Determining the densities of rocks has many different
applications, from selecting building materials to finding out about the
interior of the Earth. Determining
densities can also help to provide information about particular rock
types.
This video
shows four simple methods you can use to determine the densities of different
rock types.
The densities of rocks depend on numerous factors including
the texture and mineral content.
Sedimentary rocks tend to be less dense than metamorphic and igneous
rocks, as they are more porous. The porosity of a sedimentary rock can vary
depending on the sediment size and shape.
Fine sediment will compact more easily than course sediment. For example,
mudstone is generally denser and less porous than sandstone. Round grains will
also create more pore spaces, whereas platy minerals like kaolinite can stack
like Lego pieces reducing the pore spaces. The amount of cement between the
grains is one of the most important factors determine the porosity and density
of a sedimentary rock. A well cemented rock will be denser than a poorly
cemented rock.
Metamorphic and igneous rocks have interlocking crystals
with very little to no pore spaces (pumice and scoria being an exception). Their
densities are mostly determined by their mineral content. Felsic igneous rocks
(with high silica content) such as granite, will generally be less dense than
mafic igneous rocks, such as basalt, which have more iron rich minerals.
Rock and mineral density tables can be found here.
The density ranges of different rock types can be determined
in a lab and then used to assist with the interpretation of geophysical
surveys. Denser rocks and materials will have a higher gravitational pull than
less dense materials. Gravitational surveys can therefore be very useful
methods of exploration for resources which provide a strong gravitational
contrast. For example, a salt dome will generally have much lower
density than the rocks surrounding it and will give a low gravity reading.
Gravity methods are also useful for finding voids such as old mine workings,
hidden caves, and areas of potential sinkholes. Changes in gravity measurements
around a volcano can indicate magma movement, which may indicate a possible
eruption.
- To have a look at gravity surveys across Western Australia check out this interactive map, the Using Geoview video will help explain how to use the website.
Understanding how the densities of rocks vary within the
Earth helps geologists to determine more about the interior structure of the
Earth. From analysis of meteorites and application of the principle of gravity
settling (see blog post – Gravity Settling), we know that the density of rocks increases towards the centre
of the Earth. Laboratory studies show that when density increases, the velocity
which seismic waves are transmitted decreases. Therefore, we would expect
seismic waves to slow down as they travel deeper through the Earth. However,
this is not the case (at least, until the outer core is reached). Scientists have determined that this is due
to the rigidity and incompressibility of rocks increasing. Scientists can also
use their understanding of seismic velocity in relation to rock density to help
map out the location of mantle plumes, magma and subducting plates in a process
known as seismic tomography. This has helped with our understanding of tectonic
processes.
Seismic tomography uses the velocity of seismic waves to help map out areas of differing temperature within the Earth. Waves travelling faster than average can indicate colder regions, whereas waves travelling slower than expected can indicate hotter regions. (Wikipedia, accessed 28/08/2020)
Understanding how seismic velocities vary with rock types is
also a key principle used in seismic reflection and refraction methods, which
are commonly utilised in hydrocarbon exploration. Changes in velocities can indicate changes in
rock types or indicate fluids in pore spaces which is useful when looking for
oil.
However, because of the range in densities which rock types
can have, any geophysical studies can only be used to produce interpretations
and models. They create approximations which can be better defined with addition
of models produced from other types of survey.
- For more activities relating to rock densities look at the Year 8 WASP STEM packages