Rocks

What is a Rock? Rocks are known as an aggregate of minerals. In other words, they are made from minerals and most rocks contain several different minerals.

Types of Rocks Igneous Rocks Metamorphic Rocks Sedimentary Rocks

Igneous Rocks Igneous rocks are formed by the cooling and solidification of hot molten rock from inside the Earth. The hot molten rock is known as magma. Igneous rocks can either form under Earth’s surface or at Earth’s surface.

Intrusive (Plutonic) Igneous Rocks Intrusive (plutonic) rocks form UNDER Earth’s surface. (INSIDE = INTRUSIVE) Since they form underground, where it is quite warm, they cool very SLOWLY!! These are very COARSE rocks (can see big grains/crystals) because they cooled so slow. Therefore BIG CRYSTALS = SLOW COOLING RATE!!!!

Examples of Intrusive Igneous Rocks Granite – makes up continental crust Pegmatite Gabbro

Extrusive (Volcanic) Igneous Rocks Extrusive (volcanic) rocks form AT or ABOVE Earth’s surface. (EXTRUSIVE = EXIT) When magma makes it to Earth’s surface, usually through a volcano, it becomes known as lava. Since they form at Earth’s surface, where it’s cooler, they cool very QUICKLY!!

Extrusive (Volcanic) Igneous Rocks, Cont. These are very FINE, GLASSY, or VESICULAR (holes) rocks (cannot see the grains/crystals, they look like glass, or they have air pockets) because they cooled so fast. Therefore SMALL CRYSTALS OR NO CRYSTALS = FAST COOLING RATE!!

Examples of Extrusive Igneous Rocks Obsidian Conchoidal Fracture! Air pockets, like those found in Scoria and Pumice are a sign of super-fast cooling. Scoria Basalt – makes up ocean crust (oceans are salty = basalt)

Igneous Rock Formation

Types of Magma There are 2 general types of magma: Felsic Magma – has a lot of feldspar and silica in it. These igneous rocks are light in color, low in density, and are known to have a felsic composition (they are high in aluminum). Igneous rock examples: Rhyolite, Granite, Pegmatite Minerals in felsic rocks – feldspars, quartz

Types of Magma Mafic Magma – has a lot of magnesium and iron (Fe) in it. These igneous rocks are dark in color, high in density, and known to have a mafic composition (they are high in magnesium and iron) Igneous rock examples: Basalt, Gabbro, Peridotite Minerals in mafic rocks – amphibole, pyroxene, olivine, biotite mica

CONFUSED YET??? GUESS WHAT???!!! Everything you just learned is free of charge on page 6 of your ESRT!! So, what are you waiting for? Get it out, turn to page 6 and let’s start learning how to find all that information without using your notes!

Sedimentary Rocks

Sedimentary Rocks Sedimentary rocks form from various types of sediment. Sedimentary rocks make up 90% of SURFACE rocks and only about 1% of Earth’s crust. Which rock is it that makes up Earth’s continental crust? ________________ Which rock is it that makes up Earth’s oceanic crust? _____________________ Both of those are Igneous rocks. Granite Basalt

Three Kinds of Sedimentary Rocks 1. Clastic – fragments of other rocks. For example: shale, sandstone, conglomerate 2. Crystalline – formed from mineral grains that fall out of solution by chemical action or evaporation. For example: Rock salt and chemical limestone. 3. Bioclastic – formed from remains of plants or animals. For example: Coal and limestone from shells (coquina).

Clastic Sedimentary Rocks Sediment is deposited and then goes through the process of lithification (forming sediment into sedimentary rocks) Clastic sedimentary rocks are characterized based on their grain size. Grain sizes vary from Coarse (pebbles, cobbles, boulders) to Medium (sand) to Fine (silt and clay)

How Does The Sediment Get Sorted Into Different Sizes? Running water (rivers, streams, creeks) pick up sediment and carry it along its travels. The faster the water is moving, the larger the particle that can be carried, and vice-versa. When a river hits a larger body of water, like a lake or ocean, it’s like hitting a brick wall. This results in the river “dumping its load” – dropping all the sediment it’s been carrying. The sediment gets dumped according to size!! As sediment travels in the stream, what happens to it’s size + shape?

