Rocks

What is a Rock? Naturally occurring solid aggregate of minerals Earth’s outer solid layer, the lithosphere, is made of rock Often covered by soil or water

What is a Rock? Three types – classified by the processes that made them Type changes! This change is diagramed by the “rock cycle”

Types of Rocks Igneous– form from solidification of molten (liquid) rock material. Sedimentary– form from compaction and cementation of sediments Metamorphic– form from re-crystallization of existing rock material.

What would cause a rock to melt!?! Igneous Rocks Rocks formed by solidification from a melt What would cause a rock to melt!?! Lava- Magma that reaches the surface (extrusive) Magma- molten rock beneath the Earth’s surface (intrusive)

Extrusive Igneous - rocks formed from cooled lava Extrusive Igneous - rocks formed from cooled lava. Fine- grained, cooled quickly on the Earth’s surface. Obsidian Intrusive Igneous-rocks formed from cooled magma. Coarse-grained, cooled slowly beneath Earth’s surface. Granite

How Igneous Rocks Melt Different rocks have different melting points Magma often a slushy mix of molten rock, gasses, and minerals – partial melting Mix depends on temperature, pressure, water content and mineral composition Closer to Earth’s core – hotter it gets (1500 C) Approx Temp (C) Molten Minerals 600 Quartz, micas 800 Ca/Na 1000 Pyroxene 1200 All molten http://www.learner.org/interactives/volcanoes/activty1/tempmain.html

Extrusive Igneous Rock - Lava (Hawaii)

Extrusive Igneous Rock - Lava (Hawaii)

Extrusive Igneous Devil’s Tower, Wyoming Los Tercios Waterfall, Suchitoto, El Salvador Devil’s Tower, Wyoming

Intrusive Igneous Rock (Granite) – This granite cooled 30 kilometers under the surface

Coarse texture Glassy Texture Texture- the “feel” of a rock due to the grain size, shape, and arrangement of mineral crystals or sediments in a rock Coarse texture Glassy Texture

Grain Size- the diameter of individual grains of sediment Grain Size- the diameter of individual grains of sediment. Geologists often make thin sections – very thin slices that light can pass through – to view grain size. Thin sections under a microscope Thin section to examine grain size

Grain Size Fine grained – individual mineral grains are too small to be seen without a microscope. Coarse grained – can see individual grains.

Igneous textures Factors affecting crystal size Rate of cooling Fast rate forms many small crystals (extrusive) Very fast rate forms glass Slow rate forms large crystals (intrusive) Amount of silica (SiO2) present Amount of dissolved gases

Small crystals (fast cooling)

Large crystals (slow cooling)

High silica, warm, viscous Fine crystals Need a microscope Low silica, HOT, fluid Intermediate High silica, warm, viscous Coarse crystals Easily seen

Some Common Rock Descriptors Felsic: silicate minerals, magma, and rocks which have the lighter elements such as silicon and oxygen. Usually light in color. Quartz and Granite Mafic: silicate mineral or rock rich in magnesium and iron. Usually dark in color. Basalt.

Origin of Granitic Rocks Huge blobs w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths Can also get subduction-generated granites folded in collisions, or from deep rock burial