Rocks and Fossils

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Rocks

A rock is any naturally found solid mass or aggregate of minerals or mineraloid matter. They form the outer solid layer of the earth known as the CRUST. Depending on their origin, the minerals present, and their chemical composition, they are mainly divided into three categories – igneous rocks, sedimentary rocks, and metamorphic rocks. The study of rocks is called petrology.

Fossils

A fossil is the preserved remains or impressions of a living organism such as a plant, animal, or insect.  They can be in the form of bones, exoskeletons, hair, shells, stone imprints, oil, and coal, petroleum among others. Studying fossils helps scientists to learn about the past history of life on Earth.

Classification of Rocks

Igneous Rocks

Igneous rocks are formed when magma and lava cool and solidify into rocks. As the hot molten magma rises towards the surface of the earth, it cools down and as a result, solidifies and crystallizes. There is a variety of igneous rocks and they are mostly found beneath the earth’s crust. They are very important as their mineral and chemical composition can be used to learn about conditions like temperature, pressure, and the composition of Earth’s mantle and its tectonic activities. Depending upon the cooling process the grain sizes of minerals may vary within igneous rocks-

  • Slow cooling at great depths results in large mineral grains.
  • Sudden cooling at the surface results in small and smooth grains.
  • Intermediate conditions of cooling would result in intermediate sizes of grains.

2- Some important igneous rocks from left to right:

gabbro, andesite, pegmatite, basalt, pumice, porphyry, obsidian, granite, and tuff

Sedimentary Rocks

Rocks of the earth’s surface area exposed to weathering or erosion by the friction of waves and are thus, broken into particles of various sizes. These mineral particles are carried by water, wind, glaciers, and other agents and are deposited by sedimentation on the beds. These sediments, through compaction and fermentation, turn into rocks. This process is called lithification. Dead aquatic organisms and other suspended matter may also pile up on the floor and end up within these sedimentary rocks. In several sedimentary rocks, the layers of mineral deposits maintain their characteristics even after lithification. Sandstone, shale are some of the examples for Sedimentary Rocks.

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

 

 

  • Clastic –They are made from pieces of other rocks damaged by weathering and erosion. E.g. Sandstone, conglomerate, limestone, shale, loess, etc. 

 

 

 

 

  • Chemical – They are formed when tiny particles of minerals precipitate within a liquid in which they were dissolved. E.g. Chert, limestone, halite, potash, etc. 

 

 

 

 

 

  • Organic – They are formed from organic debris of plants and animals under the influence of temperature and pressure. E.g. Geyserite, chalk, limestone, coal, etc.

 Sandstone

 Chert

    Geyserite

Lithification of sedimentary rocks

Metamorphic Rocks

Metamorphic rocks are formed from existing rocks when they are subjected to extreme environmental conditions like high pressure and temperature. They undergo a process called metamorphism due to chemical and physical changes in them and they recrystallize into metamorphic rocks. They include anthracite, marble, slate, granulite, gneiss, schist, etc. It is to be noted that igneous, sedimentary and even metamorphic rocks themselves can undergo metamorphosis to give rise to new metamorphic rocks.

As in the case of marble, which is a metamorphic rock, is formed from sedimentary rock limestone. There are two types of metamorphism –

a) Contact metamorphism

When hot intruding magma permeates through surrounding rocks, they recrystallize due to high temperatures giving rise to new kinds.

 

 

 

 

 

 

b) Regional metamorphism

When great masses of rock over a region undergo change, mostly beneath the earth’s surface, due to extreme conditions of temperature and pressure, it is termed as regional metamorphism.

Metamorphic rocks

Other Rocks

Meteorite Rocks

Meteorite rocks are solid pieces of objects or debris from outer space. They are remnants of comets, asteroids, and meteoroids that originate in outer space and survive their passage through the earth’s atmosphere.

Fossil Shrimp and Fish in a layer of sedimentary rock from Lebanon dated back to 95 million years ago

Fossil Rocks

Fossils are prehistoric rock remains or organic traces preserved in sedimentary rocks. They are found throughout the world but are most commonly found in sedimentary rocks like sandstone, limestone, and shale. They are preserved by getting buried underneath multiple layers of sand or mud under conditions of tremendous pressure. The sand and mud eventually form sedimentary rocks and in the bargain, the organic matter is replaced by minerals producing a stone replica of the organic matter.

There may be microbes and animals preserved in amber, peat, tar or ice; strands of hair, oil, coal, imprints on rocks, etc. Fossil rocks are also formed from shells, bones, or teeth that have been exposed to surrounding minerals, and over a long period of time, they are converted into stony substances by a process called petrifaction.

Fossil birch leaf from the Cretaceous Dakota Formation in Kansas

Rock Cycle

Rocks undergo transitions through geologic time among the three main rock types- igneous, sedimentary, and metamorphic. The diagram below explains how the three rocks are related to one another and how they transform from one type to another over time.

Check Point

  1. A rockis any naturally found solid mass or aggregate of ____________.
  2. A _____________ is the preserved remains or impressions of a living organism such as a plant, animal, or insect.
  3. Sandstone, shale are some of the examples of
  4. When existing rocks undergo physical and chemical changes, they recrystallize to form _____________________ rocks.
  5. Fossil rocks are formed when dead organisms along with minerals crystallize to form stony substances by a process called ____________________.

Answer Key

  1. minerals
  2. fossil
  3. Sedimentary Rocks
  4. Metamorphic
  5. petrifaction