Rocks & Fossils

Grade 8 Science Worksheets

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Rock Formation

The earth has been making three types of rocks all the time.

  1. igneous rocks
  2. sedimentary rocks
  3. metamorphic rocks

Igneous rocks are formed by volcanic eruptions when molten rock breaks through the earth’s mantle and then cools and solidifies on the surface into rock.

Sedimentary rocks are tiny fragments of rock broken off from mountains by wind or rain. These fragments are swept away by rivers to the sea. They settle down as a layer in river and sea beds and harden over time.

Metamorphic rocks are created when existing rock types undergo transformation due to heat and pressure over time.

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Finding Fossils

Which of these rock types is best suited to preserve the remains of animal or plant life? Igneous and metamorphic rocks undergo too much heat and pressure to preserve these remains. Sedimentary rocks (such as sandstone or limestone) are softer, at lower temperature, lesser pressure, and therefore more capable of preserving animal and plant life. Such preserved life forms are termed fossils.

Imagine an animal (such as a dinosaur) that fell and drowned in soft mud many millions of years ago. The body of the animal would have rotted away but its bones would be preserved by the mud which harden into clay with the layers of sedimentary rock deposited on top. These bones when dug up and discovered are named fossils. Even plants can be found preserved in this manner.

There are mainly 5 types of fossils –

  1. Body fossils, such as described above, which are the remains of actual animals or organisms.
  2. Molds,which are impressions left in the rock by the decayed shell of an organism.
  3. Casts, where a mold gets filled with a softer element like mud which hardens.
  4. Trace fossils, which are impressions of a different kind, such as a trail or footprint or even dungleft by an animal.
  5. Petrified fossils, where the entire animal or plant form is replaced by hard minerals and turned into stone.

Many ancient creatures that lived and died have been discovered through the study of fossils, from large dinosaurs and mammoths to tiny insects and shellfish.

Fossil Fuels

We use coal to start a barbeque, oil to drive our car, gas to heat our homes and furnaces. Where do we obtain these essentials from?From fossils!

Plants and animals buried millions of years ago have high carbon content which is an essential element to burn as fuel. That’s why these are all termed fossil fuels.

Crude oil or petroleum is found in the cracks and crevices of sedimentary rock and can be accessed by drilling. This crude oil is processed to form gasoline which drives our cars.

Coal is rock made of rich carbon from plant life. It is accessed by mining and useful for burning and heating purposes.

Natural gas is also found underground and accessed by drilling. It is useful for cooking and heating.

These conventional fossil fuels have sustained us for many years now. There are good as well as bad aspects of these fuels, as they are a major cause of pollution and global warming.

Non-conventional sources of energy such as Solar, Wind and Tidal energy are slowly replacing these fossil fuels. You must learn about these as well to make our world cleaner!

Check Point

Fill in the blanks:

  1. When existing rock types undergo heat and pressure over time, they form _______ rocks.
  2. _______ rocks are more capable of preserving plant and animal life.
  3. A trail or footprint or dung left by an animal is called a _______ fossil.
  4. Some fossil fuels are _______, _______ and _______.
  5. Solar, Wind and Tidal energy are _______ sources of energy.

Answer Key

  1. Metamorphic
  2. Sedimentary
  3. Trace
  4. Crude oil, Coal, Natural Gas
  5. Non-conventional

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