SC.4.E.6.1 Identify the three categories of rocks: igneous, (formed from molten rock); sedimentary (pieces of other rocks and fossilized organisms); and metamorphic (formed from heat and pressure).

Rocks (680L)

By Milt Huling, Ph.D, Katherine Barnhart, M.S.Ed, Alison Smith & Sara Stoquert MEd

Source: Wikimedia Credit: HINI BOUALEM

You find a huge rock while on a hike. It does not look like other rocks you have seen on this hike. It is large, has a unique color, and a strange shape. What kind of rock is this? Where did it come from, you wonder? How was it formed? A scientist studies rocks to find out answers to these questions. Rocks can tell us about Earth’s past.

SC.4.E.6.1 Identify the three categories of rocks: igneous, (formed from molten rock); sedimentary (pieces of other rocks and fossilized organisms); and metamorphic (formed from heat and pressure).The Earth is a terrestrial, or rocky, planet. In some places it is hard to see Earth’s rocky surface. In most places on land the rocky surfaces is covered by plants and soil. Earth’s rocky surface even extends under lakes, rivers, and oceans.

Source: Wikimedia Credit: Chris Martin

Rocks come in many shapes and sizes. They are made of several different materials. Though there are many kinds of rocks, they can all be grouped into three categories. These three types of rocks are: igneous, metamorphic, and sedimentary. They are grouped by the way they were formed.

Source: Flickr Credit: James St. John

SC.4.E.6.1 Identify the three categories of rocks: igneous, (formed from molten rock); sedimentary (pieces of other rocks and fossilized organisms); and metamorphic (formed from heat and pressure).Some rocks form deep within the Earth’s crust where it gets very hot. It is hot enough to melt rocks. This molten, or melted, rock is pushed towards Earth’s surface. Scientists call this melted rock, magma. Magma that cools slowly can form large crystals. These crystals are large enough to see with your eyes. Most magma does not reach the Earth’s surface. Under Earth’s rocky surface, there is a huge amount of heat and pressure. When rocks form from melted rock under the intense heat and pressure, they are called igneous rocks. Granite is a great example of this type of igneous rock.

Source: Flickr Credit: Liz West

Some magma does reach Earth’s surface. When magma is pushed out of Earth’s surface, it is called lava. Lava is liquid rock. It has been exposed to such intense heat and pressure deep in the Earth that it changes from its original type of rock into a new type of rock. Sandstone becomes quartzite.

Lava usually comes from volcanoes. It cools quickly once it is exposed to air or water. Volcanoes can erupt anywhere on Earth’s surface. This includes under the water. Many ocean islands formed because of

SC.4.E.6.1 Identify the three categories of rocks: igneous, (formed from molten rock); sedimentary (pieces of other rocks and fossilized organisms); and metamorphic (formed from heat and pressure).erupting volcanoes. Water causes lava to cool quickly. It then turns into rock. This process of eruption and then cooling is what forms islands. Lava cools quickly under the water. It has less time for large crystals to form, like in igneous rock. Instead, lava forms small crystals.

Source: Wikimedia

Some rocks are formed from sediment. Sediments are small bits of many things. Sediment can be sand grains from a beach or even a desert. When sand is the main component of a sedimentary rock, it is called sandstone. Sediments may also be tiny remains of plants and animals. Coal is a sedimentary rock composed mostly of decayed plants. Sedimentary rocks are formed by sediments becoming buried. The buried sediments get squeezed together. Over time, they form into a rock.

SC.4.E.6.1 Identify the three categories of rocks: igneous, (formed from molten rock); sedimentary (pieces of other rocks and fossilized organisms); and metamorphic (formed from heat and pressure).

Source: Flickr Credit: James St. John

Questions

1. What is the relationship between magma and lava? Use the text to help you choose the best answer.

A. Magma and lava are both liquid rock.B. Magma is found underneath the surface of the Earth and when it

is pushed out, it turns into Lava.C. Lava turns into magma once it is heated up and under pressure.

2. Think about what you read about how rocks form underground. Write to explain how you can tell a rock has formed underground.

3. Why do you think obsidian does not have any crystals? Write your answer below and use text evidence to support your answer.

4. Based on what you read, how do sedimentary rocks differ from igneous rocks?

A. Sedimentary rocks are formed by small pieces called sediments while igneous rocks are formed by heat and pressure.

SC.4.E.6.1 Identify the three categories of rocks: igneous, (formed from molten rock); sedimentary (pieces of other rocks and fossilized organisms); and metamorphic (formed from heat and pressure).

B. Sedimentary rocks are formed by small pieces called sediments while igneous rocks are formed when hot molten lava is cooled down.

C. They are not different, in fact sedimentary rocks are formed the same way that igneous rocks are formed.

5. Look at the sentences below from the passage. What is the best definition of the underlined word? Use context clues to help you find the answer.

Sediments may also be tiny remains of plants and animals. Coal is a sedimentary rock composed mostly of decayed plants.

A. Growing plantsB. Ripening plantsC. Rotting plants