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Lesson2e . Lava Tubes and Density. Lava tubes. Lava pouring out of Lava Tube. Collapsed Lava Tube on Earth . Lava flows in channels also. Lunar Rille (means “groove”). Topographical map of Mars. Olympus Mons. Lava Tubes on Mars – Pavonis Mons.

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Lava Tubes and Density


Surface A is likely to be older. We can see a number of impact craters in surface A. But surface B seems to be missing craters.

  • The area in the image is about the same for both regions.
  • We must assume that lava has covered up any craters that might have been present in surface B. This makes B younger than A.
a very big question to answer
A very big question to answer
  • We see that the Moon and Mars have both had active volcanism.
  • When a planet is still very hot inside various tectonic processes occur that allows the heat to escape the interior. We say the planet is geologically active.
  • When a planet cools to the point that tectonism stops, we say the planet is geologically dead.
  • If we don’t see tectonic processes occurring at the moment, what do we have to do in order to determine when a planet was geologically active?

We need to determine the age of the features we see.

  • Radiometric data of lava would be the best but we can not currently travel to Mars and return rock samples. Or the other planets or moons.
  • An important tool to use is crater density on the surface. The greater the density of craters the older the surface must be.
density an important parameter
Density -- an important parameter
  • There are two types of densities we will use in this class.
  • Mass density (mass/volume) and Number density (number/volume).
  • A density doesn’t have to be a volume density.
  • It can be an area density. (kg/m2)
  • And it doesn’t have to be mass.
population density
Population Density
  • Alaska has a population of 686,000 people
  • It has an area of 663,000 square miles.
  • That is roughly 1 person/square mile.
  • What does this tell us about the lives of Alaskans?

Not much. It only tells us there is a lot of room for people to expand into.

  • This is an average density. In fact most Alaskans live near or in cities where the population density is much higher.
white blood cell counts
White blood cell counts.
  • When a doctor checks your white blood cell counts for an infection, they draw some blood and then use a very small volume of blood to count the number of white cells.
  • This is a number of white blood cells/volume.
  • This density tells them if you have an infection or not.
  • What assumptions is built into this analysis?
let s compute the mass density of the earth
Let’s compute the mass density of the Earth.
  • Earth mass = 5.97 x 1024 kg.
  • Earth radius = 6.96 x 106 m.
  • Density = mass/volume
  • The Earth is approximately a sphere.
  • So all we need is the volume equation for a sphere.
a volume is 3 d
A volume is 3-D





V = L x L x L = L3

V = 4πR3/3


The surface area of a sphere is: 4πR2

  • The surface area of a cube is: 6L2
  • The volume of a sphere is (4/3)πR3
  • The volume of a cube is L3
let s compute the mass density of the earth1
Let’s compute the mass density of the Earth.
  • Earth mass: M = 5.97 x 1024 kg.
  • Earth radius: R = 6.96 x 106 m.
  • Density = mass/volume = M/V
  • The Earth is approximately a sphere.
  • So all we need is the volume equation for a sphere.
  • V = (4/3) π R3

V = (4/3)π(6.96 x 106 m)3

  • Dearth = (5.97 x 1024 kg)/(1.09 x 1021 m3)
  • Dearth = 5,477 kg/m3
  • Dearth = 5.48 g/cm3

We can test this by comparing to rocks we find on the surface.

this is easy to do
This is easy to do.
  • Weigh the rock to find its mass.
  • Find the volume of the rock by the amount of water it displaces.
  • The average density of rocks on the surface of the Earth is Drock = 3 g/cm3
  • The density we calculated for the Earth was

This is very different. How can we explain this?

  • Dearth = 5.48 g/cm3

The density we calculated for the Earth was the average density. It doesn’t mean that the density is the same everywhere inside.

  • Here is what we have:
  • Average density of Earth is 5.48 g/cm3
  • Average density of surface rocks is 3 g/cm3
  • Inside the Earth there must be something that is much more dense than the surface rock in order to increase the average.

Research on the composition of meteorites shows that many are composed of high fractions of iron.

  • The Earth and the other planets are built up from the accumulation of meteors and asteroids.
  • The planets should have a lot of iron.
  • The density of iron is about 8 gm/cm3
  • You can see that mixing iron densities with rock densities will give us something closer to the average.

The iron has to be in the interior of the Earth and not much on the surface.

  • In fact, the core of the Earth is mostly iron.
  • Why is the majority of Earth’s iron in the core?
  • When the Earth was forming it was suffering many impacts from meteors and asteroids.
  • These impacts heated the Earth so that it was completely molten.
  • When it was in liquid form, it was possible for the dense elements (like iron) to sink, and the least dense elements (like silicon) to rise.
  • This is why the surface rocks are low density.
compressed vs uncompressed
Compressed vs. Uncompressed

The average density we have been discussing is the Earth’s compressed density. It depends on the material in the Earth and how much that material is compressed by the Earth’s gravity.

If we want to know what planets are made of, we do not want to take into account the force of gravity. This is the uncompressed density. It only depends on the material in the planet.


Let’s see what we can deduce about the four Galilean moons, Io, Europa, Ganymede, and Callisto.

other evidence
Other evidence
  • Surface of Europa, Ganymede and Callisto is composed of water ice. (Density = 1 g/cm3)
  • What can you conclude about the Galilean moons?