Types of Rocks: Igneous, Sedimentary, Metamorphic
Types of Rocks: Igneous, Sedimentary, Metamorphic
- Unit 1: Earth & Rocks
- Unit 2: Tectonics & Space
- Unit 3: Chemistry & Atmosphere
- Unit 4: Ecosystems & Biology
🎯 In this topic you will
- Identify and describe the three main types of rocks: igneous, sedimentary, and metamorphic
- Explain how each rock type is formed and transformed through the rock cycle
- Distinguish between intrusive and extrusive igneous rocks
- Describe how sediment forms and becomes sedimentary rock
- Understand how heat and pressure create metamorphic rocks
- Recognize the dynamic nature of Earth’s surface through rock transformation
🧠 Key Words
- igneous rock
- sedimentary rock
- metamorphic rock
- rock cycle
- magma
- intrusive
- extrusive
- lithification
- fossil
- metamorphism
- foliation
Show Definitions
- igneous rock: A type of rock that forms when molten magma or lava cools and solidifies.
- sedimentary rock: Rock formed from compressed layers of sediments over time, often containing fossils.
- metamorphic rock: Rock that has changed in structure or composition due to heat and pressure without melting.
- rock cycle: The continuous transformation of rocks between igneous, sedimentary, and metamorphic forms.
- magma: Molten rock located beneath Earth's surface that may cool to form igneous rocks.
- intrusive: Describes igneous rocks formed from magma that cools slowly underground, producing large crystals.
- extrusive: Describes igneous rocks formed from lava that cools quickly on or near Earth’s surface.
- lithification: The process of turning sediment into solid rock through compaction and cementation.
- fossil: The preserved remains or traces of ancient organisms found in sedimentary rocks.
- metamorphism: The process by which existing rock is transformed by heat, pressure, or chemical processes.
- foliation: A layered or banded appearance in metamorphic rocks caused by mineral alignment under pressure.
🪨 What Are the Three Main Types of Rocks?
All rocks on Earth belong to one of three main categories: igneous, sedimentary, or metamorphic. These types are not separate and unchanging — instead, they are part of a continuous system called the rock cycle, where rocks are constantly being formed, broken down, and transformed over time. The type of rock depends on how it was formed, and each has distinct characteristics that tell a story about its origin and history.
⚠️ Common Mistake
Don't assume the three rock types are fixed and unrelated — they can transform into each other through the rock cycle.
🌋 Formation of Igneous Rocks
Igneous rocks form when molten rock, known as magma or lava, cools and solidifies. If the molten rock cools beneath the surface of the Earth, it forms intrusive igneous rocks, which cool slowly and tend to have large, visible crystals. A good example of this is granite, which is often used as a building material. On the other hand, if the lava cools at or near the surface, it forms extrusive igneous rocks like basalt or obsidian, which cool quickly and have much smaller or even invisible crystals. Some extrusive rocks may even contain gas bubbles, which get trapped as the lava cools and hardens. Igneous rocks are typically hard and resistant to weathering, and they make up much of the Earth's crust and ocean floor.
🧪 Did you know?
Some volcanic igneous rocks, like pumice, are so full of gas bubbles that they can float on water!
🏞️ How Sedimentary Rocks Are Made
Sedimentary rocks form from the accumulation and compaction of sediments, which are small particles of rock, mineral fragments, or organic material. These sediments are transported by wind, water, or ice, and are deposited in layers over long periods of time, often in rivers, lakes, or oceans. Over time, the layers become compressed and cemented together to form rock. This process is known as lithification. Sedimentary rocks often contain fossils, as dead plants and animals can be trapped and preserved within the layers. Examples include sandstone, limestone, and shale. Unlike igneous rocks, sedimentary rocks tend to be softer and may have visible layers or fragments. Their structure often provides valuable clues about Earth’s past environments, including changes in climate and the movement of ancient seas.
📌 Important Concept
Lithification: Sedimentary rocks are formed through the process of lithification, where layers of sediment are compacted and cemented together over time, often preserving fossils and environmental clues.
🪨 Transforming Rocks into Metamorphic Forms
Metamorphic rocks are formed when existing rocks — either igneous or sedimentary — are subjected to high pressure, high temperature, or both, usually deep within the Earth’s crust. This process, called metamorphism, changes the structure and mineral composition of the original rock without melting it. The result is a new type of rock with different physical and chemical characteristics. For instance, limestone can become marble, and shale can become slate. Metamorphic rocks often show features like foliation, which is a banded or layered appearance caused by the reorientation of minerals under pressure. They are generally harder and more compact than their original forms, and are commonly found in mountain ranges where tectonic forces are strong.
🔁 The Rock Cycle in Action
Each rock type is part of a larger story — a record of Earth’s dynamic processes. Igneous rocks reveal volcanic activity and cooling deep underground. Sedimentary rocks preserve ancient environments and the life that once existed. Metamorphic rocks show how heat and pressure reshape materials over time. These rocks are not static: they can change from one type to another through the rock cycle, making Earth’s surface a constantly evolving system.
❓ QUESTIONS
1. What are the three main types of rock?
👀 Show answer
2. How does an intrusive igneous rock differ from an extrusive one?
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3. What is lithification?
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4. What causes a rock to become metamorphic?
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5. How are fossils typically formed in rocks?