information could be used to provide the farmers where to farm because with
excessive amounts of natural obstacles, crops are hard to harvest. Also with the type of soil found
on the site, it could also be used to advise farmers where to farm depending on the ground.
In some cases, metamorphic rocks are formed from the pressure that has been forced from the
tectonic movements. Because of this, rocks could possess different unique grain lines and many
other characteristics. With this information, tectonic movements could be studied and provide
safety for the dangers of natural disasters like earthquakes.
In conclusion, rocks are an extremely important part throughout our lives. Studying rocks could
lead us into the history back with many environmental changes were visible, when the first
tectonic movement was detected and the fossils identified with numerous kinds of rocks. Also
rocks are used as valuable resources in our daily lives in construction sites and making cement
and statues.
Rocks have been a major key for our past and will also lead us throughout the future.