How Does a Rock Crystallize from a Melt? Si-O Tetrahedra Form First

Home , Igneous textures, Porphyritic

1 Rock types and the rock cycle 2 Rock types and the rock cycle 3 Rock types and the rock cycle 4 Rock types and the rock cycle 5 Characteristics of magma • Parent material of igneous rocks is magma

6 Characteristics of magma • Parent material of igneous rocks is magma • Forms from partial melting of rocks inside the Earth

7 Characteristics of magma • Parent material of igneous rocks is magma • Forms from partial melting of rocks inside the Earth • Magma that reaches the surface is called lava

8 Magma to lava 9 How are magmas produced? 10 How are magmas produced? 11 How are magmas produced? 12 How are magmas produced? 13 How are magmas produced? 14 How are magmas produced? 15 How are magmas produced? 16 How are magmas produced? 17 The nature of magma Consists of three components: Melt _ liquid _ mobile ions

Solids - silicate minerals (mostly)

Volatiles - dissolved gases -

H2O, (CO2), (SO2) 18 How does a rock crystallize from a melt? Si-O tetrahedra form first

19 How does a rock crystallize from a melt? Si-O tetrahedra form first Tetrahedra connect with each other and other ions 20 How does a rock crystallize from a melt? Si-O tetrahedra form first Tetrahedra connect with each other and other ions Minerals form in a specific order 21 How are igneous rocks classified?

1)Texture

2) Chemistry

22 Extrusive versus intrusive rocks Rocks formed from lava at the surface are extrusive, or volcanic rocks 23 Extrusive versus intrusive rocks Rocks formed from lava at the surface are extrusive, or volcanic rocks Rocks formed from magma that crystallizes at depth are intrusive, or plutonic rocks 24 Question: If intrusive rocks form at depth, how do we know about them? 25 Answer: Intrusive igneous rocks are uplifted by tectonic forces and stripped of the overburden by erosion 26 Outcrops of intrusive igneous rocks 27 Rate of Cooling 28 Rate of Cooling Slow rate promotes growth of fewer but larger crystals

Fast rate promotes growth of many, small crystals 29 Types of igneous textures Phaneritic

– coarse-grained = coarsely crystalline – Crystals can be identified without a microscope – Slowly crystallized

30 Types of igneous textures Aphanitic

fine-grained = finely crystalline – Microscopic crystals – Crystals formed in a short time – May contain vesicles (holes from gas bubbles)

31 Texture = coarsely crystalline (phaneritic) 32 Phaneritic texture

33 Aphanitic texture 34 Vesicular basalt

35 Texture in a lava that cools extremely quickly? 36 Glassy texture (obsidian) 37 Types of igneous textures Glassy texture

– Very rapid cooling – obsidian 38 Pumice (frothy glass) 39 Types of igneous textures Porphyritic

Large crystals – phenocrysts - embedded in a matrix of smaller crystals - the groundmass 40 Porphyritic andesite (andesite porphyry) 41 Porphyritic andesite 42 Porphyritic texture 43 Types of igneous textures Pyroclastic

– Various fragments ejected during a violent volcanic eruption

– Textures often appear more similar to sedimentary rocks 44 Types of igneous textures Pegmatitic

Exceptionally coarse grained (coarsely crystalline)

Form in late stages of crystallization of granitic magmas 45 How are igneous rocks classified? 1) Texture

2) Amount of silica (SiO2) in the whole rock 46 47 Rhyolite (granitic/felsic composition) 48 Diorite (andesitic/intermediate) 49 Gabbro (mafic/basaltic)

50 51 Relationships among magma types / appearances and volcanic eruption / landform types