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Geology - Igneous Rocks

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    Study Guide Geology – Igneous Rocks 1 . Extrusive Rock Types Extrusive rocks form when lava erupts onto Earth’s surface and cools quickly. The type of rock that forms depends mainly on the chemical makeup of the magma , especially how much silica it contains. Silica strongly affects a lava’s color, thickness (viscosity), and mineral content. 1.1 Basalt, Andesite, and Rhyolite These three rocks are the most common extrusive igneous rocks , and they differ mainly in silica content and mineral makeup. Basalt • Usually the first lava to erupt • Contains many ferromagnesian minerals (rich in iron and magnesium)

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    Study Guide • Has about 25 – 50% silica • Dark green, gray, or black • Very common on the ocean floor Andesite • Has more silica than basalt • Contains more plagioclase feldspar and fewer ferromagnesian minerals • Light greenish - gray in color • Common in volcanic arcs Rhyolite • The most silica - rich extrusive rock • Contains 65% or more silica , mostly in feldspar and quartz • Very few ferromagnesian minerals • Light - colored: tan, pink, or cream Dacite • Composition falls between andesite and rhyolite • Slightly less potassium feldspar and quartz than rhyolite • Slightly more ferromagnesian minerals • Light grayish - green • Often hard to tell apart from rhyolite in the field

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    Study Guide 1.2 Ma fi c, Intermediate, and Felsic Extrusive Rocks Geologists often group extrusive rocks using broader chemical terms: Mafic rocks • About 50% silica • High in iron, magnesium, and calcium • Dark - colored • Example: basalt Felsic rocks • High in silica, potassium, sodium, and aluminum • Low in iron, magnesium, and calcium • Light - colored • Examples: rhyolite and dacite • Most viscous magmas due to high silica Intermediate rocks • Chemical composition between mafic and felsic • Example: andesite 1.3 Ultrama fi c Extrusive Rocks A much rarer group of extrusive rocks is the ultramafic category.

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    Study Guide • Made almost entirely of ferromagnesian minerals • Contain less than 45% silica • No feldspar or quartz • Thought to originate deep in the mantle • Have very low viscosity , so they flow easily A common example is komatiite , which is made mostly of olivine and pyroxene , with smaller amounts of feldspar. 2. Rock Textures A rock’s texture gives important clues about how it formed. Texture refers to the size, shape, and arrangement of mineral crystals or rock fragments within the rock. By carefully examining texture, geologists can often tell whether a rock formed from lava, explosive eruptions, or other processes. 2.1 Grain Size and Cooling Rate Most extrusive rocks cool quickly at Earth’s surface, so they are usually fine - grained . This means their mineral grains are smaller than 1 millimeter and are often too small to see without a microscope. Lava flows often show a pattern related to cooling: • The outer edge of a lava flow cools very fast, forming a chilled margin with an extremely fine - grained (aphanitic) texture. • Moving toward the center of the flow, cooling is slower, so grain size gradually increases. • Thick lava flows may develop medium - to coarse - grained centers .

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    Study Guide 2.2 Porphyri Ɵ c and Glassy Textures A porphyritic texture contains large crystals , called phenocrysts , surrounded by a much finer - grained background called the groundmass . • The large crystals formed earlier , while the magma was still underground. • The remaining magma then erupted and cooled quickly, forming the fine - grained matrix. Obsidian , also known as volcanic glass, forms when lava cools so rapidly that crystals do not have time to grow. It is: • Very fine - grained • Hard and glassy • Composed mainly of silica 2.3 Lava Flow Surface Textures Basaltic lava flows can develop distinctive surface textures: • Pahoehoe (pa - hoy - hoy): ○ Has a smooth, ropy surface ○ Forms when the outer surface cools quickly while molten lava continues flowing underneath • Aa (ah - ah): ○ Has a rough, jagged, rubbly surface ○ Forms when a partially solidified crust breaks apart as the lava keeps moving forward 2.4 Vesicular Rocks and Gas Bubbles Magmas deep underground often contain dissolved gases because of high pressure. When magma erupts at the surface, pressure drops suddenly, allowing gas to escape.

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    Study Guide This creates vesicles , which are holes or cavities left behind by gas bubbles. • Pumice ○ Extremely vesicular ○ So full of gas bubbles that it can float on water • Scoria ○ A vesicular basalt ○ Dark - colored, rough, and full of holes ○ Contains more gas space than solid rock 2.5 Pyroclas Ɵ c and Fragmental Textures Explosive volcanic eruptions eject pyroclastic material , which includes fragments of lava, ash, and crystals. When this material becomes compacted and hardened, it forms rocks with fragmental textures . Pyroclastic material is classified by size: • Dust and ash – very fine - grained • Cinders – about pea - sized • Lapilli – about walnut - sized • Bombs and blocks – can be 1 meter across or larger • Blocks are solid fragments of hardened lava. • Bombs are blobs of molten lava that harden while flying through the air. Small crystals — often feldspars — that formed before eruption may also be mixed into these deposits. • Tuff – made of fine - grained pyroclastic material (for example, ash tuff or crystal tuff)

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    Study Guide • Welded tuff – ash and glass fragments were still hot enough to fuse together • Tuff breccia (agglomerate) – contains large volcanic bombs or blocks 2.6 Special Extrusive Textures Some extrusive rocks show textures linked to where and how they cooled: • Columnar jointing ○ Found in flood basalts ○ Cooling and contraction form vertical, parallel columns — often six - sided • Pillow structures ○ Form in submarine lava flows ○ Rounded blobs of lava harden instantly in cold seawater ○ Help geologists identify rocks formed on the ocean floor and determine which way was “up” in the original flow

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