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04/18/2003
Along with paleobiology, paleomagnetism is a principal way to figure out where an exotic foreign terrane originated. Here's what's involved: The Earth's magnetic field originates in the planet's hot flowing liquid outer core. As the Earth spins, the hot iron metal in the outer core develops a current -- a flow of charged particles (ions). This in turn produces the planet's electromagnetic field. When a basaltic lava flow erupts somewhere in the world, it first forms magnetite crystals (Fe3O4) as the basalt cools. At the moment, these little crystals are free to orient themselves with the surrounding magnetic field because they are temporarily surrounded by the rest of the liquid basaltic lava. The magnetic fields of the crystals become aligned with the lines of magnetic force of the magnetic field at their location. The INCLINATION of the crystal fields is directly tied to the latitude where the eruption occurred. Because of the shape of the magnetic field, crystals that erupt near the Earth's equator lie parallel to the Earth's surface within their volcanic rock. Crystals that erupt at the poles are aligned perpendicular to the Earth's surface and the layer of rock that contains them. Between the Equator and the Poles, the inclination increases in angle. Thus, when ancient lava flows erupted, they recorded their position relative to the equator and the poles....and this enables geologists to estimate the latitude where they originated...and from that, how far they've travelled before DOCKING. Paleomagnetism has been used to study the origins of the terranes of the Northwest. The first foreign terrane to arrive here is called THE INTERMONTANE terrane ("between the mountains"), which sits between the Rockies to the East and the Cascades to the West. The Intermontane superterrane (superterrane #1 for our class) is made up of 5 distinct terranes that originated somewhere to the west and south perhaps as far as equatorial Asia (we know this from the fossils and magnetism). Before the Intermontane superterrane arrived, its pieces AMALGAMATED (combined together) during the period 190-165 million years ago. We know the age of this grouping together from -- dating the metamorphic rocks caught between the colliding pieces and dating the "stitching" plutons (igneous bodies) that "glued" the pieces together. The entire superterrane -- the size of California -- arrived here about 160-150 million years ago with a great collision. The Intermontane superterrane includes a few prominent terranes -- Quesnellia and Cache Creek terranes. The Quesnellia terrane includes the rocks of northeastern Washington's Okanogan Highlands. These rocks are ancient seafloor rocks (basalts, sandstones, and mudstones) that have been highly metamorphosed by their numerous collisions. These rocks contain the gold that has produced a number of goldmines in NE Washington.
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