Igneous rock as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are also geologically important because: In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic and hypabyssal) or extrusive (volcanic). Intrusive igneous rocks are formed from magma that cools and solidifies within the crust
Lava dome extrusive products of lava domes. Domes may reach heights of several hundred meters, and can grow slowly and steadily for months (e.g. Unzen volcano), years (e.g. Soufrière Hills volcano), or even centuries (e.g. Mount Merapi volcano). The sides of these structures are composed of unstable rock debris. Due to the intermittent buildup of gas pressure, erupting domes can often experience episodes of explosive eruption over time. If part of a lava dome collapses and exposes pressurized magma, pyroclastic flows can be produced. Other hazards associated with lava domes are the destruction of property from lava flows, forest fires, and lahars