Indirect sources of information about the interior
Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"
Meaning
Indirect sources are clues about the interior that we read from outside, without touching the material. Temperature, pressure and density all rise with depth. Meteors are not pieces of the earth, but they are made of similar material, and iron meteorites look like the earth's core. Gravity anomalies show how mass is spread unevenly. Magnetic surveys show where magnetic materials lie in the crust. The most important clue is seismic waves, the waves sent out by earthquakes. How they speed up, bend, bounce or stop reveals the earth's layers.
Example
S-waves from an earthquake never reach stations more than 105° from the epicentre. From this, scientists worked out that the outer core, which starts about 2,900 km down, is liquid. No one has ever drilled anywhere near that depth.
Don't confuse with
- Direct sources of information about the interior: these are actual rock samples from mines, drilling and volcanoes. Volcanic material is a direct source.
Related concepts
- Direct sources of information about the interior
- Kola Superdeep Borehole
- Geothermal gradient
- Gravity anomaly