Earth and Its Structure

The inclination of the earth's axis to plane of the orbit is 23½°.

The epicentre is the point on Earth's surface directly above the focus (hypocentre), which is the actual location inside Earth where the earthquake originates. The epicentre is therefore the first point on the surface to experience earthquake waves, not the focus/hypocentre which is underground.

The India-Australia-New Zealand Plate (also called the Indo-Australian Plate) is one of the major tectonic plates covering approximately 0.2% of Earth's surface. The Philippine Plate, Arabian Plate, and Cocos Plate are smaller, minor plates. Major plates include: Pacific, North American, South American, Eurasian, African, Indo-Australian, and Antarctic plates.

Compressional Plate Boundary or Destructive Plate Boundary occurs when an oceanic plate is forced under (or subducts) a continental plate.

The lithosphere is Earth's rigid outer layer consisting of the crust and the uppermost solid mantle (approximately 100 km thick). It is broken into tectonic plates. The asthenosphere below it is the ductile part of the mantle. Options A, B, and C are incorrect because they include the lower mantle, core, or combinations that don't constitute the lithosphere.

The structure of Earth's interior is indeed deduced using seismic waves (Assertion A is true). However, Reason R is false because it reverses the facts: P-waves (Primary) can travel through solids, liquids, AND gases, while S-waves (Shear) can ONLY travel through solids. This difference in behavior is actually what helps scientists understand Earth's internal structure.

Snell's law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass.

The Mohorovicic discontinuity (Moho) is the boundary between Earth's crust and the mantle, discovered by Andrija Mohorovicic in 1909. It is characterized by a sharp increase in seismic wave velocity (from about 6-7 km/s to 8+ km/s) due to the change in composition from crustal rocks to denser mantle rocks (peridotite).

Correct Answer: (A) is true but (R) is false.
The Earth was discovered to have a solid inner core distinct from its liquid outer core in 1936, by the seismologist Inge Lehmann, who deduced its presence by studying seismograms from earthquakes in New Zealand; she observed that the seismic waves reflect off the boundary of the inner core and can be detected by sensitive seismographs on the Earth's surface.
Typical values for P-wave velocity in earthquakes are in the range 5 to 8 km/s. The precise speed varies according to the region of the Earth's interior, from less than 6 km/s in the Earth's crust to 13 km/s through the core.
 

The temperature at Earth's core is estimated to be approximately 5,430°C, similar to the surface of the Sun. This extreme heat is primarily due to: (1) residual heat from Earth's formation (accretion and differentiation), (2) heat from radioactive decay of elements like uranium, thorium, and potassium, and (3) latent heat from solidification of the inner core.

The age of Earth is estimated to be approximately 4.54 billion years (with an error margin of about 1%), based on radiometric dating of meteorites and Earth's oldest rocks. This method measures the decay of radioactive isotopes like uranium-238 into lead-206. The oldest terrestrial minerals (zircons) are about 4.4 billion years old, consistent with this age.

Earth's shape is an oblate spheroid - slightly flattened at the poles and bulging at the equator. This is caused by Earth's rotation. The centrifugal force from rotation is strongest at the equator, causing material to bulge outward there. The equatorial diameter is about 43 km greater than the polar diameter. Revolution (orbit around the Sun) and atmospheric pressure do not affect shape.

Gondwanaland was the southern supercontinent that existed during the Paleozoic and Mesozoic eras. It included present-day South America, Africa, India, Australia, and Antarctica. China was part of Laurasia, the northern supercontinent, not Gondwanaland.

Earth's mean radius is approximately 6,371 kilometers (about 3,959 miles). This value represents the average distance from Earth's center to its surface. The other options are either too small (like 1,290 km, which is closer to the Moon's radius) or don't match the accepted scientific value.

The asthenosphere (spelled 'aethenosphere' in this question) is the upper, mechanically weak layer of the mantle, extending from about 100-200 km depth to 660 km. It lies beneath the lithosphere. The lithosphere is the rigid outer layer, while photosphere relates to the Sun, and atmosphere is the gaseous envelope surrounding Earth.