Sponge - Why do astronomers believe the lunar highlands are older than the maria?

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Sponge - Why do astronomers believe the lunar
highlands are older than the maria?
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Surface Rocks -There are basically two kinds of
surface rock on the Moon.

• Highland rock - 2.9 g/cm3, rich in aluminum,
  lighter in color, crust material
• Maria rock - 3.3 g/cm3, richer in iron, darker in
  color, mantle material

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Many rock samples are impact breccia, a rock that
consists of many smaller rock fragments stuck
together due to meteoric impacts.
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The maria are believed to have been formed by
great volcanic upwellings of mantle material
about 4 billion years ago. These may have been
caused by lunar impacts with very large space
debris.
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Meteoric collisions caused the layer of
pulverized ejecta that covers the Moon to an
average depth of 20 m.
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This soil is also called lunar dust or
regolith, which means fine rocky layer. It is
thinnest on the maria (10 m), thicker on the
highlands (over 100 m in some places).(The term
regolith is also used for the soil on other moons
and planets.)
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The bootprints left in the regolith by the Apollo
astronauts will remain for millions of years due
to the slow erosion.
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Lunar regolith contains no organic matter or
water, unlike Earth soil.
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Most all lunar craters are meteoric, but some
are volcanic.
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Volcanic rilles are ditches where molten lava
once flowed.
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No rocks on the Moon have been found to be
younger than 3 billion years, so this was the
time of the last volcanic activity.
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Lava flows
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