The correct answer is 30 km. Explanation & Key Takeaways: • Earth’s crust: • The interior of the earth is made up of several concentric layers which are the crust, the mantle, the outer core and the inner core. • The crust is the outermost layer of the earth making up 0.5-1.0% of the earth’s volume and less than 1% of Earth’s mass. • Materials that initially stayed in their liquid phase during this process, called “incompatible elements,” ultimately became Earth's brittle crust. • The lower layer of the crust consists of basaltic and ultra-basic rocks. • Density increases with depth, and the average density is about 2.7 g/cm3 (the average density of the earth is 5.51 g/cm³). • The thickness of the crust varies in the range of 5-30 km in the case of the oceanic crust and 50-70 km in the case of the continental crust. • The mean thickness of the oceanic crust is approximately 7 km, while the mean thickness of the continental crust is about 35-40 km. Important Notes: +--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Layer | Characteristics | +--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Crust | The uppermost layer over the earth’s surface is called the crust. It is the thinnest of all the layers. It is about 35 km. on the continental masses and only 5 km. on the ocean floors. The main mineral constituents of the continental mass are silica and alumina. It is thus called sial (si-silica and al-alumina). The oceanic crust mainly consists of silica and magnesium; it is therefore called sima (si-silica and ma-magnesium) The crust makes up only 1% of the volume of Earth. | | Mantle | Just beneath the crust is the mantle which extends up to a depth of 2900 km. below the crust. The mantle is semi-liquid, sort of like a malleable plastic, and makes up 84% of Earth's volume. | | Core | The innermost layer is the core with a radius of about 3500 km. It is mainly made up of nickel and iron and is called NiFe (Ni – nickel and Fe – ferrous i.e. iron). The central core has very high temperature and pressure. The core makes up only 15% of the volume of Earth. | +--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+