Inner core
Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"
Meaning
The inner core is the solid ball at the very centre of the earth. It starts at about 5,150 km depth and goes down to the centre at about 6,371 km. It is made mostly of iron and nickel, which is why the core is called nife. The inner core is about 5,000-6,000 °C, hot enough to melt iron at the surface. It stays solid because the rock and metal above press down on it with huge force. We know it is solid from seismic waves (shaking waves sent out by earthquakes). Inge Lehmann found it in 1936 when she noticed faint P-waves bouncing off a solid inner ball.
Example
Some weak P-waves from an earthquake show up on seismographs inside the P-wave shadow zone (the band 105°-145° from the epicentre). In theory no direct P-waves should arrive there. These waves have bounced off the solid inner core.
Don't confuse with
- Outer core: same iron-nickel material, but it is liquid (2,900-5,150 km). The inner core is solid.
- Lithosphere: the solid, rigid outer shell of the earth (crust plus uppermost mantle). It has nothing to do with the inner core.