Volcanic Rock
Volcanic or igneous rocks are among the most common rock types on the earth's surface, particularly in the oceans, and on land at tectonic plate boundaries. These rocks are important because their mineral and chemical makeup is used to learn about the composition, temperature and pressure that exists within the Earth’s mantle. They also tell us much about the tectonic environment, given that they are closely linked to the convection of tectonic plates.
Igneous rocks by definition are produced under intense heat, through the cooling and solidification of magma (or lava). As hot, molten rock rises to the surface, it undergoes changes in temperature and pressure that cause it to cool, solidify, and crystallize.
All told, there are over 700 known types of igneous rock, the majority of which are formed beneath the surface of the Earth’s crust, which classifies them as plutonic rock.
Plutonic Rocks
Igneous rock formed below the earth’s surface is called intrusive or plutonic rock. Intrusive igneous rock is formed when magma cools and solidifies within small pockets contained within the earth’s crust. As this rock is surrounded by pre-existing rock, the magma cools slowly, which results in it being coarse grained with mineral grains that are big enough to be identifiable with the naked eye. The most common types of plutonic igneous rock are granite, gabbro, or diorite. The central cores of major mountain ranges consist of large bodies of intrusive igneous rocks because they are the result of magma cooling within preexisting solid rock on the surface.
Extrusive Rock
Igneous rock formed on the surface as a result of volcanic activity is called extrusive or simply volcanic rock, so named because it is the result of magma pouring onto the surface of the planet and cooling. When it reaches the surface, either on a continental shelf as a volcano or on the ocean floor as a submarine volcano, it becomes lava, by definition.
The viscosity of lava depends upon the temperature composition and crystal content of the molten rock itself; so lava can flow slowly, forming short steep flows; or it can flow rapidly, forming long, thin flows. It can also explode violently, dispersing magma into the air that falls back to the surface as ash and tuffs. Compared to intrusive rock, this type of igneous rock cools and crystallizes at a much faster rate due to it being exposed to air or water, which results in it being fine-grained.
The initial composition of most volcanic rocks is basaltic and fine-grained. Basalt is the most common type of volcanic rock on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, such as Iceland and Hawaii. Sometimes, the cooling of lava is so rapid as to prevent the formation of even small crystals after extrusion, resulting in rock that may be mostly glass, such as obsidian.
Obsidian rock
Vesicles
Volcanic rock may have a vesicular texture. Vesicular texture is characterized by the rock being pitted with many cavities (vesicles) at its surface and inside. As magma rises to the surface the pressure on it decreases. When this happens the gasses dissolved in the magma come out of solution, forming gas bubbles inside it. When the magma finally reaches the surface as lava and cools, the rock solidifies around the gas bubbles and traps them inside, preserving them as holes filled with gas, called vesicles.
Vesicles in igneous rock
Please answer the questions below, and send your answers to the CO through their profile, before you log a find. No need to wait for confirmation before you log. Short answers are fine.
1. Feel the volcanic rock at the GZ. Now feel the texture of the large non-volcanic boulder next to it. What might have caused the texture of these two rocks to be so different?
2. What is the difference between plutonic and extrusive rock?
3. What are phenocrysts, and do you see any in the volcanic rock at the GZ?
4. A photo of your party at the GZ is fine, but not required, and remember DO NOT stand, sit or climb on these rocks. Short answers to questions are fine.
FTF: climbingdivers!
References: dictionary.com, Wikipedia, http://www.universetoday.com/82009/how-are-igneous-rocks-formed/. Photo credit, igneous rocks are formed: Wikipedia Commons/Jasmin Ros.