How Rocks
Form
Explore the three major rock types, the processes that create and transform them, and the rock cycle that connects them all.
Earth & Space Science
What Is a Rock?
A rock is a naturally occurring solid made of one or more minerals. Minerals are the building blocks: tiny crystals of substances like quartz, feldspar, and mica. Rocks form through specific geological processes involving heat, pressure, cooling, or compaction over time.
The Three Rock Types
Igneous
Formed when molten rock (magma or lava) cools and solidifies. "Igneous" comes from the Latin word for fire.
Sedimentary
Formed when small particles (sediments) are deposited, compacted, and cemented together over long periods.
Metamorphic
Formed when existing rocks are changed by heat and pressure deep underground without fully melting.
Why Does Rock Formation Matter?
Understanding how rocks form helps scientists read Earth's history. Each rock is a record of the conditions that created it: the temperature, the pressure, and the environment at that time.
1
Fossils in sedimentary rocks tell us about ancient life and past climates.
2
Igneous rocks from volcanoes reveal how Earth's interior works and help predict eruptions.
3
Metamorphic rocks deep in mountains show where tectonic plates collided millions of years ago.
Igneous Rock Formation
Sedimentary Rock Formation
Metamorphic Rock Formation
The Rock Cycle
Intrusive vs. Extrusive Igneous Rocks
Intrusive (Plutonic)
Forms below Earth's surface. Magma cools slowly over thousands of years. Produces large, visible crystals. Example: granite.
Extrusive (Volcanic)
Forms on or near Earth's surface. Lava cools quickly in days to weeks. Produces small or no crystals. Example: basalt.
Types of Sedimentary Rock
Clastic
Made from broken fragments of other rocks. Examples: sandstone (sand grains), shale (clay particles), conglomerate (pebbles).
Chemical
Forms when minerals crystallize from water solutions. Examples: rock salt (halite), some limestone from evaporating seawater.
Organic
Made from remains of living things. Examples: coal (compressed plant matter), fossiliferous limestone (shells and skeletons).
Contact vs. Regional Metamorphism
Contact
Rock is heated by nearby magma intrusion. Affects a small area around the heat source. Mainly driven by temperature.
Regional
Large-scale transformation from tectonic forces. Affects huge regions deep in mountain belts. Driven by both heat and pressure.
How to Identify a Rock Type
1
Look at the texture. Are there visible crystals, layers, or fragments cemented together?
2
Check for layers or bands. Visible horizontal layers suggest sedimentary; wavy bands suggest metamorphic foliation.
3
Examine crystal size. Large interlocking crystals = igneous (intrusive) or metamorphic. Tiny or glassy = igneous (extrusive).
4
Look for fossils. Fossils appear almost exclusively in sedimentary rocks.
5
Consider the environment. Found near a volcano? Likely igneous. Found in flat plains? Likely sedimentary.
Example: Granite to Sandstone
Example: Limestone to Marble
Example: The Full Cycle
1
Magma erupts from a volcano and lava cools quickly on the surface, forming basalt (extrusive igneous rock).
2
Weathering and erosion break the basalt into fragments. Rivers carry these fragments to an ocean basin, where they settle as sediments.
3
Compaction and cementation over millions of years transform the sediments into sedimentary rock.
4
Tectonic forces push the sedimentary rock deep underground, where heat and pressure transform it into metamorphic rock.
5
If pushed even deeper, the rock melts back into magma, and the cycle begins again.
Common Mistakes
✕ "Metamorphic rocks melt"
Metamorphic rocks change by heat and pressure but do NOT fully melt. If they melt, they become magma and form igneous rocks.
✕ "The rock cycle goes in one direction"
Any rock type can become any other type. The cycle has no fixed order: igneous can become metamorphic directly without being sedimentary first.
✕ "Bigger crystals = extrusive"
It is the opposite. Large crystals form from slow cooling underground (intrusive). Fast cooling on the surface produces tiny or no crystals.
Summary Comparison
| Property | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
| How it forms | Cooling of magma/lava | Compaction & cementation | Heat & pressure |
| Texture | Crystalline or glassy | Grainy, layered | Banded or foliated |
| Fossils? | Never | Often | Rarely (destroyed) |
| Examples | Granite, Basalt | Sandstone, Shale | Marble, Slate |
| Where found | Volcanoes, deep crust | River beds, ocean floors | Mountain cores |
Question 1 of 5
Which type of rock forms when magma cools and solidifies?
Igneous rocks form when molten rock (magma underground or lava on the surface) cools and solidifies. The word "igneous" comes from the Latin word for fire.
Question 2 of 5
A rock has large, visible crystals that interlock tightly. This rock most likely formed by:
Large, visible crystals are the hallmark of intrusive igneous rocks. Slow cooling underground gives mineral crystals plenty of time to grow, producing rocks like granite with clearly visible crystal grains.
Question 3 of 5
What distinguishes metamorphic rock formation from igneous rock formation?
The critical distinction: metamorphic rocks are transformed by heat and pressure without fully melting. If the rock melts completely, it becomes magma and would produce an igneous rock instead. Metamorphic rocks rarely contain fossils (they are usually destroyed), and both types can form underground.
Question 4 of 5
A scientist finds a rock with visible layers containing fossil shells. This rock is most likely:
Sedimentary rocks are the only type that commonly contain fossils, because they form from sediments deposited gently enough to preserve remains. Layers are another key feature of sedimentary rocks. Limestone often contains fossil shells and marine organisms.
Question 5 of 5
In the rock cycle, which process transforms sedimentary rock into metamorphic rock?
Heat and pressure are the forces that drive metamorphism. When sedimentary rock is pushed deep underground by tectonic forces, the intense heat and pressure rearrange its minerals into a new metamorphic rock, without melting it completely. Compaction and cementation form sedimentary rock, not metamorphic.