rock origins 1 and 2 Flashcards
Plate tectonics is a dynamic process.
Plate tectonics is a dynamic process.
As you begin to think about how plate tectonics relates to the formation of rocks, keep in mind the results of the movement of plates along the asthenosphere. As plates plow their way across the surface, mountains are formed, new crust is formed, old crust is destroyed in subduction zones, earthquakes occur, and volcanoes are created.
Think about that as you consider what must occur for rocks to form. Igneous rocks are made from cooling magma or lava. Metamorphic rocks are the result of other types of rock being subjected to great heat and great pressure. Sedimentary rocks form as sediment is compacted or cemented, or as shallow seas evaporate
The connection between plate tectonics and igneous rocks is clear.
Igneous rocks form from the cooling of molten material. As you learned before, this molten material can be located within the crust as magma, or on the surface of earth in the form of lava.
Where does lava occur, and how does that relate to plate tectonics? Lava is typically associated with volcanoes. Many volcanoes are associated with subduction zones. Subduction zones are convergent plate boundaries where a dense slice of oceanic crust subducts under a lighter slice of continental crust. Subduction-zone volcanoes tend to be explosive. Rocks such as pumice and obsidian are often formed from eruptions of subduction-zone volcanoes.
Extrusive rocks are formed from volcanic eruptions.
If you’ve seen pictures of or visited the beaches in Hawaii, you may have seen the black sand beaches there. Black sands are the result of the weathering of the igneous rock basalt that forms from the eruption of the volcanoes that formed the Hawaiian Islands. The Hawaiian Islands are the result of a hot-spot volcano. The Pacific plate has moved north-northwest over a hot spot in the mantle, located under the plate. The result is the Hawaiian island chain.
Basalt also makes up the oceanic crust now forming at spreading centers. Divergent plate boundaries, such as the Mid-Atlantic Ridge, allow lava to spill out on the ocean floor, cooling quickly into basalt.
Plate tectonics is also responsible for intrusive igneous rocks.
Plate tectonics is also responsible for intrusive igneous rocks.
It’s easy to explain the formation of extrusive, or volcanic, igneous rocks as resulting from plate tectonics. But what about the formation of intrusive igneous rocks such as granite or gabbro?
Magma located deep within the crust will slowly rise toward the surface (remember that hot materials rise). If that magma is unable to reach the surface, it will harden in the crust. These intrusive rocks are often found in ancient volcanic necks or vents that formed and then became extinct.
These rocks are now seen on the surface as a result of the process of erosion and weathering. Later in this lesson, you will investigate how this is related to plate tectonics as well.
Plate tectonics uplifts and exposes rocks to weathering and erosion.
Plate tectonics uplifts and exposes rocks to weathering and erosion.
When exposed to the elementswsuch as water, wind, rain, and snowwrocks will begin to break down. Rocks on the tops of mountains often undergo a considerable about of weathering. Weathered material is eroded, or carried away, to places where it is deposited and could become clastic sedimentary rocks such as sandstone, shale, conglomerate, or breccia.
Plate tectonics causes changes that produce chemical sedimentary rocks.
Chemical sedimentary rocks, such as rock salt and rock gypsum, are formed from the evaporation of a shallow sea. Shallow seas have occupied different regions on earth during earth’s history. For example, White Sands, New Mexico, is a huge deposit of rock gypsum. The gypsum was deposited 250 million years ago when a shallow sea covered the area. The area was uplifted, due to plate tectonics, 70 million years ago as the Rocky Mountains formed. Parts of it began to collapse 10 million years ago, forming the basin that is there today. The sea that was there in the past, the deposits of gypsum, and the features seen in the area today are the result of plate tectonics.
Plate tectonics is the source of heat and pressure required to metamorphose rock.
Plate tectonics is the source of heat and pressure required to metamorphose rock.
The heat and pressure that it takes to change a rock into a different rock is intense. Such pressures can be generated through the collision of plates. Convergent plate boundaries, where two continental plates collide, create the heat and the pressure needed to change granite into gneiss.
Continental-continental plate boundaries form mountains. The Alps, Himalayas, Appalachian Mountains, and Rocky Mountains are all the result of convergent plate boundaries. Careful examination of the rocks in these areas will expose many metamorphic rocks, which have often been subjected to extreme heats and pressures.
Convergent plate boundaries can also explain metamorphic rocks such as marble or quartzite.
Convergent plate boundaries can also explain metamorphic rocks such as marble or quartzite.
