The Differences Between Metamorphic and Igneous Rocks

Metamorphic rocks are formed from the transformation of existing rocks, such as sedimentary or igneous rocks. This transformation occurs through a process called metamorphism, which is caused by heat and pressure. Metamorphism can occur when rocks are buried deep within the Earth's crust, subjected to the heat and pressure of colliding tectonic plates, or exposed to the heat and pressure of volcanic activity.

Igneous rocks: Igneous rocks are formed from the cooling and solidification of magma or lava. Magma is molten rock that is found beneath the Earth's surface, while lava is molten rock that has reached the Earth's surface. Igneous rocks can form either intrusively, beneath the Earth's surface, or extrusively, on the Earth's surface.

The Differences Between Metamorphic and Igneous Rocks

How They Form

Metamorphic rocks: The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (1500 bars), causing profound physical and/or chemical change. The temperatures were not enough to melt the rock, otherwise, an igneous rock would have formed.

Igneous rock: is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

Tectonic Setting

The tectonic sitting of the metamorphic rock

Metamorphic rocks: result from the forces active during plate tectonic processes. The collision of plates, subduction, and the sliding of plates along transform faults create differential stress, friction, shearing, compressive stress, folding, faulting, and increased heat flow. The tectonic forces deform and break the rock, creating openings, cracks, faults, breccias, and zones of weakness along which magmas can rise. Generally speaking, the greater the tectonic forces, the higher the pressures and temperatures affecting a rock mass and the greater the amount of resulting structural deformation and metamorphism.

 

The Tectonic setting of the igneous rocks Credit: Rob Van der Voo

Igneous rocks: form from magmas, and most magmas are associated with plate tectonics. Mafic (basaltic) and ultramafic magmas form along the divergent midoceanic ridges and are major components of new oceanic crust. More felsic magmas, such as andesites and rhyolites, are associated with the edges of continental crust at subduction zones along converging plate boundaries. Whether a magma is intermediate or felsic may depend on the relative amounts of oceanic crust and continental crust in the subduction zone that melt to form the magma.

Mineral Composition

Metamorphic minerals form under specific temperature and pressure conditions.

Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as index minerals, include sillimanite, kyanite, staurolite, andalusite, and some garnet.

Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz, may be found in metamorphic rocks, but are not necessarily the result of the process of metamorphism. These minerals formed during the crystallization of igneous rocks.

 

Minerals composition of the igneous rocks

Igneous rock: Felsic Igneous rock, highest content of silicon, with predominance of quartz, alkali feldspar and/or feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light coloured, and have low density. While mafic Igneous rock, lesser content of silicon relative to felsic rocks, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks.ultramafic rock, lowest content of silicon, with more than 90% of mafic minerals.

Classification

Metamorphic rock types, their texturture, grain size, and characteristics.

Metamorphic rock is classified by texture and composition. The texture of a metamorphic rock can be either foliated and appear layered or banded, or non-foliated and appear uniform in texture without banding. Foliated rocks contain many different kinds of minerals, but non-foliated rocks contain only one main mineral, which contributes to their more uniform appearance.

 

Classification of Igneous Rocks Take a look and see how igneous rocks are classified according to dominant minerals, accessory minerals, color, and texture.

Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.

Textures

Texture of the metamorphic rock

The five basic metamorphic textures with typical rock types are slaty (includes slate and phyllite; the foliation is called "slaty cleavage"), schistose (includes schist; the foliation is called "schistosity"), gneissose (gneiss; the foliation is called "gneissosity"), granoblastic (includes granulite, some marbles and quartzite), and hornfelsic (includes hornfels and skarn).

 

Textures of the igneous rocks

Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.

Economic importance

Metamorphic rocks: Metamorphic rocks are important economic resources, such as marble, slate, and quartzite. Marble is used in building materials and sculptures, slate is used in roofing and flooring, and quartzite is used in countertops and abrasives.

Igneous rocks: Igneous rocks are also important economic resources, such as granite, basalt, and gabbro. Granite is used in building materials and countertops, basalt is used in road construction and landscaping, and gabbro is used in railroad ballast and building materials.

Best Examples

Granite and Gneiss.

Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. It is often foliated (composed of layers of sheet-like planar structures). The foliations are characterized by alternating darker and lighter colored bands, called "gneissic banding".

Granite is the most famous igneous rocks. Which is a type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. By definition, granite is an igneous rock with at least 20% quartz and up to 65% alkali feldspar by volume. 

Read also: Granite vs Gneiss: The Difference Between Gneiss and Granite
The Differences Between Sedimentary and Metamorphic Rocks