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Rocks Building Blocks of the Earth’s Crust Earth’s Layers Chemical Composition of the Earth Element Iron Oxygen Silicon Magnesium Nickel Sulfur Calcium Aluminum Symbol Fe O Si Mg Ni S Ca AL Total Percent 34.6% 29.5% 15.2% 12.7% 2.4% 1.9% 1.1% 1.1% 98.5% Chemical Composition of the Earth’s Crust Element Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium Symbol O Si Al Fe Ca Na K Mg Total Percent 46.6% 27.7% 8.1% 6.0% 3.6% 2.8% 2.6% 2.1% 98.5% Where the Rocks Are … The Crust What Is a Rock? A rock is a mixture of minerals! What is a mineral? • naturally occurring (not man-made) • inorganic (not part of or a product of a (once) living organism ) • solid (not made of liquid or gas) • chemical compound • with a definite chemical structure which gives it unique physical properties. Minerals are the building blocks of rocks. Minerals • minerals are compounds and compounds are composed of elements • What are the most common elements in the crust? Oxygen (O) and Silicon (Si) • the most common minerals are silicates (contain silicon and oxygen) – the most common silicate – quartz Physical Properties of Minerals • Hardness – what minerals can scratch it or be scratched by it • Cleavage – how does it break … along planes? • Streak – what does the powdered form look like • Color – what colors are common • Density – the mass of a given volume Cleavage Calcite Halite Rock Classification Rocks are divided into three major groups based on the methods of formation 1. Igneous rocks … from cooling lava or magma 2. Sedimentary rocks … from compacted sediments 3. Metamorphic rocks … meta (change) morphic (form) … rocks changed by extreme forces Rocks Classification • Rocks are further subdivided by –mineral composition • what minerals are present in large quantities – texture • what is the size of the mineral crystals (grains ) Igneous Rocks Igneous Rocks Formed when molten rock cools and solidifies Lava Magma geollab.jmu.edu/Fichter/ IgnRx/Intrusiv.html Volcano Explosive vs Non-explosive Magma vs. Lava Magma Lava • intrusive – below the surface • gathers in large bodies called plutons • may become lava • cools slowly – forms large crystals • extrusive above the surface • expelled from a fissure or a volcano • begins as magma • cools rapidly – forms small or no crystals Mineral Composition of Igneous Rocks • • • • Rich in silicates lots of feldspar, quartz, and other silicates lighter color more viscous – extrusives form steep volcanoes continental crust • • • • Poor in silicates lots of iron and magnesium rich minerals darker color less viscous – extrusives form large flows oceanic crust Types of Common Igneous Rocks Extrusive (Volcanic) Intrusive (Plutonic) Rich in Silicates Rhyolite Granite Poor in Silicates Basalt Gabbro Igneous Rocks Rhyolite Granite Gabbro Basalt Other Important Volcanics • Pyroclastics – also called ejecta - thrown from the volcano – cool very quickly Obsidian – volcanic glass, a unique rock containing NO MINERALS Pumice – frothy volcanic glass Sedimentary Rocks Sedimentary Rocks • formed from sediments (particles) that settle in great amounts and are cemented together under great pressure • sedimentary rocks cover some 80% of the Earth's crust • generally, the only type of rock to contain fossils Sedimentary Rock Formation – particles deposited by water, wind, or ice Chemical – particles precipitate from solution Organic – growth in place (eg, coral reefs) Clastic Clastic –basic sedimentary rock. –accumulations of pieces of broken up (eroded) rock (clasts) which have piled up and been "lithified“ (turned to rock) by compaction and cementation Queen’s Throne, Utah Clastic Sediments • formed from the erosion of other rocks • most sediments are carried by rivers and are deposited in great amounts in the ocean • under great pressure, these particles are cemented together • classified by particle size Clastic Particles and the Corresponding Rocks Particle Gravel Sand Silt Clay Rock Conglomerate Sandstone Shale Siltstone Claystone Chemical –some form when water conditions change causing solids to precipitate out of solution –some form when standing water evaporates, leaving dissolved minerals behind Limestone Chemical Sediments Deposit Calcium Carbonate Calcium Magnesium Carbonate Sodium Chloride (Salt) Rock Limestone Dolomite Halite Organic – accumulation of sedimentary debris caused by organic processes – shells, bones, and teeth (calcium) – these bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock – Limestone can form this way; also, lignite, bituminous coal Metamorphic Rocks Metamorphic Rocks • metamorphics get their name from "meta" (change) and "morph" (form) • the rock is moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions • in most cases, this involves burial which leads to a rise in temperature and pressure Metamorphic Rocks • the metamorphic changes in the minerals always move in a direction designed to restore equilibrium • any rock can become a metamorphic rock, including another metamorphic rock (can become a different metamorphic rock) • a metamorphic rock exposed to too much heat will melt and become ??? • common metamorphic rocks include slate, schist, gneiss, and marble Metamorphic Rock - Gneiss www.nearctica.com/geology/ mineral/metamor.htm Types of Metamorphism Regional • areas of great amounts of pressure (during mountain-building) • usually associated also with high temperatures Contact • near magma chambers (plutons) Types of Metamorphism Regional • areas of great amounts of pressure (during mountain-building), usually associated also with high temperatures • foliation of rocks present … pressure causes tabular minerals in the rock to grow parallel to each other and perpendicular to the direction of pressure (stress), generating a mineral alignment termed a foliation • rocks often look folded or wavy Foliation Slate Metamorphic Rock www.aqd.nps.gov/grd/usgsnps/ rxmin/rock3.html Types of Metamorphism Contact • near magma chambers (plutons) • rocks in “contact” or near-contact with the magma are baked • new minerals are formed • rocks do not conduct heat well (they are good insulators) so the zone of contact metamorphism is usually relatively narrow • marble results from the contact metamorphism of limestone Metamorphic Rock Marble The Rock Cycle www.canadianrockhound.com/ junior/rock_cycle.html http://www.ecomlink.org/MAP/Caribbean.gif Likely geological origins of Caribbean islands • Many are extrusive volcanic islands – Eruption eventually above the surface of the water: Montserrat, Mt. Pelee, many others – Eruption below the surface and uplifted: St. John, St. Thomas • Some are flat limestone islands – Aruba, Anegada, Anguilla, Bonaire, Curacao • St. Croix is sedimentary, but has more complex origin • Barbados is an accretionary wedge.
