Understanding Earth's Layers and Rock Types

The question asks to identify the Earth layer composed of lower density felsic rocks like andesite and granite. To answer this, we need to understand the composition and characteristics of different layers of the Earth.

Composition of Earth's Layers

Earth is broadly divided into several layers: the crust, the mantle, and the core. The crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is further divided into continental crust and oceanic crust.

• Continental Crust: This forms the continents. It is typically thicker (30-70 km) and primarily composed of lower density rocks. The dominant rock types found here are igneous rocks like granite and sedimentary and metamorphic rocks derived from them. Granite and andesite are examples of felsic rocks, which are rich in feldspar and silica, making them lighter in color and lower in density compared to mafic rocks.
• Oceanic Crust: This forms the ocean floors. It is thinner (5-10 km) and denser than continental crust. It is mainly composed of mafic rocks like basalt and gabbro, which are rich in magnesium and iron.
• Mantle: Lies beneath the crust, extending down to about 2,900 km. It is composed mostly of silicate rocks that are richer in iron and magnesium than the crust, making it denser.
• Core: Located at the Earth's center. It is primarily composed of iron and nickel.
  • Outer Core: Liquid layer about 2,300 km thick.
  • Inner Core: Solid sphere with a radius of about 1,220 km. Both are very dense due to the high pressure and composition.

Analyzing the Options

Let's examine each option based on its known composition:

• Outer core: Composed mainly of liquid iron and nickel. This is very dense, not lower density felsic rock.
• Continental crust: Composed primarily of lower density felsic rocks like granite and andesite. This matches the description.
• Inner core: Composed mainly of solid iron and nickel. This is extremely dense, not lower density felsic rock.
• Oceanic crust: Composed primarily of denser mafic rocks like basalt and gabbro, not lower density felsic rocks like granite.

Based on the composition, the layer made of lower density felsic rocks, such as andesite and granite, is the continental crust.

Comparison of Crust Types

The table clearly shows that the continental crust is characterized by lower density felsic rocks, which aligns with the question's description.

Conclusion

The layer described as being made of lower density felsic rocks, such as andesite and granite, is the continental crust.

Revision Table: Earth's Major Layers

Additional Information on Crustal Rocks

Rocks are classified based on their composition, texture, and how they are formed. Felsic rocks are light-colored igneous rocks that are rich in feldspar and silica (quartz). Granite is a common example of a felsic intrusive igneous rock. Andesite is a felsic or intermediate extrusive volcanic rock, often associated with volcanic arcs above subduction zones.

Mafic rocks, in contrast, are dark-colored igneous rocks rich in magnesium and iron. Basalt is a common mafic extrusive rock, forming most of the oceanic crust. Gabbro is the intrusive equivalent of basalt.

The difference in composition between continental and oceanic crust plays a crucial role in plate tectonics. The less dense continental crust floats higher on the mantle compared to the denser oceanic crust. When oceanic and continental plates collide, the denser oceanic plate typically subducts beneath the continental plate.