In Marathon County we find the igneous rocks basalt, gabbro, granite (of various kinds), rhyolite, and andesite. We must interpret what past events and environments these represent based on what we know about the origin of these rocks in the world today (see below).
At various locations in Earth rock material is melted down to form a liquid
mix of elements and crystals called "magma". When magma solidifies "igneous"
rocks are formed.
1. Mid-Ocean Ridges (Divergent Plate Boundaries), such as are found in all the world's major oceans. Along MORs the oceanic lithosphere (i.e. the near-surface mantle and ocean crust) is pulling apart and new ocean crust is formed to fill the gap, or rift. In fact, this is how oceans are created in the first place. The rifting apart causes the upper mantle to melt and for this reason ocean crust is similar, although not identical, to the mantle in composition. This composition may be described simply as "oceanic" -- low in quartz and rich in iron and magnesium. Other terms used include "mafic" (i.e. magnesium and iron rich).
2. Subduction Zones (Convergent Plate Boundaries), where the oceanic lithosphere is sinking into Earth's interior and being melted down. Such zones include the Pacific's "Ring of Fire" and the small group of islands that form the eastern boundary of the Caribbean Sea. These are zones of violent volcanic and earthquake activity, either on the margins of continents (e.g. Cascades, Andes) or off-shore on islands arcs (Japan, Aleutians, Summatra etc.). The rocks formed here have more quartz in them (are more "continental") than is the case at MORs. "Continental" igneous rocks are also commonly described as "felsic", or rich in feldspar and silica (quartz) crystals.
Subduction Zones and Mid-Ocean Ridges thus have complementary functions (MORs create oceanic crust while Subduction Zones destroy it). Both zones together represent the boundaries of Earth's Tectonic Plates. A map of these zones can be viewed HERE in these pages.
3. Finally, magma is also produced at Hot Spots in Earth's interior, such as exists deep below Hawaii. These are randomly distributed across the face of Earth. As with MORs, the molten material is derived from the mantle and is thus rich in iron and magnesium. Hot Spots originate as plumes of buoyant, but not molten, material at Earth's Core. The plumes melt as the pressure drops when they approach the surface.
If the magma is "extruded" onto the surface of Earth then the activity is said to be "volcanic" or "extrusive". The rocks formed in such circumstances are fine-grained (have small, often invisible crystals) because they cooled rapidly. This is true regardless of whether the rocks are oceanic or continental in general composition. Basalt is an oceanic and hot spot lava, while andesite and rhyolite are continental (subduction) lavas.
If the magma remains below surface while cooling then the activity is said to be "plutonic" or "intrusive". The rocks formed in such circumstances are coarse-grained (have large, visible crystals) because they cooled slowly. This is true regardless of whether the rocks are oceanic or continental in general composition. Gabbro is an oceanic plutonic rock, whereas granite and syenite are continental (subduction) intrusives.
Landforms caused by extrusion include volcanic cones (of various dimensions and shapes) and lavas flows. Again, these may be oceanic or continental in composition. In the subsurface, plutons (the forms caused by intrusion of magma) may be of various shapes and dimensions. The largest and most massive of these are batholiths; smaller forms include horizontal and vertical sheets of material called dikes and sills, respectively. Batholiths, dikes and sills are all a part of Marathon County's geology.