Geology 5 (Igneous Rocks)
TLDRThe video explores igneous rocks, which form from cooled magma or lava. It covers textures like glassy, fine-grained, and coarse-grained, explaining how cooling rate affects crystal size. It then discusses compositions, explaining the differences between felsic rocks like granite and rhyolite versus mafic rocks like basalt and gabbro. The goal is to get viewers excited to learn more about volcanoes, as understanding igneous rocks is key for anyone studying volcanology.
Takeaways
- 😀 Igneous rocks form from the cooling and crystallization of molten rock or magma
- 🔥 Magma is composed of melt, crystals, and volatiles (dissolved gases)
- 🌋 The texture of igneous rocks depends on cooling rate, silica content, and dissolved gases
- 💎 Slow cooling produces fewer, larger crystals (phaneritic texture)
- ⚡ Fast cooling produces many tiny crystals (aphanitic) or glass
- 🖼 Porphyritic texture has large phenocrysts in finer grained groundmass
- 💨 Pyroclastic textures have volcanic rock fragments with vesicles from gases
- 🏔 Felsic magma like granite is viscous; mafic magma like basalt flows readily
- 👀 Granite and rhyolite have the same chemistry but different crystal sizes
- 🍫 The mafic minerals in basalt and gabbro determine their characteristic dark color
Q & A
What is the definition of an igneous rock?
-An igneous rock is one that forms from the cooling and crystallization of molten rock or magma. The word 'igneous' comes from the Latin word 'ignis' meaning fire.
What are the three main components that make up magma?
-The three main components of magma are: 1) Melt - the liquid portion composed of mobile ions, 2) Solids - any solid silicate minerals that have already crystallized, and 3) Volatiles - gases like water vapor, carbon dioxide, and sulfur dioxide dissolved in the melt.
What causes the different crystal sizes in igneous rocks?
-The main factors affecting crystal size are: 1) Rate of cooling - slower cooling allows larger crystals to form, 2) Amount of silica - more silica leads to stickier magma and smaller crystals, 3) Amount of dissolved gases - can inhibit crystal growth.
What is the difference between extrusive and intrusive igneous rocks?
-Extrusive or volcanic rocks form from lava erupting and cooling on the Earth's surface. Intrusive or plutonic rocks form from magma cooling at depth within the crust without reaching the surface.
What causes porphyritic textures in igneous rocks?
-Porphyritic textures form when some minerals solidify early while the magma is still molten, forming large crystals (phenocrysts) surrounded by a matrix of smaller crystals (groundmass) that crystallize later as the magma continues to cool.
Why do mafic magmas typically erupt more violently than felsic magmas?
-Mafic magmas like basalt have lower silica, are less viscous, can flow more easily, and allow gases to escape steadily. Felsic magmas like rhyolite are silica-rich, more viscous, and trap gases, building pressure that leads to explosive eruptions.
What causes the differences between obsidian, pumice, and granite?
-Obsidian, pumice, and granite all share the same felsic, granitic composition but cool at different rates - obsidian is volcanic glass that cooled too fast for crystals to form, pumice cooled fast but had gases that left holes and made it lightweight, while granite cooled slowly underground allowing large visible crystals to grow.
Why do mafic magmas require higher temperatures to melt than felsic magmas?
-Mafic rocks have less silicon dioxide, higher iron/magnesium content. This bonds the minerals more tightly, requiring temperatures about 600°C higher to start melting than felsic rocks.
What are the main differences between basalt and gabbro?
-Basalt and gabbro have the same chemical composition but gabbro cooled slowly underground forming coarse-grained crystals while basalt erupted on the surface, cooling rapidly with microscopic crystals.
How can you identify quartz and potassium feldspar in a granite sample?
-Quartz appears as small gray grains without cleavage planes. Potassium feldspar often appears pink or orange-pink in color and has good cleavage in two directions at 90° angles.
Outlines
😃 Introducing Igneous Rocks and Their Characteristics
Paragraph 1 introduces igneous rocks, which form from the crystallization of molten magma. It explains that igneous rocks provide insights into volcanoes and the earth's interior. The key characteristics like texture, composition, and cooling rate are discussed.
🤔 Factors Affecting Magma Crystallization and Igneous Rock Textures
Paragraph 2 explains how factors like cooling rate, silica content, and volatiles affect the texture and crystal size in igneous rocks. Rapid cooling produces glass or fine grains, while slow cooling allows large crystals to grow.
📉 Igneous Rock Classification Based on Mineral Composition
Paragraph 3 discusses how igneous rocks are classified into granitic, basaltic, and ultramafic based on their mineral composition. Granitic rocks are SiO2-rich while mafic rocks have more Fe, Mg, and Ca.
🔬 Microscopic Crystal Size Determines Extrusive vs. Intrusive Rocks
Paragraph 4 explains how the microscopic crystal size distinguishes extrusive volcanic rocks from intrusive plutonic rocks of the same composition. Extrusive rocks cool rapidly on the surface while intrusive rocks cool slowly underground.
🌋 Violent Pyroclastic Eruptions Produce Igneous Rock Fragments
Paragraph 5 describes pyroclastic igneous rocks like pumice and scoria that form from violent volcanic eruptions. They contain gas vesicles and may resemble sedimentary rocks.
🔮 Crystal Formation in Obsidian, Pumice and Granite
Paragraph 6 contrasts obsidian, pumice and granite, which have identical chemistry but different crystal sizes or lack of crystals due to varied cooling rates.
🌡️ Factors Controlling Crystal Size in Igneous Rocks
Paragraph 7 summarizes how cooling rate, silica content and volatiles affect crystal size and texture in igneous rocks.
🏔️ Rock Suite: Granite, Rhyolite, Obsidian, Andesite and Basalt
Paragraph 8 examines examples of granitic, andesitic and basaltic igneous rocks, comparing intrusive/extrusive pairs and porphyritic textures.
Mindmap
Keywords
💡igneous
💡magma
💡lava
💡texture
💡aphanitic
💡phaneritic
💡porphyritic
💡silica
💡mafic
💡volcano
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