NGS Rocks & Minerals -Section 4: Rock Groups

What are the most common type of rocks found on Earth?

• Igneous rocks

• Sedimentary rocks

• Metamorphic rocks

• Fossil rocks

Answer explanation

The most common type of rocks found on Earth's surface are Sedimentary Rocks.

Think of it this way:

• Igneous rocks are like freshly baked cookies, hot and straight from the oven (volcano!). They cool and harden quickly.

• Metamorphic rocks are like those cookies that got squished in your lunchbox – they've been under intense heat and pressure, completely changing their shape and sometimes even flavor (minerals).

• Sedimentary rocks are like building a giant sandcastle. Tiny pieces of sand (sediments) get piled up layer by layer, and over time, they harden into a solid rock.

Since so much of Earth's surface is constantly being eroded (worn down) and then rebuilt by things like rivers, wind, and rain, we see a lot of these "sandcastle" rocks all around us!

Bonus Fact: Have you ever seen the Grand Canyon? Those amazing layers are mostly made up of sedimentary rocks!

What is crystallization in the context of igneous rocks?

• The process of erosion

• The cooling and hardening of magma

• The melting of rocks

• The formation of sediment

Answer explanation

Crystallization in the context of igneous rocks is the cooling and hardening of magma.  

Imagine hot melted chocolate (magma). As it cools down, it starts to form those little sugary crystals. That's basically what happens with magma! As it cools, the melted minerals within it start to arrange themselves into organized patterns, forming crystals.  

The size and type of crystals depend on how quickly the magma cools:  

• Slow cooling: Large, visible crystals (like in granite)  

• Fast cooling: Small, often invisible crystals (like in basalt)  

So, crystallization is the key process that turns hot, liquid magma into the solid rocks we see around us!

Which type of igneous rock forms when magma cools slowly underground?

• Sedimentary rock

• Metamorphic rock

• Extrusive rock

• Intrusive rock

What is the texture of an extrusive igneous rock?

• Randomly arranged grains

• Small crystals that are hard to see

• Bands of different minerals

• Large and interlocking crystals

Answer explanation

Explanation:

Extrusive igneous rocks form when magma erupts onto the Earth's surface as lava and cools quickly. This rapid cooling doesn't give the minerals much time to grow into large crystals. As a result, extrusive igneous rocks typically have:  

• Fine-grained texture: The crystals are so small that they're difficult or impossible to see with the naked eye.  

Think of it this way: Imagine trying to build a giant sandcastle very quickly. You wouldn't have time to carefully shape each grain of sand, so your castle would be rough and have small grains. Similarly, when lava cools quickly, the minerals don't have time to form large, well-defined crystals.

Which of the following is an example of an intrusive igneous rock?

• Obsidian

• Granite

• Sandstone

• Coal

Answer explanation

Intrusive igneous rocks form when magma cools and solidifies slowly beneath the Earth's surface. This slow cooling allows the minerals to grow into large, interlocking crystals. As a result, intrusive igneous rocks typically have:

• Coarse-grained texture: The crystals are large enough to be seen with the naked eye.

Think of it this way: Imagine baking a loaf of bread in a slow oven. The bread has plenty of time to rise and form a large, airy loaf. Similarly, when magma cools slowly beneath the Earth's surface, the minerals have plenty of time to grow into large crystals.

Here are some examples of intrusive igneous rocks:

• Granite: A common intrusive igneous rock with a coarse-grained texture and a light color. It is often used for countertops and other building materials.

• Diorite: A dark-colored intrusive igneous rock with a coarse-grained texture. It is often used for sculptures and other decorative objects.

• Gabbro: A dark-colored intrusive igneous rock with a coarse-grained texture. It is often used for roadbeds and other construction materials.

What is sediment?

• A small, solid material from rocks or living things

• A type of metamorphic rock

• A process of rock formation

• A type of igneous rock

Answer explanation

Sediment is a small, solid material from rocks or living things that has been transported and deposited by wind, water, or ice. Sediment can be found in many different places, including:

• Rivers and streams: Sediment is carried by rivers and streams and deposited in floodplains and deltas.

• Beaches: Sediment is deposited on beaches by waves and currents.

• Deserts: Sediment is carried by wind and deposited in dunes and sandstorms.

• Glaciers: Sediment is carried by glaciers and deposited in moraines and drumlins.

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What is erosion?

• The process of sediment settling

• The process of pressing sediment together

• The process of wind and water scraping away rock

• The crystallization of minerals

Answer explanation

Explanation:

Erosion is the process where wind, water, ice, and gravity wear away at the Earth's surface, moving soil and rock from one place to another.  

Think of it like this: Imagine a river flowing through a valley. As the water flows, it carries small pieces of rock and soil with it. Over time, the river can carve a deep canyon by slowly wearing away the rock. This is erosion in action!  

Here are some examples of erosion:

• Water erosion: Rivers, streams, and rain can erode soil and rock.  

• Wind erosion: Wind can blow away soil and sand, especially in deserts.

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• Ice erosion: Glaciers can carve out valleys and transport large rocks as they move.  

• Gravity erosion: Gravity can cause landslides and mudslides, moving large amounts of soil and rock downhill.

   

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What is deposition?

• The crystallization of minerals

• The process of sediment settling out of water or wind

• The process of pressing sediment together

• The process of wind and water scraping away rock

Answer explanation

Deposition is the process of sediment settling out of water or wind. It is the opposite of erosion, which is the process of sediment being carried away by water or wind.

Deposition can occur when water or wind slows down. For example, when a river flows into a lake or ocean, the water slows down and the sediment it is carrying settles to the bottom. This can form deltas and other landforms.

Deposition can also occur when wind slows down. For example, when wind blows over a desert, it can carry sand with it. When the wind slows down, the sand settles to the ground, forming dunes.

Deposition is an important process in the formation of sedimentary rocks. Sedimentary rocks are formed when sediment is deposited and then compacted and cemented together.

What is compaction?

• The process of pressing sediment together

• The process of sediment settling out of water or wind

• The crystallization of minerals

• The process of wind and water scraping away rock

Answer explanation

Compaction is a crucial step in the formation of sedimentary rocks. Here's how it works:  

1. Layers of sediment: Over time, layers of sediment (like sand, mud, or gravel) build up on top of each other.  

2. Weight and pressure: As more and more layers accumulate, the weight of the overlying material exerts immense pressure on the lower layers.  

3. Squeezing out water: This pressure squeezes the sediment together, forcing out much of the water that was trapped between the particles.  

Think of it like stacking books on a table. The bottom books will be squeezed more and more as you add more books on top.

Compaction is one of the two main processes involved in lithification, which is the process of turning loose sediment into solid rock. The other process is cementation.

What is cementation?

• The process of sediment settling out of water or wind

• The process of pressing sediment together

• The crystallization of dissolved minerals that glue sediment particles

• The melting of rocks

Answer explanation

Cementation is like nature's own glue! Here's how it works:

1. Dissolved minerals: Water flowing through the spaces between sediment grains carries dissolved minerals.  

2. Crystallization: As the water evaporates or the minerals reach a certain concentration, they start to crystallize.

3. Binding the sediment: These crystals grow and interlock, effectively gluing the sediment particles together.  

Common cementing minerals include:

• Calcite: A type of calcium carbonate, often found in limestone.  

• Silica: A very hard and resistant mineral, often found in sandstone.  

• Iron oxide: Gives some rocks a reddish or brownish color.  

Cementation, along with compaction, is a crucial part of lithification, the process of transforming loose sediment into solid rock.