Meteorites and the rock cycle
Geologists sort Earth rocks into igneous, sedimentary and metamorphic. Meteorites can be described in the same way, which tells us what happened on their parent asteroids.
- Describe how igneous, sedimentary and metamorphic rocks form.
- Explain why chondrites are unlike any rock on Earth.
- Link crystal size to cooling rate in igneous meteorites.
Links to: KS3 chemistry, Earth and atmosphere: the rock cycle and the formation of igneous, sedimentary and metamorphic rocks.
Earth's three rock types
Igneous rocks form when molten rock, magma or lava, cools and crystallises. The slower it cools, the bigger the crystals: granite cooled slowly underground and has large crystals; basalt cooled quickly at the surface and has tiny ones.
Sedimentary rocks form from fragments that are deposited, compacted and cemented. Metamorphic rocks form when existing rocks are changed by heat and pressure without melting. On Earth these processes are linked in the rock cycle, which constantly recycles the crust.
Chondrites: a rock type of their own
The most common meteorites, chondrites, fit none of these boxes neatly. They are made of material that came together in space: millimetre-sized chondrules, which were molten droplets, metal grains and fine dust, all collected into an asteroid. In a way that is like a sedimentary rock, except the 'sediment' was gathered by gravity in the disc of gas and dust around the young Sun.
Many chondrites were then heated inside their asteroids, sometimes to about 800 to 950 °C, without melting. Their grains grew and their chondrules blurred, just as heat changes rocks in metamorphism on Earth. Scientists grade this heating with a number from 3, barely heated, to 6, strongly heated.
Igneous meteorites
Some asteroids grew hot enough to melt. The rocks that crystallised from their magma are igneous, and are called achondrites because they have no chondrules. Eucrites, from the asteroid Vesta, are basalts that formed as lava flows or shallow intrusions, much like basalt on Earth. Most iron meteorites are igneous too: they are metal that melted, sank to an asteroid's core and crystallised as it cooled extremely slowly.
There is no rock cycle on a small asteroid. It has no plate tectonics, rain or rivers, which is why its rocks have survived for 4.5 billion years.
Make a table with four rows: igneous, sedimentary, metamorphic and chondrite. In columns, write how each forms, an Earth example or meteorite example, and one feature you could see with a hand lens. Then answer: why do meteorites preserve older rocks than Earth does?
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1How do igneous rocks form?
Igneous rocks crystallise from magma or lava. Eucrites and iron meteorites are igneous.
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Question 2A rock has large crystals. What does this tell you?
Slow cooling gives crystals time to grow large, as in granite.
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Question 3What are chondrites mainly made of?
Chondrites are aggregates of material from the disc around the young Sun, never melted on their asteroid.
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Question 4Which process is like the heating of chondrites inside their asteroids?
Heating without melting changes the texture of the rock, as metamorphism does on Earth.
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Question 5Why have asteroid rocks survived for 4.5 billion years when Earth's oldest rocks have mostly been destroyed?
Earth constantly recycles its crust. Small asteroids have no such processes, so their rocks are preserved.