Large crystals in granite, tiny ones in basalt
1 min 50 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Igneous rock classification, with the past questions on it.
What Dr. Mars says (the words of the video)
Why does granite have large crystals, and basalt tiny ones?
Dr. Mars breaks a piece of granite.
A fresh surface shows the crystals.
His hand lens shows large crystals.
Each colour is a different mineral.
Now he breaks a piece of basalt.
The same lens shows tiny crystals.
Most are too small to see by eye.
Both are igneous rocks: they form when melted rock cools.
Underground it is magma. At the surface it is lava.
Granite formed from its own magma, deep underground.
It cooled slowly, so crystals had time to grow large.
Later, the land rises and the rock above the granite wears away, uncovering it.
Basalt formed from another magma, as lava at the surface.
It cooled fast, so crystals had little time to grow.
Granite is intrusive: it cooled inside Earth.
Basalt is extrusive: it cooled at the surface.
Look for intrusive and extrusive on the Rock Cycle Infographic in the Reference Tables.
Slow cooling: large crystals. Fast cooling: tiny crystals.
Check yourself! Where do the largest crystals grow: A, B, or C?
Look for the slowest cooling.
Answer: C! This magma stays deep and cools slowly.
The granite Dr. Mars broke formed this way.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.