How Does The Sediment Get Sorted Into Different Sizes? The bigger sediment gets deposited FIRST, since it’s bigger, heavier and sinks first. Then, the sediment gets gradually SMALLER the farther you go from where the river hits the lake or ocean. Therefore, coarse clastic rocks get made where the larger sediment gets deposited and the medium to fine clastic rocks get made further away from that point.

Clastic Sedimentary Rocks Conglomerate – made of pebbles and/or cobbles cemented together by sand, silt and clay. Coarse-grained Sandstone – sand that has been cemented together. Medium-grained Shale – made from clay. Fine-grained

Crystalline Sedimentary Rocks Also called chemical sedimentary rocks. Chemical action or evaporation causes minerals to drop out of solution. This process creates chemical or crystalline sedimentary rocks. For example: Calcite will precipitate out of water and build-up to form chemical limestone (it will fizz with acid since it has calcite in it!!!)

Crystalline (Chemical) Sedimentary Rocks Gypsum Rock – made when the mineral gypsum precipitates out of water. Chemical Limestone

What happens when salty water evaporates and just salt is left behind? Bonneville Salt Flats of the Great Salt Lake, Utah. The lake bed is covered with rock salt which gives it the white color. The salt is mined by the Morton Salt Company.

Dead Sea, Israel This sea is 10 times saltier than the oceans! (this is at the surface!) At the bottom, the water is saturated!

Bioclastic (Organic) Sedimentary Rocks Also called Organic sedimentary rocks. Formed from once-living things like plants and animals. For example: Plants die and decay and form coal. Another example: Shells can get broken up and cemented back together to form organic limestone (also called coquina).

Bioclastic (Organic) Sedimentary Rocks Coal Organic Limestone or Coquina

Metamorphic Rocks

Metamorphic Rocks Metamorphic rocks are igneous, sedimentary, or other metamorphic rocks that have undergone extreme heat and/or pressure. The extreme heat and/or pressure has caused the original rock to change form (metamorphosis). The “new” rock is usually much different from its original form.

Types of Metamorphism There are 2 types of metamorphism: 1. Regional Metamorphism 2. Contact Metamorphism

Regional Metamorphism Occurs when large areas of rock are under intense heat AND pressure. Most often happens during mountain building events (orogeny). It is important to keep in mind that the heat and pressure are VERY intense and this happens over VERY LARGE areas.

Regional Metamorphism of Shale As you know, shale is a sedimentary rock. During metamorphism, the pressure squeezes the mineral crystals closer together. Heat then rearranges the mineral crystals. The first level of metamorphism of shale results in the metamorphic rock slate. This “new” rock slate shows low-grade foliation – layers.

After heat and pressure, mineral crystals are aligned into layers - FOLIATION Before heat and pressure is applied – mineral crystals are random

Regional Metamorphism of Shale If further heat and pressure is applied to slate, the second level of metamorphism will result in the rock phyllite. Phyllite is shiny and has medium foliation. If more heat and pressure are applied to phyllite it will result in a third level of metamorphism and the rock schist. Schist has medium to high foliation and is very shiny.

Regional Metamorphism of Shale Finally, if more heat and pressure is applied to schist, it results in the highest grade of metamorphism and the rock gneiss. Gneiss shows mineral banding (high foliation). So much heat and pressure has been applied that the minerals have aligned themselves into bands of colors.

Phyllite (low to medium grade metamorphism) Shale (Sedimentary rock) Slate (low grade metamorphism) Gneiss (very high grade metamorphism – shows banding) Schist (medium to high grade metamporphism)

Contact Metamorphism Contact metamorphism occurs on a much smaller level and ONLY includes heat! NO PRESSURE. This happens when hot magma comes in direct contact with existing rock and the heat changes the rocks form. Contact metamorphism is not strong enough to create foliation.

Regional or Contact Metamorphism Rocks Marble Quartzite Metaconglomerate Serpentine