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Earth and Space Science - The Rock Cycle and Earth's Layers

Grade 8IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Earth consists of three main compositional layers: the Crust (rich in SiO2SiO_2), the Mantle (rich in MgMg and FeFe silicates), and the Core (primarily FeFe and NiNi).

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Mechanical layers categorize the Earth by physical properties: the Lithosphere (brittle outer shell), the Asthenosphere (ductile, plastic-like layer where convection occurs), the Mesosphere, and the Outer and Inner Cores.

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Density (ρ\rho) increases with depth; the average density of the crust is approximately 2.7−3.0 g/cm32.7 - 3.0 \text{ g/cm}^3, while the inner core exceeds 13.0 g/cm313.0 \text{ g/cm}^3.

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Igneous rocks form from the cooling and solidification of molten rock. Intrusive rocks form from magma below the surface (slow cooling, large crystals), while extrusive rocks form from lava on the surface (fast cooling, small crystals).

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Sedimentary rocks form through a process of weathering, erosion, deposition, and lithification (compaction and cementation of sediments like CaCO3CaCO_3).

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Metamorphic rocks are created when existing rocks are subjected to high heat (TT) and pressure (PP) without melting, leading to recrystallization and foliation.

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The Rock Cycle is a continuous process where any rock type can be transformed into another through geological processes such as melting, uplift, and metamorphism.

📐Formulae

ρ=mV\rho = \frac{m}{V}

P=ρghP = \rho gh

ΔT=Tbottom−Tsurface\Delta T = T_{bottom} - T_{surface}

Geothermal Gradient=ΔTΔd\text{Geothermal Gradient} = \frac{\Delta T}{\Delta d}

💡Examples

Problem 1:

A geologist finds a sample of granite with a mass (mm) of 540 g540 \text{ g} and a volume (VV) of 200 cm3200 \text{ cm}^3. Calculate the density (ρ\rho) of the rock and determine if it is likely from the continental crust (2.7 g/cm32.7 \text{ g/cm}^3) or the oceanic crust (3.0 g/cm33.0 \text{ g/cm}^3).

Solution:

ρ=540 g200 cm3=2.7 g/cm3\rho = \frac{540 \text{ g}}{200 \text{ cm}^3} = 2.7 \text{ g/cm}^3

Explanation:

Since the calculated density is 2.7 g/cm32.7 \text{ g/cm}^3, the sample matches the average density of the continental crust, which is primarily composed of granitic rocks.

Problem 2:

If the temperature increases by 25∘C25^{\circ}C for every 1 km1 \text{ km} of depth (geothermal gradient), calculate the temperature (TT) at a depth (dd) of 4 km4 \text{ km} if the surface temperature is 15∘C15^{\circ}C.

Solution:

T=Tsurface+(gradient×d)T = T_{surface} + (\text{gradient} \times d) T=15∘C+(25∘C/km×4 km)=115∘CT = 15^{\circ}C + (25^{\circ}C/\text{km} \times 4 \text{ km}) = 115^{\circ}C

Explanation:

To find the temperature at depth, we multiply the depth by the rate of temperature increase and add it to the starting surface temperature.

Problem 3:

Identify the process required to turn a Metamorphic rock into an Igneous rock.

Solution:

Metamorphic Rock →\rightarrow Melting →\rightarrow Magma →\rightarrow Cooling/Solidification →\rightarrow Igneous Rock

Explanation:

For any rock to become igneous, it must first be melted into magma or lava due to extreme TT, and then undergo crystallization as it cools.