When volcanoes erupt, hot lava drips on the side of volcanoes then cools and hardens. This becomes igneous rocks like granite. Igneous rocks can also form deep inside the Earth from magma.
Arguably, the start of the rock cycle begins when molten rock cool from volcanoes. Because without igneous rocks, there wouldn’t be the rock cycle.
How do igneous rocks form?
Igneous rocks form after cooling and solidifying from magma or lava. So when molten rock cools, they turn into a solid and become igneous rocks. First, you have to understand the difference between lava and magma. And it’s all about location.
For a volcano when it’s deep inside the Earth, it’s magma. Once it erupts, it’s lava. Then, it cools and solidifies. But it matters where it cools and solidifies.
Because of the immense pressure that igneous rocks undergo, it erases any fossil evidence. Likewise, this is true for the immense heat from metamorphic rocks. So if you see a rock with a fossil, you can automatically assume that it is a sedimentary rock.
What are the two types of igneous rocks?
There are two types of igneous rocks. Based on texture and composition, we can classify them into two categories:
- INTRUSIVE: Intrusive (or plutonic) rocks form inside the Earth from magma.
- EXTRUSIVE: Extrusive rocks cool and solidify on the surface.
For intrusive (or plutonic) rocks, magma is hot inside Earth so it cools slowly. When it cools, minerals form slowly. So crystals grow very large for intrusive igneous rocks. For example, gabbro, granite and pegmatite are intrusive rocks.
For extrusive rocks, because they cool quickly, the crystals are smaller. Some types of extrusive igneous rocks can even have pockets of air bubbles inside them. For example, extrusive rocks include basalt, andesite and obsidian.
Where do igneous rocks form?
Hidden beneath Earth’s oceans, underwater volcanoes spew out lava at mid-oceanic ridges (rift valleys).
Because divergent plates move apart from each other at these mid-oceanic ridges, magma flows upwards from the mantle beneath.
When the lava hardens, it becomes dark igneous rock or “basalt” at rift volcanoes.
Because divergent plates fill in the gaps with basalt, oceanic crust turns out to be very young geologically.
Over time, the plates grow at oceanic crust and older rock is pushed away from mid-oceanic ridges.