Rocks A. Igneous Rocks

WISK: How are igneous rocks formed? What texture(s) would an extrusive igneous rock have? What texture(s) would an intrusive igneous rock have?

Plutonic Igneous Rocks: forms underground magma never reaches the surface cools slowly form large crystals medium to coarse grained texture Get exposed at the surface only when rock is uplifted and exposed

Volcanic Igneous Rocks: Forms on the surface Lava comes out of a volcano cools rapidly forms small crystals glassy or fine-grained Covers the surface as lava flows or pyroclastic debris

Classification of Igneous Rocks: They are classified according to their chemical composition Felsic Rich in silica( >65%) Intermediate (53-65% silica) Mafic Poor in silica (45-52%) Ultramafic Very poor in silica (< 45%) The amount of silica affects the color of the rock More silica the lighter the rock, less silica the darker

Mineral Composition

Igneous Rock Composition

Silica content and Viscosity Viscosity is a fluids resistance to flow low viscosity = flows easily high viscosity = doesn’t flow easily, thick pasty mass High silica content (felsic) = high viscosity explosive eruptions: pyroclastic Low silica content (mafic) = low viscosity less violent eruptions: lava flows E.g. Hawaii: pahoehoe and aa lava

Basaltic lava in Hawaii Top: pahoehoe Highly fluid lava Low viscosity Left: aa Slow moving lava High viscosity Right: pahoehoe

Highly viscous lava tends to block the vents of volcanoes. This causes pressure to build up. Which in turn released by a violent explosion

While lava that has a low viscosity tends to flow away from the vent and this leads to less violet eruptions e.g. Surtsey Island

Vesicles As lava rises and cools to form volcanic igneous rock, Pressure drops which causes the gases that are in the lava to expand. These gases form vesicles (cavities) which are the remains of gas bubbles. Plutonic rocks rarely contain vesicles because they are formed at depth where high pressure stops the gases from expanding.

Crystallization: Magma is a mixture of different substances. They all have different melting points. So as a magma cools, some substances will form crystals be for other substances. They have a head start in growth and tend to be bigger. If the magma already has crystal present in it and then is cooled rapidly then you will see large crystals embedded in finer-grained material. This is called a porphyritic texture

Bowen’s Series This shows the order of crystallization of minerals in magma & lava

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Igneous Rocks What is the difference between magma and lava? What are the two main types of igneous rocks? How can you tell them apart? Why is it not common for plutonic rocks to have vesicles? Vocabulary: magma, lava, intrusive, extrusive, plutonic, volcanic, pyroclastic, vesicles, felsic, mafic, Bowen’s series H/W questions on page 53: 1, 2, 3,11, 12, 15, 22