Scientists<span> can tell by observing the seismic waves that are recorded all over the surface of the </span>earth<span> from distant earthquakes. The seismic waves are reflected (bounced off) </span>layers<span> of different density, and they are refracted (bent) when they enter </span>layers<span> of different density.</span>
That would be Pyroclastic material
<em>it took <u>130 years </u> to build Sagrada familia... :) it build from <u>1852 to 1926...</u>
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A shield volcano<span> is a type of volcano usually built almost entirely of fluid magma flows</span>
These subs are useful in studying the ocean floor, where it is so deep that divers would be crushed by the water pressure. For example, research submersibles are used to study shipwrecks like the RMS Titanic.
