Describe the direct and indirect sources …

There are two sources for information about interior of the earth – a) Direct Sources and b) Indirect Sources:
•Direct Sources: Mining, drilling and volcanic eruption are examples of direct sources. During the process of mining and drilling rocks and minerals are extracted which gives information that there are layer system in the crust. Crust is made of many kinds of rocks and minerals. Volcanic eruption suggests that there is some zone inside the earth which is very hot and in liquid condition. Direct sources are not very reliable because mining and drilling can be done only up to some depth only.
Indirect Sources: Seismic waves, gravitational field, magnetic field, falling meteors etc are example of indirect sources. They are very important for know about earth‘s interior. Movement of seismic wave suggests that there are three layers in the earth and each layer has different density. Density increases toward the center of the earth.

The direct sources are those that provide first-hand information about events whereas the indirect sources involve the reports and analysis of direct information by other people. In scientific disciplines, experiments are the most common direct source; in other disciplines, surveys and interviews, or research into written and oral records of events provide direct access to information. Most of our information about earth's interior has come from seismic studies. Seismic waves produced during an earthquake are recorded by a seismograph when these waves reach the surface of the earth. There are two types of seismic waves- body waves and surface waves.