I'm studying minerals..I understand that if molten rock cools slowly undergound, then large mineral cystals start forming. AND if that rock cools quickly at the surface, small cystals form. My ? is ..sometimes rock has both large and small mineral crystals ..how can that be explained?
Lost...
thanks for responding..Kira G.
The rocks with both small and large crystals is probably a conglomerate rock. Check this site for information.
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Great question, Kira! The presence of both large and small mineral crystals in a rock can be explained by a geological process called "differential cooling."
Differential cooling occurs when the molten rock, also known as magma, cools at different rates in different parts of the rock. This can happen due to various factors such as variations in temperature, pressure, or the presence of surrounding materials.
Let's break down the process step by step:
1. Magma Formation: Magma is formed beneath the Earth's surface from the melting of rocks due to high temperatures and pressure. The exact composition of the magma depends on the minerals present in the original rock.
2. Cooling: As the magma rises towards the surface or moves deeper underground, it starts to cool down. The rate of cooling can vary depending on the specific conditions. Generally, cooling proceeds more slowly deep underground and faster near the surface.
3. Crystal Formation: As the magma cools, the minerals in it begin to solidify and form crystals. However, the rate of cooling plays a crucial role in determining the size of the crystals.
Now, let's consider two scenarios:
- Slow Cooling: If the magma cools slowly deep underground, there is more time for the minerals to grow and form larger crystals. This occurs because the slower cooling allows for the atoms within the minerals to arrange themselves in an ordered pattern over an extended period.
- Fast Cooling: On the other hand, if the magma cools rapidly at or near the surface, there is less time for the minerals to grow due to the shorter cooling period. As a result, the crystals that form are much smaller.
4. Differential Cooling: In some cases, the magma may cool at different rates within the same rock. This can happen if the magma body is relatively large or if it undergoes changes in the cooling conditions over time. As a result, different regions of the magma experience different cooling rates.
When differential cooling occurs, the regions that cool more slowly will have larger crystals, while the areas that cool more quickly will have smaller crystals. Therefore, a single rock can exhibit a combination of large and small mineral crystals.
It's important to note that the specific minerals present in the rock can also influence crystal size. Some minerals naturally tend to form smaller crystals regardless of cooling rate, whereas others have a higher tendency to form larger crystals.
I hope this explanation helps you understand how rocks can have both large and small mineral crystals. If you have any more questions, feel free to ask!