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Rocks on Earth

Rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.

The earth’s crust is composed of rocks. A rock is an aggregate of one or more minerals. Rock may be hard or soft and in varied colors. 

For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white. Rocks do not have definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.

You need minerals to make rocks, but you don't need rocks to make minerals.
All rocks are made of minerals.

There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation. 

They are: 

(i) Igneous Rocks — solidified from magma and lava;

(ii) Sedimentary Rocks — the result of deposition of fragments of rocks by exogenous processes; 

(iii) Metamorphic Rocks — formed out of existing rocks undergoing recrystallisation.

Types of Rocks

Igneous Rocks

Igneous means made from fire or heat.

As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks. The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma cools and solidifies.

When volcanoes erupt and the liquid rock comes up to the earth's surface, then new igneous rock is made. When the rock is liquid & inside the earth, it is called magma

When magma in its upward movement cools and turns into solid form it is called igneous rock. The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.

When the magma gets hard inside the crust, it turns into granite. 

Most mountains are made of granite. It cools very slowly and is very hard. When the magma gets up to the surface and flows out, like what happens when a volcano erupts, then the liquid is called lava.

Lava flows down the sides of the volcano. When it cools & turns hard it is called obsidian, lava rock or pumice - depending on what it looks like.
  • Igneous rocks form when molten lava (magma) cools and turn to solid rock.  The magma comes from the Earth’s core which is molten rock .  The core makes up about 30% of the Total Earth Mass (31.5%)
  • Obsidian is nature’s glass. It forms when lava cools quickly on the surface. It is glassy and smooth.
  • Pumice is full of air pockets that were trapped when the lava cooled when it frothed out onto the surface.  It is the only rock that floats.  

There are 5 kinds of igneous rocks, depending on the mix of minerals in the rocks. 

    • Granite contains quartz, feldspar & mica
    • Diorite contains feldspar & one or more dark mineral. Feldspar is dominant.
    • Gabbro contains feldspar & one or more dark mineral. The dark minerals are dominant.
    • Periodotite contains iron and is black or dark.
    • Pegmatite is a coarse-grained granite with large crystals of quartz, feldspar and mica.
    Igneous rocks are classified based on texture. Texture depends upon size and arrangement of grains or other physical conditions of the materials. If molten material is cooled slowly at great depths, mineral grains may be very large. Sudden cooling (at the surface) results in small and smooth grains.

    Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks. Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.

    Sedimentary Rocks

    The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling. Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments. Such fragments are transported by different exogenous agencies and deposited. These deposits through compaction turn into rocks. This process is called lithification. 

    In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification. Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.

    When mountains are first formed, they are tall and jagged like the Rocky Mountains on the west coast of North America. Over time (millions of years) mountains become old mountains like the Appalachian Mountains on the east coast of Canada and the United States. When mountains are old, they are rounded and much lower.

    What happens in the meantime is that lots of rock gets worn away due to erosion.  Rain, freeze/thaw cycle, wind and running water cause the big mountains to crumble a little bit at a time.
    Eventually most of the broken bits of the rock end up in the streams & rivers that flow down from the mountains. These little bits of rock & sand are called sediments.

    When the water slows down enough, these sediments settle to the bottom of the lake or oceans they run into. Over many years, layers of different rock bits settle at the bottom of lakes and oceans.
    Think of each layer as a page in a book. One piece of paper is not heavy. But a stack of telephone books is very heavy & would squish anything that was underneath. Over time the layers of sand and mud at the bottom of lakes & oceans turned into rocks. These are called sedimentary rocks.
    Some examples of sedimentary rocks are sandstone and shale. Sedimentary rocks often have fossils in them.

    Plants & animals that have died get covered up by new layers of sediment and are turned into stone. Most of the fossils we find are of plants & animals that lived in the sea. They just settled to the bottom.

    Other plants & animals died in swamps, marshes or at the edge of lakes. They were covered with sediments when the size of the lake got bigger.
    When large amounts of plants are deposited in sedimentary rocks, then they turn into carbon. This gives us our coal, oil, natural gas and petroleum.

    A large sea once covered the central part of Canada and the climate was very tropical. In time, sedimentary rocks formed there. That is why we find dinosaur fossils in Alberta and the area is a good source of natural fuels.
    Sedimentary rocks cover 75% of the earth’s surface. Most of the rocks found on the Earth’s surface is sedimentary even though sedimentary rocks only make up less than 5% of all the rocks that make up Earth.
    When rocks are exposed to the elements – air, rain, sun, freeze/thaw cycle, plants – erosion occurs and the little bits of rock worn away get deposited as sediments.

    Over time, these sediments harden as they get buried by more sediments and turn into sedimentary rocks. Sedimentary rocks are usually formed in layers called strata.

    There are 6 main kinds of sedimentary rocks depending on the appearance of the rock.

    Conglomerate rock has rounded rocks (pebbles, boulders) cemented together in a matrix.
    Sandstone is a soft stone that is made when sand grains cement together. Sometimes the sandstone is deposited in layers of different colored sand.
    Shale is clay that has been hardened and turned into rock. It often breaks apart in large flat sections.
    Limestone is a rock that contains many fossils and is made of calcium carbonate &/or microscopic shells.
    Gypsum, common salt or Epsom salt is found where sea water precipitates the salt as the water evaporates.
    Breccia has jagged bits of rock cemented together in a matrix. 

