where did geologists based their interpretion of the earth's crst?

Answer 1

Based on...
...borings
...visible rock strata that have been overturned/brought to the surface through geologic processes (mountain building/ earthquakes, etc)
...volcanic activities (study what is spewed out of a volcano)
...seafloor spreading
...knowledge of plate tectonics

Answer 2

The information in the above answer is correct. It applies only to the upper few km of the earth's crust, and the crust is anywhere from 50 to 200 km thick.

Most of the information we know about the earth's internal structure comes from geophysics. We know the mass of the earth from calculating the distance from the sun and its orbiting speed, and we know there are layers in the earth from seismic data. Combining these data with surficial data we have a pretty good idea of the very complex structure within the earth.

For example, we know that the average density of the entire earth is about 5.5, but the earth's crust is only about 2.7. Therefore, density (or specific gravity) of the core of the earth must be greater than 5.5. We also know that part of the earth's interior must be liquid, because certain types of seismic waves (earthquakes) do not travel through the liquid mantle.

Hope that helps!