What are Rocks?

What are Rocks? 

Rocks are made up of minerals. They can be all one kind of mineral, for example sandstone which is made of sand or they can made of many different types of minerals like granite. Rocks form the Earth’s crust and include the surface forms we see every day.

There are 3 main types of rocks: Igneous, Sedimentary, and Metamorphic.

Igneous Rocks form from magma (molten rock) that comes from deep inside the earth. Magma that cools below the earth’s surface is called intrusive rock. Magma that cools above the surface, such as lava from a volcano, is called extrusive rock. Metal ores and gemstones often form in the veins and fissures of igneous rocks.


granite (intrusive)

pegmatite (intrusive)

obsidian (extrusive)

basalt (extrusive)


Granite is an Igneous rock. It was formed when magma hardened within the Earth's crust.

Sedimentary Rocks cover about three-quarters of the earth’s surface. Sedimentary rocks form through the erosion, movement, and deposition of minerals. Over time, eroded rocks and minerals build-up and compress together over millions of years to form rock. Most sedimentary rocks form in layers and some may contain fossils.






Limestone is a Sedimentary rock that contains tiny grains of the mineral calcite.

Metamorphic Rocks are rocks that have undergone change. Any type of rock can be metamorphosed—igneous, sedimentary, and other metamorphic rocks. Metamorphism occurs under great heat, pressure over a significant period of time, often resulting in changes in the rock or landform.


shale turns to slate

limestone turns to marble

sandstone turns to quartzite

granite turns to gneiss (nice)


Marble is an Metamorphic rock. It was transformed from a Sedimentary rock by heat and pressure.

Rock Cycle

The Rock Cycle is a continuous process in which rocks are created and destroyed. There is no end or beginning. Igneous rocks may be created through volcanic eruption or by slow cooling of a magma system. Over time the igneous rocks may be broken down into sediments. Those sediments may be deposited into layers and solidified into a sedimentary rock. Layers of sediment may cause metamorphism to occur, changing the rock into metamorphic. This is one example of the process, but many more examples exist.