🪨 What Are Metamorphic Rocks?

The word metamorphosis means ‘change’. Metamorphic rocks are existing rocks that have been changed by heat or pressure. When rocks are heated, the minerals inside them can melt and form new crystals.

Sometimes, rocks are changed by great pressure that squeezes them tightly, creating new thin layers. Other times, rocks are changed by both heat and pressure together, which gives them new minerals and many layers.

🔥 What Causes the Heat and Pressure?

Layers of sedimentary rock can be buried deep beneath the Earth’s surface. Hot magma from the mantle rises up and forms a large body called an intrusive igneous mass. This hot, liquid rock pushes the surrounding solid rocks aside, creating intense heat and pressure.

This heat and pressure cause nearby rocks to change into metamorphic rocks. The deeper the rocks are buried or the closer they are to magma, the more they change in structure and mineral composition.

🪨 Types of Metamorphic Rock

Let’s explore what kinds of metamorphic rocks form when sandstone, limestone, shale, and granite are heated until they melt and are squashed by pressure.

✨ Quartzite

When sandstone is heated, all the sand grains melt and form new quartz crystals. This change creates the metamorphic rock called quartzite. It looks similar to sandstone but shines with crystals and is much harder.

Quartzite is so hard that it is sometimes used to make sculptures. The photograph shows a quartzite statue of Ankhrekhu made around 1850 BCE — almost 4000 years ago.

🏛️ Marble

When limestone is heated, it changes into a metamorphic rock called marble. The minerals melt and form new crystals, making marble very hard and shiny. Marble can be white, red, blue, or green depending on the substances in the original limestone.

Marble is a beautiful rock that lasts for a very long time. People use it to carve statues, make gravestones, and decorate buildings and temples. Different colors of marble have been used in famous buildings around the world.

The white marble shown in the photograph has been cut into blocks ready for carving statues or making furniture.

📏 Slate

When shale is put under intense pressure, it forms thin layers — this is the metamorphic rock called slate. Layers of sediment build up on the sea bed, and the bottom layers are pressed by the weight of the layers above. When these layers are deeply buried, the heat and pressure turn the rock into slate.

Slate is strong and long-lasting, so people often use it for roof tiles and floor tiles. The photograph shows slate roof tiles used in building construction.

🪨 Gneiss

When granite is heated, its minerals melt and form new crystals. The pressure then arranges these crystals into bands or stripes, forming a metamorphic rock called gneiss.

Gneiss looks similar to granite but has striped patterns of light and dark minerals. It is a very hard rock and is often crushed to make roads or used in construction.

🌍 The Rock Cycle

At Stage 5, you learned about the water cycle — how water is reused again and again on Earth’s surface. The rock cycle is similar! It shows how the rocks of the Earth are used and changed over and over again. One type of rock can turn into another type through different processes that happen over millions of years.

The rock cycle takes a very long time — sometimes hundreds of millions of years! It’s a repeating journey where melting, cooling, erosion, and pressure all play a role in forming new rocks from old ones.

🔁 Understanding the Rock Cycle Diagram

The diagram of the rock cycle is a model showing how rocks can change from one form to another. It does not represent real time or size but shows possible orders of events that occur in the cycle.

Each arrow in the diagram has a special meaning:

• 🟧 Orange arrows: melting
• 🔵 Blue arrows: cooling
• 🟩 Green arrows: weathering and erosion (breaking down)
• 🟣 Purple arrow: sedimentation
• 🟨 Yellow arrows: burial, heat, and pressure

These arrows help us understand how rocks move through different stages — from molten magma to solid rock, to broken sediments, and back again!

💎 Following the Rock Cycle

Begin at magma — this is where the rock cycle starts. Magma cools (shown by the blue arrow) to form igneous rock. From there, the cycle continues in several possible directions, depending on what happens to the rock next.

Sometimes, weathering breaks up the igneous rock (green arrow), and erosion carries the pieces away to form sediments. If the igneous rock stays deep underground, heat and pressure (yellow arrow) may change it into metamorphic rock. Or, if it melts again (orange arrow), it turns back into magma — completing the cycle.

🔥 From Sedimentary Rock to Metamorphic Rock

Look at the arrow between sedimentary rock and metamorphic rock — this arrow represents burial, heat, and pressure. Layers of sediment build up on the sea bed, and the lower layers become buried under many more layers above them.

Deep below the Earth’s surface, it becomes much hotter, and the weight of the overlying layers increases the pressure. Together, this heat and pressure change sedimentary rock into metamorphic rock, creating new textures and minerals inside the rock.