    Depending upon the mode of formation, sedimentary rocks are classified into three major groups:

    (i) mechanically formed — sandstone, conglomerate, limestone, shale, loess etc. are examples;
    (ii) organically formed— geyserite, chalk, limestone, coal etc. are some examples;
    (iii) chemically formed — chert, limestone, halite, potash etc. are some examples.

    Metamorphic Rocks

    The word metamorphic means ‘change of form’.  The word comes from the Greek "meta" and "morph" which means to change form. 

    Metamorphic rocks were originally igneous or sedimentary, but due to movement of the earth's crust, were changed. 

    These rocks form under the action of pressure, volume and temperature (PVT) changes. Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.

    Metamorphism is a process by which already consolidated rocks undergo recrystallisation and reorganisation of materials within original rocks.
    If you squeeze your hands together very hard, you will feel heat and pressure. When the earth's crust moves, it causes rocks to get squeezed so hard that the heat causes the rock to change.

    Marble is an example of a sedimentary rock that has been changed into a metamorphic rock. 

    Mechanical disruption and reorganisation of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism.

    The materials of rocks chemically alter and recrystallise due to thermal metamorphism.

    There are two types of thermal metamorphism —
    contact metamorphism and 
    regional metamorphism.

    In contact metamorphism the rocks come in contact with hot intruding magma and lava and the rock materials recrystallise under high temperatures. Quite often new materials form out of magma or lava are added to the rocks.
    In regional metamorphism, rocks undergo recrystallisation due to deformation caused by tectonic shearing together with high temperature or pressure or both. In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.

    Metamorphic rocks are classified into two major groups — foliated rocks and non-foliated rocks.
    • Foliated metamorphic rocks have layers, or banding.
      • Slate is transformed shale. It splits into smooth slabs.
      • Schist is the most common metamorphic rock. Mica is the most common mineral.
      • Gneiss has a streaky look because of alternating layers of minerals.
    • Non-foliated metamorphic rocks are not layered.
      • Marble is transformed limestone.
      • Quartzite is very hard.


    Rocks do not remain in their original form for long but may undergo transformation. Rock cycle is a continuous process through which old rocks are transformed into new ones.

    Each type of rock is altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the three rock types are related to each other, and how processes change from one type to another over time.

    Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) form from these primary rocks. Igneous rocks can be changed into metamorphic rocks. The fragments derived out of igneous and metamorphic rocks form into sedimentary rocks. 

    Sedimentary rocks themselves can turn into fragments and the fragments can be a source for formation of sedimentary rocks. The crustal rocks (igneous, metamorphic and sedimentary) once formed may be carried down into the mantle (interior of the earth) through subduction process (parts or whole of crustal plates going down under another plate in zones of plate convergence) and the same melt down due to increase in temperature in the interior and turn into molten magma, the original source for igneous rocks.

    Transition to igneous

    When rocks are pushed deep under the Earth's surface, they may melt into magma. If the conditions no longer exist for the magma to stay in its liquid state, it will cool and solidify into an igneous rock. A rock that cools within the Earth is called intrusive or plutonic and will cool very slowly, producing a coarse-grained texture.

    As a result of volcanic activity, magma (which is called lava when it reaches Earth's surface) may cool very rapidly while being on Earth's surface exposed to the atmosphere and are called extrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian. Any of the three main types of rocks (igneous, sedimentary, and metamorphic rocks) can melt into magma and cool into igneous rocks.

    Transition to metamorphic

    Rocks exposed to high temperatures and/or pressures can be changed physically or chemically to form a different rock, called metamorphic. Regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building events within orogenic belts. These rocks commonly exhibit distinct bands of differing mineralogy and colors, called foliation. 

    Another main type of metamorphism is caused when a body of rock comes into contact with an igneous intrusion that heats up this surrounding country rock. This contact metamorphism results in a rock that is altered and re-crystallized by the extreme heat of the magma and/or by the addition of fluids from the magma that add chemicals to the surrounding rock (metasomatism). Any pre-existing type of rock can be modified by the processes of metamorphism.


    Transition to sedimentary

    Rocks exposed to the atmosphere are variably unstable and subject to the processes of weathering and erosion. Weathering and erosion breaks the original rock down into smaller fragments and carries away dissolved material. This fragmented material accumulates and is buried by additional material. While an individual grain of sand is still a member of the class of rock it was formed from, a rock made up of such grains fused together is sedimentary. 

    Sedimentary rocks can be formed from the lithification of these buried smaller fragments (clastic sedimentary rock), the accumulation and lithification of material generated by living organisms (biogenic sedimentary rock - fossils), or lithification of chemically precipitated material from a mineral bearing solution due to evaporation (precipitate sedimentary rock). Clastic rocks can be formed from fragments broken apart from larger rocks of any type, due to processes such as erosion or from organic material, like plant remains. Biogenic and precipitate rocks form from the deposition of minerals from chemicals dissolved from all other rock types.



    1. this is good info except the words should be more easier


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