Chapter 6 IGNEOUS ROCKS IGNEOUS ROCKS Solidi ed molten rock which freezes at high temperature Earth is mostly igneous rock Magma Subsurface melt Lava Melt at the surface Magma erupts via volcanoes Intrusive igneous rocks Cool slowly underground Extrusive igneous rocks Cool quickly at the surface Lava Cooled liquid Pyroclastic debris Cooled fragments Melted rock can cool above or below ground MAGMA COMPOSITIONS Most abundant component is silica Dissolved gases are important 0 2 0 3 by weight Major magma types based on silica SiO2 Felsic Feldspar and silica Ma c Mg and Fe rich 66 to 76 silica 45 to 52 silica SOURCE COMPOSITION Mantle source ultrama c and ma c magmas Crustal source ma c intermediate and felsic magmas Source rock dictates initial magma composition MAGMA FORMATION Partial melting in crust upper mantle Melting is from Decompression Volatile addition Heat transfer DECOMPRESSION Base of the crust is hot enough to melt mantle rock Due to high pressure the rock does not melt A drop in pressure initiates decompressional melting ADDITION OF VOLATILES Volatiles cause rocks to melt at much lower T Water Carbon dioxide Adding volatiles to hot dry rocks initiates meltingHEAT TRANSFER MELTING Rising magma carries mantle heat with it This raises the T in nearby crustal rock which then melts Upon melting rocks rarely dissolve completely instead only a portion of the rock PARTIAL MELTING melts Si rich minerals melt rst Si poor minerals melt last Partial melting then yields a silica rich magma Removing a partial melt from its source creates Felsic magma Ma c residue MAGMA MOVEMENT Magma tends to rise upward because it is less dense than rock Overlying rock pushes down squeezing magma upwards Speed of magma movement governed by viscosity VISCOSITY Higher viscosity Slower ow Higher silica content Higher viscosity Higher temperature Lower viscosity Higher gas content Lower viscosity IGNEOUS ENVIRONMENTS Extrusive settings Cool at or near the surface Cool rapidly Chill too fast to grow big crystals Intrusive settings Cool at depth Lose heat slowly Crystals grow large PILLOW BASALT GABBRO Basalt is usually extrusive cools quickly crystals don t have time to grow ne grained Gabbro is usually intrusive cools slowly large crystals coarse grained EXTRUSIVE FLOWS ERUPTIONS INTRUSIVE ACTIVITY Magma intrudes into other rocks in 2 ways As planar tabular bodies dikes and sills and As balloon shaped blobs plutons Size varies widely plutons can be massive TABULAR INTRUSIONS Tend to have uniform thickness and can be traced laterally Two major subdivisions Sill Parallels rock fabric Dike Crosscuts rock fabric PLUTONIC ACTIVITY Most magma is emplaced at depth in the Earth A large deep igneous body is called a pluton Deeper features are exposed by uplift and erosion Intrusive rocks are more resistant to erosion Intrusive rocks often stand above the landscape Unroo ng takes long periods of geologic time EROSION MAY EXPOSE PLUTONIC ROCKSINFLUENCE ON LANDSCAPE