Content

• Steps
• Method 1 of 2: Sedimentary and Metamorphic Rocks
• Method 2 of 2: Identifying Igneous Rocks
• Warnings
• What do you need

When you hold volcanic rock in your hands, the stone in your hand is one of the most ancient objects in the world. Volcanic rocks are formed from lava, magma, or ash from a volcanic eruption. Learn how to identify volcanic rocks and you can distinguish volcanic rocks from other two types of rocks - sedimentary and metamorphic.

Steps

Method 1 of 2: Sedimentary and Metamorphic Rocks

1. 1 Distinguish igneous rocks from sedimentary rocks on the basis that there are no fossils, shells or rounded grains in igneous rocks. All igneous rocks are characterized by intertwined crystals. In some igneous rocks, these crystals are large enough to be seen without a lens. Other igneous rocks are formed of crystals so small that the structure of the rock appears to be uniform. Sedimentary rocks do not have a crystalline structure, but rather granular (detrital). In sedimentary rocks, grains can be seen through a magnifying glass.
2. 2 Note the layering of the metamorphic rocks. In igneous rocks, bedding is absent. In some common metamorphic rocks, bedding is absent, for example, marble, consisting of calcite, and quartzite, consisting of grains of quartz. Magpathic rocks will never be composed of calcite or quartz grains alone.

Method 2 of 2: Identifying Igneous Rocks

1. 1 Classify igneous rocks into seam of the main types: volcanic, or effusive, which form when lava, dust and ash flow or explode from a volcano, and intrusive, or deep, which form when magma or molten rock cools and solidifies beneath the earth's crust.
   • Classify igneous rock into one of two categories: rock formed from flows of molten rock (lava); and tephra, or pyroclastic material, formed when ash and dust are thrown or exploded out of a volcano and deposited on the ground.
2. 2 Identify the type of igneous rock — pegmatite, plywood, aphanite, porphyrite, vitreous, vesicular, or pyroclastic — by crystal size or texture. Igneous rocks with large crystals formed more slowly beneath the earth's surface. Rocks with smaller crystals formed faster after the lava erupted and cooled. The vitreous igneous rocks formed so quickly that the crystals did not have time to form. Large crystals are visible to the naked eye. Small crystals are not.
   • Pegmatite igneous rocks have very large crystals, more than 2.5 cm.
   • Fanerite igneous rocks are composed of intertwined crystals, smaller than crystals in pegmatite rocks, but still visible.
   • Aphanite igneous rocks are fine-grained, with most of their crystals too small to be seen.
   • Porphyritic igneous rocks have two different crystal sizes.
   • Igneous rocks that formed too quickly for crystals to form in them have a so-called glassy texture. This means that the rock cooled too quickly and could not form crystals. It has a disordered structure. Obsidian is the only vitreous igneous rock that can be distinguished by its dark color (although it is transparent in thin layers).
   • Vesicular igneous rocks, such as pumice, appear bubbly and form before gases escape from the solidifying lava. They also form upon very rapid cooling.
   • Pyroclastic igneous rocks have a texture formed by volcanic fragments ranging in size from very fine (ash) to coarse (lapilli) or very coarse (bombs or clasts).

Warnings

• In some countries, such as the United States, it is prohibited to take mineral or biological samples from national parks without permission. Contact National, State and Local Reserves to find out if you can take samples with you

What do you need

• Magnifying glass