It makes sense that gneiss is found in many mountainous regions. Gneiss is the metamorphic version of granite, and granite is found in continental crust. So how do rocks such as marble (changed from limestone) or quartzite (changed from sandstone) develop? These, too, can form in convergent plate boundaries.
The uplift of mountains is not always tidywthat is, it is not necessarily just two pieces of continental crust colliding. There may be sections of beaches or even some oceanic material that is pushed up with one of the plates as well. It is possible to find oceanic fossils in some of the rocks at the top of the Himalayas.
Limestone and sandstone can form metamorphic rock.
Limestone and sandstone that get caught up in the collision of two plates are subjected to the same heat and pressure. The limestone is changed into marble, and the sandstone is changed into quartzite.
If a mountain stops growing, the forces of weathering and erosion begin to take over. The rocks on the mountain begins to wear away, forming sediment, which washes downhill to be deposited and formed into sedimentary rock some time in the future.
Plate tectonics explains the formation of all three rock types.
The connection between plate tectonics and extrusive igneous rocks is clear. We have all seen pictures of volcanoes erupting lava onto the surface, which is how some igneous rocks form. Plate tectonics also creates the heat and pressure needed for metamorphic rocks to form, as well as the conditions needed for rock to weather into sediment that will later form sedimentary rocks. The history of a rock can be traced to the basic movement of plates on earth’s surface.
Each rock type has unique characteristics that provide clues to its formation.
So far in this unit, you’ve learned how plate tectonics explains the formation of igneous, sedimentary, and metamorphic rocks. Here, you’ll look at the formation of these three types of rocks again, but in a different way.
Each type of rock has distinguishing characteristics, such as a unique type of texture or a banded appearance. These characteristics can tell more about the environment in which the rock formed.
Sedimentary rocks have many unique characteristics.
Sedimentary rocks have many unique characteristics.
Many features found only in sedimentary rocks are clues to how the rock formed. For example, some sedimentary rocks may form when very fine sediment settles out into a shallow lake or pond. The shale that forms in this environment may have features such as ripple marks or mud cracks. Ripple marks indicate that the water in that environment was moving. In fact, in some cases it’s possible to infer the direction of movement. If the rock shows mud cracks, the mud that compacted to become sedimentary rock had dried out at one time, just like the mud in a puddle in your yard will dry up over time.
Limestone can contain some lumpy features.
Limestone can contain some lumpy features.
Limestone sometimes contains lumps of silica called chert. Limestone is composed of calcite, so the presence of lumps of chert is unusual. These lumps, called nodules, are thought to have been deposited from a solution very slowly over time. It may have formed around a small bit of a fossil. Chert that is gray or black is known as flint.
Limestone has also been known to contain geodes. A geode is a small hollow mass of silica.
Sometimes, if you break open a geode, you will find quartz or calcite crystals lining the inside. Geodes form from groundwater in this way: Groundwater acts to dissolve some of the calcite in limestone, leaving small cavities. The groundwater then deposits quartz or calcite crystals in the cavities.
Fossils are almost exclusive to sedimentary rocks.
Fossils are almost exclusive to sedimentary rocks.
Some metamorphic rocks retain the fossils of their precursor rocks, and in a few cases, ash from volcanoes has fossilized organisms. However, these cases are very rare. Most fossils are found in sedimentary rock. So, if you find a fossil, you can be sure you’re looking at a sedimentary rock.
Sedimentary rocks form from sediment. As the sediment piles up, the animals or plants that die in the area will become buried in the sediment. For the most part, the soft parts of the organism will decay. But if an organism has hard parts, it is quite possible that those parts will become rock with the sediment. A fossil is any remain, impression, or evidence of life found in rock.
The fossils in a sedimentary rock can tell a lot about where the rock formed. A deposit of shale with fern fossils may have formed in a swamp. Limestone with clam or mussel fossils probably formed in a beach or tidal area.
Sedimentary rocks are often arranged in layers.
Sedimentary rocks are often arranged in layers.
Stratification, or the arrangement of visible layers, is a property unique to sedimentary rocks.
When a change occurs in the type of sediment being deposited in an area changes, then the type of rock formed will change too. Say you have a layer of fine-clay sediment deposited on top of a layer of coarse-clay sediment. That means there will be two different types of shale formed when the rocks form. If a layer of sand is deposited on top of those layers, then sandstone will be found on top of the clay.
Sedimentary rocks are mostly deposited in horizontal layers. Windblown sediment or delta deposits may form crossbedding. These rock layers are not always horizontal.
