Geology of the Hawaiian Islands

Transcription

1 Geology of the Hawaiian Islands Class 8 5 February 2004 Any Questions? Rocks are classified into 3 major categories: Igneous Metamorphic Sedimentary Igneous rocks Solidified from molten magma Most common rocks forming the Hawaiian Islands Granite Sedimentary Rock Metamorphic rocks Former sedimentary or igneous rocks Have been subjected to high pressures and/or temperatures that have changed (metamorphosed) them Rocks Aggregates of minerals 1

2 Minerals Naturally occurring inorganic solids Possess a definite internal structure and a specific chemical composition Minerals Made up of elements Some minerals, such as gold, silver, sulfur are made entirely of one element Most are combinations of two or more elements Atoms Building blocks of minerals Atoms Building blocks of minerals Arranged in specific ways that give each mineral its unique characteristics Common salt NaCl (Sodium Chloride) Silicon (Si) + Oxygen (O) Most common arrangement of atoms in igneous minerals Produces a tetrahedron with Si in the middle Tetrahedral arrangement of SiO 4 Silicon (Si) + Oxygen (O) Most common arrangement of atoms in igneous minerals Produces a tetrahedron with Si in the middle SiO 4 bonds with other elements to form chains or sheets of silica tetrahedra with the other atoms in between 2

3 K, Na, Ca, Mg, Mn, Fe, etc. Silicate minerals Most common minerals in the earth's crust Divided into light and dark-colored minerals Light colored minerals, such as quartz and feldspar contain K, Na, Al Dark minerals, such as pyroxene contain either Mg, Fe, or both Hand Specimens Physical Properties Hardness Density Crystal Form Cleavage SiO 4 tretrahedra Calcite (CaCO 3 ) Common mineral in sedimentary rocks Sulfur Common mineral associated with volcanoes Extrusive (volcanic) rocks Erupted at the earth's surface Exposed to surface temperatures Cool very rapidly from > 1000 C to less than 100 C Intrusive (plutonic) rocks Do not reach the surface Solidify in the crust In the magma chamber Minerals begin to crystallize as the magma cools First mineral to crystallize in Hawaiian rocks is usually olivine Next is plagioclase feldspar Phenocrysts Large crystals form if magma stays in chamber long enough 3

4 Phenocrysts Crystals float" in fine-grained "ground mass" (or matrix) Porphryritic texture. Picture is about 4 mm across Continued Slow Crystallization As magma cools, crystallization continues Olivine and Plagioclase crystals grow large No fine-grained groundmass if all magma is allowed to crystallize without eruption to the surface only large crystals left Bowen s Reaction Series Bowen s Reaction Series Fe + Mg Removed Fractional Crystallization Change in color indicates change in composition due to fractional crystallization Early minerals settle to bottom of magma chamber because they are more dense than the magma Igneous rocks grouped on basis of: Mineral composition Due to original chemistry of magma plus environment of crystallization Texture Mostly due to cooling history fine-grained = volcanic coarse-grained = plutonic Mafic Rock Formed mostly of minerals crystallized early in Bowen's reaction series High percentages of magnesium and iron (fe( fe) Most of the Hawaiian volcanic rocks are mafic Most common kind of mafic rock is called basalt Ultramafic rock Formed entirely of the early formed minerals that settled to the bottom of the magma chamber and crystallized there 4

5 Felsic Rock Has mostly minerals formed late High in feldspar and quartz (Si( Si) Bowen s Reaction Series Plutonic and volcanic equivalents: Basalt Gabbro Andesite Diorite Rhyolite Granite Hawaiian volcanoes Produce several kinds of mafic rocks Most common is basalt Formed of Ca-plagioclase and pyroxene Igneous rocks arranged into three general series: Tholeiitic Alkalic Nephelinitic Tholeiitic rocks Relatively poor in Si Poor in alkalies (Na + K) Poor in Si Rich in alkalies Alkalic rocks Nephelentic rocks Even more alkalic than alkalic rocks No plagioclase nepheline instead This is what the Sugar Loaf flow is made of Xenoliths (foreign rocks) Fragments of rocks within host rock Example: ultramafic fragments in basalt 5

6 Xenoliths (foreign rocks) Some Hawaiian ultramafic xenoliths have a mineral called garnet Garnets form at very high P and T What do they tell us? The xenolith may have come from >50 km deep in the mantle Metamorphic rocks Not common in Hawaii Have undergone change in mineral composition due to change in P/T Example: High P/T minerals in a lava change when extruded onto the surface with lower P and T Intrusive Bodies Intrusive igneous rocks Those that do not make it all the way to the surface Cool and crystallize in the earth's crust Sierra Nevada Batholith El Capitan Granite in Yosemite Valley 6

7 Pluton in Wyoming Stock former volcano Devil s Tower vent of old volcano (plug) Ship Rock volcanic plug and dike Dikes Very common in Hawai`i Range in size from a few cm to meters Sills Sheetlike intrusive bodies that lie parallel to layers of host rock Also common in Hawai`i Questions?? Homework #3 Web assignment See GG103 page Measuring Tilt on volcanoes Due a week from Today (12 February) First Exam one week from Tuesday (February 17 th ) Will be less than 1 hour long Will cover all material so far including Homework and Field Trip Review next Thursday come with questions 7