Welcome to Earth Science 2017-2018

Igneous Rocks Chapter 5

EES News and Notes We’re done with the first quarter (YAY!) Our next quiz will be over minerals (CH 4) and it will take place on Friday, November 3. A new quarter means a fresh start, so please be sure that you are taking notes, studying notes and asking questions to ensure you are understanding everything!

#Volcanoes #Magma2Lava What Are Igneous Rocks? #Volcanoes #Magma2Lava

Igneous Rock Formation Igneous rocks form when lava or magma cools and crystallizes. Magma: molten rock below the Earth’s surface. Lava: magma that flows out onto Earth’s surface.

What do these numbers mean? 800°C – 1200°C This is the temperature of the upper mantle and lower crust. Where do we get this intense heat? Theories include remaining energy from Earth’s molten formation and decay of radioactive elements.

Composition of Magma Group Silica Content Example Location Basaltic 45 – 52% Hawaiian Island Andesitic 52 – 66% Cascade Mountains, Andes Mountains Rhyolitic More than 66% Yellowstone National Park

Composition of Magma Qs What were the most common elements in the Earth’s crust? Well, the composition of magma contains similar instances of the same materials. Silica content affects melting temperature and viscosity! Oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K) and sodium (Na). Viscosity = resistance to flow (milk shake has higher viscosity than water)

How does Magma form? Four main factors: Temperature Pressure Water content Mineral content

Mineral Content Different minerals have different melting points. How might this affect the ability of rocks to melt? Granite has more water and silicon = melting at lower temperatures. Rocks rich in Fe and Mg = melt at higher temperatures.

Partial Melting Not all minerals in rocks have the same melting point. Partial Melting: Process where some minerals melt at lower temperatures while others melt at higher temperatures. If some of a rock melts but the other part does not, elements can get mixed in and make a whole new composition of rock!

Bowen’s Reaction Series Canadian scientist that demonstrated that as magma cools and crystallizes, minerals for in predictable patterns. Two main branches: Right Branch: Continuous, gradual change of mineral compositions in feldspars. Left Branch: Abrupt changes of mineral type in the iron-magnesium group characteristics.

Calcium rich Sodium rich

Iron-rich Minerals Left branch of Bowen’s reaction series Abrupt changes (quick). EX: Magma is rich in Fe and Mg as it begins to cool. It turns into Olivine, then keeps cooling and converts into pyroxene, then to amphibole and, eventually, biotite mica.

Feldspars Right branch of Bowen’s reaction series. Continuous change of composition (smooth). Cooling starts with feldspars that are rich in calcium, and as they continue to cool, rich in sodium.

Fractional Crystallization Process by which the first minerals that crystalized during cooling are the last to melt. Reverse order of partial melting If the composition of the magma changes, the remaining magma becomes a concentrated silica.

Fractional Crystallization If olivine converts to pyroxene during cooling, why is olivine present in some rocks? Crystals (solids) settle out in the bottom of a magma body when squeezed through tight spaces. As more crystals are separated, the magma becomes more concentrated in silica, aluminum and potassium. Page 116 has an awesome illustration of this!

What do you notice is changing?

Classification of Igneous Rocks Think granite: it’s all over the place!

Mineral Composition of Igneous Rock When rocks cool, they can either cool slowly inside the crust or rapidly outside of the crust. Intrusive rocks are rocks from magma that has cooled below the Earth’s surface. Extrusive rocks are rocks from magma that has cooled atop the Earth’s surface.

Mineral Composition of Igneous Rocks Igneous rocks are classified according to their mineral compositions. Basaltic rocks (mafic rocks) are dark colored, have lower silica content and contain mostly plagioclase and pyroxene. Granitic rocks (felsic rocks) are light colored, have high silica content and contain mostly quartz and feldspar.

Basaltic vs. Granitic

Texture The size, shape and distribution of the crystals or grains that make up a rock. Rhyolite, a fine-grained igneous rock, is extrusive. Granite, a coarse-grained igneous rock, is intrusive.

Crystal Size and Cooling Rates Extrusive rocks (on the surface) cool quickly and create small sized crystals. Sometimes, the cooling is so fast, no crystals form, resulting in volcanic glass (obsidian). Intrusive rocks (below the surface) cool slowly, creating crystals that are larger than 1 cm on average.

Porphyritic Rocks Rocks with a texture that is characterized by large, well- formed crystals surrounded by finer-grained crystals of the same minerals or different minerals. How does this happen? Slow cool under the crust forms big crystals  launch upward rapidly and cool remaining crystals quickly!

FOOD SIMILARITIES! To help remember porphyritic rocks, think of the texture of chocolate chip cookie dough: What similarities do you notice between this texture and the texture of the rocks?

Vesicular Rocks Spongy appearance in rocks is referred to as vesicular texture. This occurs when trapped gasses in magma escape due to the lessening of the surrounding pressure. If the lava is thick enough to prevent escape, the holes left behind are called vesicles. vesicles

Thin Sections To ID minerals, geologists take thin slices of rock (about 0.03 mm thick) to pass light through it. Using a petrographic microscope, mineral grains can be seen and exhibit distinct properties.

Igneous Rocks as Resources Remember, ores are valuable minerals that form in igneous intrusions. When cooling slowly, trace elements of value—gold, silver, copper, etc.—can cool in the cracks of the surrounding rock. The results is metal-rich veins, such as the gold veins to the right.

Pegmatites Vein deposits can contain other valuable minerals besides metals. Ores of rare elements, like lithium and beryllium, often form in pegmatites. Famous example: pegmatites that are on T. Jefferson’s face at Mount Rushmore National Park!

Kimberlites Diamond, a rare mineral, is found in rare, ‘ultrabasic’ rocks known as kimberlites. Most likely form at the mantle at depths of 150-300 km. Must form under intense pressure. These structures extend many km into the crust’s depth, but are only 100-300 km in diameter.

Igneous Rocks in construction Interlocking grain textures make igneous rocks incredibly strong. Many materials in igneous rock are resistant to weathering. Granite is also very beautiful, which is why it makes such great countertops, floors and statues!