Intermediate

Definition

In geology, the term 'intermediate' refers to a classification of igneous rocks and their compositions that fall between mafic (rich in magnesium and iron) and felsic (rich in silica) categories. Intermediate rocks typically contain a moderate amount of silica, usually ranging from about 53% to 65%, and often exhibit characteristics that reflect both mafic and felsic influences, such as their mineral composition and color.

5 Must Know Facts For Your Next Test

1. Intermediate rocks typically contain minerals like plagioclase feldspar, amphibole, and biotite, contributing to their unique appearance and properties.
2. Examples of intermediate igneous rocks include andesite and dacite, both of which can form in subduction zone settings where oceanic plates meet continental plates.
3. Intermediate rocks have a higher viscosity compared to mafic rocks, which can affect volcanic activity, leading to explosive eruptions when these rocks melt and rise to the surface.
4. The classification of rocks into intermediate, mafic, or felsic is important for understanding the geological processes involved in their formation, especially in volcanic regions.
5. The color of intermediate rocks is often gray or brown, providing a visual distinction from both darker mafic rocks and lighter felsic rocks.

Review Questions

• Compare the mineral compositions of intermediate rocks to those of mafic and felsic rocks.
  • Intermediate rocks have a mineral composition that includes both mafic and felsic components. They commonly contain plagioclase feldspar along with minerals such as amphibole or biotite, which are also found in mafic rocks. In contrast, mafic rocks are rich in olivine and pyroxene with less silica, while felsic rocks contain higher proportions of quartz and alkali feldspar. This blend of minerals gives intermediate rocks characteristics that bridge the two classifications.
• Discuss how the viscosity of intermediate magmas impacts volcanic activity compared to mafic and felsic magmas.
  • The viscosity of intermediate magmas is generally higher than that of mafic magmas but lower than that of felsic magmas. This difference affects how these magmas erupt; intermediate magmas can lead to more explosive volcanic eruptions due to their ability to trap gas as they ascend. In contrast, mafic magmas tend to have less explosive eruptions because they flow more easily, while felsic magmas can cause highly explosive eruptions due to their high viscosity. Thus, understanding the viscosity helps predict the nature of volcanic eruptions.
• Evaluate the role of subduction zones in the formation of intermediate igneous rocks and how this relates to plate tectonics.
  • Subduction zones play a crucial role in forming intermediate igneous rocks through the process of melting and mixing of oceanic and continental crust materials. As an oceanic plate subducts beneath a continental plate, it introduces water into the mantle, lowering the melting temperature of the surrounding rock. This results in partial melting that creates intermediate magma compositions. This process not only highlights the interaction between tectonic plates but also explains why many volcanic arcs associated with subduction zones produce intermediate rocks like andesite, showcasing the link between plate tectonics and geological rock classification.

Related terms

Mafic: Mafic rocks are igneous rocks that are rich in magnesium and iron, characterized by darker colors and a lower silica content, usually less than 53%.

Felsic: Felsic rocks are igneous rocks that are high in silica content (more than 65%), generally lighter in color, and contain minerals like quartz and feldspar.

Andesite: Andesite is a common example of an intermediate igneous rock that is typically found in volcanic arcs and is characterized by its gray or brown color and fine-grained texture.