Analyzing Statements on Earth's Crust Properties

Let's carefully examine each statement provided regarding the Earth's crust.

Statement 1: The Earth’s crust is brittle in nature.

The Earth's crust is the outermost solid shell of our planet. It sits atop the mantle. Materials can behave differently under stress depending on factors like pressure, temperature, and composition. At the relatively lower temperatures and pressures found in the upper part of the crust, rocks tend to fracture and break when subjected to stress. This property is known as brittleness.

• When tectonic forces build up, this brittle nature allows for faults to form and movement along these faults, which is the cause of most earthquakes.
• Deeper within the Earth, where temperatures and pressures are higher, rocks become more ductile and deform by flowing rather than breaking.

Therefore, describing the Earth's crust, particularly the upper crust, as brittle is geologically accurate. This statement is correct.

Statement 2: The mean thickness of the oceanic crust is 15 km, whereas that of the continental crust is around 30 km.

Let's look at the typical thicknesses of oceanic and continental crust.

• Oceanic Crust: This type of crust underlies the ocean basins. It is relatively thin compared to continental crust, typically ranging from 5 to 10 km thick. It is primarily composed of basaltic rocks.
• Continental Crust: This type of crust forms the continents and continental shelves. It is significantly thicker, typically averaging around 30 to 40 km. Under large mountain ranges, the continental crust can be even thicker, reaching up to 70-80 km. It is primarily composed of less dense, granitic rocks.

Now let's compare these typical values to the statement:

• The statement claims the mean thickness of oceanic crust is 15 km. This figure is significantly higher than the typical range of 5-10 km.
• The statement claims the mean thickness of continental crust is around 30 km. This figure falls within the typical average range of 30-40 km for continental crust.

Since the stated thickness for the oceanic crust (15 km) is incorrect, the entire statement is incorrect.

Conclusion on Correctness of Statements

Based on the analysis:

• Statement 1 is correct because the Earth's crust is indeed brittle, especially in its upper layers.
• Statement 2 is incorrect because the mean thickness of oceanic crust is typically 5-10 km, not 15 km, although the figure for continental crust is roughly correct.

Therefore, only Statement 1 is correct.

Identifying the Correct Option

We found that only Statement 1 is correct.

• Option 1 says "1 only". This matches our finding.
• Option 2 says "2 only". This is incorrect as Statement 2 is false.
• Option 3 says "Both 1 and 2". This is incorrect as Statement 2 is false.
• Option 4 says "Neither 1 nor 2". This is incorrect as Statement 1 is true.

The correct option is the one indicating that only Statement 1 is correct.

Revision Table: Earth's Crust Facts

Additional Information: Crust and Lithosphere

It's important to distinguish between the crust and the lithosphere.

• Crust: This is purely the chemical layer defined by its composition (oceanic is basaltic, continental is granitic).
• Lithosphere: This is a mechanical layer comprising the crust and the uppermost, rigid part of the mantle. This entire layer is brittle and forms the tectonic plates that move across the Earth's surface. The lithosphere is thicker than the crust alone.

The brittle nature discussed in Statement 1 applies primarily to the lithosphere, which includes the crust and the rigid upper mantle. However, the crust itself, being the outermost part of this rigid layer, is indeed brittle, making Statement 1 correct in this context.