1. why is earths crust the least dense layer?
2. why we know very little about the mantle?
3. why is the outer core of earth liquid and the inner core is solid?
4. how are scientists are able to study the center of the earth?

1. Because it is thick and has relatively low density, continental crust rises higher on the mantle than oceanic crust, which sinks into the mantle to form basins. When filled with water, these basins form the planet's oceans.The lithosphere is the outermost mechanical layer, which behaves as a brittle, rigid solid.

3. Earth's inner core is solid because it is under such intense pressure from the weight of the entire planet that its atoms are held together by force. ... ... By contrast, the outer layers of Earth, while being slightly cooler (but still incredibly hot), have less pressure and thus their atomic form reverts to liquid.

4. Scientists have learned about Earth's internal structure by studying how these waves travel through the Earth. The technique is straightforward — it involves measuring the time it takes for both types of waves to reach seismic stations from the epicenter of an earthquake.