Explanation:

Volcanic eruptions are a crucial source of obtaining direct information about earth's interior. As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis. (However, it is difficult to ascertain the depth of the source of such magma.)
Seismic activity also tells us about the interior of earth especially about plate margins and fault lines. Study of different types of earthquake waves also gives us information about earth’s interior.
The gravity values also differ according to the mass of material. The uneven distribution of mass of material within the earth influences this value. Such a difference is called gravity anomaly.  Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.   
The material and the structure observed in the meteors are similar to that of the earth. They are solid bodies developed out of materials same as, or similar to, our planet. Hence, this becomes yet another source of information about the interior of the earth.