SHIELDS
The term Shield refers to a very large, relatively stable, and relatively flat expanse of very old rocks. Shields constitute the earliest slabs of solidified molten crust. A shield is from the nuclei of the landmasses. A shield’s rocks are igneous and metamorphic in origin and contain the oldest rocks on Earth.
Example:
- Indian shield: Peninsular region
- Australian Shield: Western half of Australia
- African shield: Perhaps the largest at present
- Brazilian shield
- Venezuelan shield
- Canadian shield: Laurentian shield
- Antarctic shield: Under the ice of Eastern Antarctica
Economic Significance
Magma contains ferrous and non-ferrous minerals. Cratons, which are blocks of magma when exposed as shields, bring these mineral resources to the surface.
- Mining: Rich resources propel primary and secondary economic activities like mining.
- Industrialization: Helps in industrialization by providing a much-needed raw material base for setting up industries.
- Export: Earn forex reserves through exports.
- Economic development: Employment opportunities and overall economic development in the region by acting as a growth pole.
- Energy generation: The Canadian Shield area’s topography with numerous rivers and waterfalls has helped in the generation of hydroelectric power.
Examples (Economic significance)
- Canadian Shield: It is rich in natural resources, including minerals, forests, and freshwater. Mining began in the mid-19th century and was key to Canada’s economic development. Various minerals and precious stones have been mined or continue to be mined on the Shield, including gold, silver, copper, zinc, nickel, iron, uranium, and diamonds.
- Singhbhum Shield: It is the area where Jharkhand is situated. Since it’s a shield region, there is an abundance of minerals and ores like the occurrence of iron, chromite, manganese, and silver ores. Hence Jharkhand is a state of Mines, Minerals, and Industries.
- Baltic shield: With glacier retreat, shield depressions have turned into lakes supporting inland transport in Sweden and Finland.
- Dharwad Shield: With laterite soil, it has promoted the development of building materials like bricks.
- Seismic Waves: An earthquake releases pulses of energy called seismic waves.
Difference between Body and Surface waves
| Body Waves | Surface Waves |
|---|---|
| They are generated due to the release of energy at the focus and move in all directions traveling through the body of the earth. Hence, the name – body waves. | When body waves interact with surface rocks, a new set of waves is generated called surface waves. |
| They travel only through the interior of the earth. | These waves move along the earth’s surface. |
| Body waves are faster than surface waves and hence they are the first to be detected on a seismograph. | Surface waves are also transverse waves in which particle movement is perpendicular to the wave propagation. |
| There are two types of body waves: primary waves and secondary waves. | Hence, they create crests and troughs in the material through which they pass. |
| Surface waves are considered to be the most damaging waves. | |
| Two common surface waves are Love waves and Rayleigh waves. |
Difference between P-Waves and S-Waves
| P-Waves | S-Waves |
|---|---|
| P-waves are known as Primary waves. They are the first waves to arrive at the surface. | S-waves arrive after some time after the happening of an earthquake and they are called secondary waves. |
| The characteristics of P-waves are like sound waves. They travel through all three mediums—solid, liquid, and gas. | A significant characteristic of these S-waves is that they travel only through a solid medium. |
| These waves tend to vibrate parallel to the direction of wave propagation. This causes density differences in the material through which they travel. | The direction of vibration of this S-wave is perpendicular to the direction of wave propagation, thereby creating crests and troughs in the material of their transmission. |
| These waves are responsible for elongating and squeezing material. |
Difference Between Love-Waves and Rayleigh-Waves
| Love-Waves | Rayleigh-Waves |
|---|---|
| This kind of surface wave causes horizontal shifting of the earth during an earthquake. | These waves follow an elliptical motion. |
| They are much slower than body waves but are faster than Rayleigh. | A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. |
| They exist only in the presence of a semi-infinite medium overlain by an upper finite thickness. | Because it rolls, it moves the ground up and down and side-to-side in the same direction that the wave is moving. |
| Confined to the surface of the crust, Love waves produce entirely horizontal motion. | Most of the shaking felt from the earthquake is due to the Rayleigh wave, which can be much larger than the other waves. |
SHADOW ZONE
- The shadow zone is that portion of the earth in the earthquake that does not receive any direct P-waves. It is from the angular distances of 104 to 140 degrees from the given earthquake.
- Shadow zones are formed when the liquid core stops the S waves and refracts the P waves.
- A zone between the epicenter’s 105-145 degrees is found to be a shadow zone for both the waves. It appears as a band away from the epicenter.
- Beyond 105 degrees of the epicenter, the entire zone is free of the S-waves.
- S-waves have larger shadow zones than P-waves.