Rocks continuously change and transform over geological time through a dynamic process called the rock cycle. This cycle illustrates how one type of rock can convert into another. A common question is whether sedimentary rock material can become igneous rock. This article explains how such a transformation is possible.
Defining Sedimentary and Igneous Rocks
Sedimentary rocks form from the accumulation and compaction of sediments. These sediments are formed through the weathering and erosion of existing rocks, which break down into smaller fragments. These particles are transported and deposited in layers. Over time, the weight of overlying material compacts the sediments, and dissolved minerals act as cement, binding them together to form solid rock. Examples include sandstone, shale, and limestone.
Igneous rocks form from the cooling and solidification of molten rock. This molten rock is known as magma when it is beneath the Earth’s surface and lava when it erupts onto the surface. When magma cools slowly within the Earth’s crust, it forms intrusive igneous rocks with large mineral crystals, like granite. When lava cools rapidly on the Earth’s surface, it forms extrusive igneous rocks, which have fine-grained textures or a glassy appearance, such as basalt.
The Path to Transformation: From Sediment to Melt
For sedimentary rock material to become igneous rock, it begins with deep burial within the Earth’s crust. As tectonic plates move, sedimentary layers can be pushed downwards to great depths. At these depths, the rock material experiences increasing pressure and elevated temperatures from the Earth’s internal heat.
Under these extreme conditions, sedimentary rocks first undergo metamorphism, where their mineral composition and texture change without complete melting. If burial continues and temperatures and pressures rise further, the rock material can reach its melting point. When the sedimentary rock material melts, it transforms into magma.
Formation of New Igneous Rock
Once the sedimentary material has melted and become magma, it follows the typical path of igneous rock formation. This magma tends to rise through the Earth’s crust and begins to cool. The rate at which this molten material cools determines the size of the mineral crystals that form within the new igneous rock.
If the magma cools slowly deep within the Earth, it allows sufficient time for large, visible crystals to grow, forming intrusive igneous rocks. If the magma reaches the Earth’s surface and erupts as lava, it cools much more rapidly. This fast cooling limits crystal growth, resulting in extrusive igneous rocks with very small crystals or a glassy appearance. The material that originated as sedimentary rock has now completed its transformation into a new igneous rock.