Week 3 posting

Lecture 15: ROCK TYPES / EFFLUENT AND EFFUSIVE VOLCANISM The previous lecture discussed the most significant factors of endogenic, otherwise known as, building processes. Plate tectonics determine the specific building features found in a given location; earthquakes (found at divergence, convergence and hot spot locations), and folding or faulting, also known as diastrophism (found in locations of collision, most significantly along continental​continental margins of convergence).

Recognizing which forces produce specific features makes the map very predictable, and will help us to understand where exogenic, or degradation, processes occur as gravity moves materials downhill.

Geographers and geologists have categorized rock types and age of materials based upon the rock cycle. The best map available to demonstrate this idea is page 6​7 in the atlas. Please note the following:

a. Alpine system materials ​(shown in purple in atlas): these materials correlate with locations of volcanism at plate boundaries and some hot spots. They are composed of igneous rock, both intrusive and extrusive.

b. Sedimentary materials ​(shown in green and beige​Caledonian and Hercynian in atlas): these materials correlate with areas that have been worn down by the atmosphere (degradation by gravity, wind, water, and ice) or built by collision (folded and faulted).

c. Shield materials ​(shown as Laurasian or Gondwana shield): these materials are the oldest materials still found in the crust. We consider them to be remnant features, created by tectonic activity that no longer occurs in these regions. These shields are crystalline, very hard and resistant, metamorphic rocks (see also p.361, 9th ed or p.351, 10th ed).

This map shows us two important ideas: 1) It shows us the age of the materials​. If you recall from chapters 12 and 13, there are three types of rock materials located in the crust. These materials are categorized by their formation processes, which also reflect their degree of resistance to erosion.

a. IGNEOUS ROCK ​​ this material is created through volcanism. It is found in alpine regions, isolated volcanic areas, and rifted shield locations (see atlas pg. 6​7). This rock material is the ​youngest ​material found on the earth, since it has been created through the recycling of tectonic plates (subduction) or upwelling of magma from the earth's interior (divergence and hot spot activity). There are two types of volcanic rock: ​INTRUSIVE and EXTRUSIVE ​and this categorization is determined whether the magma cools inside or outside of the volcano. See image of composite cone volcano, p. 374, 10th ed.

Extrusive igneous ​rock is material that forms outside of the volcano. It is strewn out in explosive volcanism (associated with convergence), and example of this material is pumice. In effluent volcanism (associated with divergence), basalt is the extrusive material most often found in volcanic regions.

Intrusive igneous ​rock is material that cools inside of the volcano. Due to the slower rate of cooling, this rock material is much more resistant to weathering than extrusive. Examples of intrusive igneous rock are granite and obsidian.

b. SEDIMENTARY ROCK​: sedimentary rocks, such as sandstone and limestone are rocks that have been ​weathered by degradation processes​. Sedimentary rocks are often layered, each layer representing a distinct period of degradation of igneous or metamorphic rock. From the atlas (pg. 6​7) you will see that sedimentary rock is found in river locations, along the Mississippi River, the Amazonian rivers, and the Nile. It is also located in regions that exist on the margins of areas, which have been glaciated, such as the Great Lakes, and the European lowlands (Belgium, Netherlands, and Denmark). The Bay Area is also a region of sedimentary rock, mostly found within the Bay itself, due to transportation by incoming rivers and streams. Foster City is another good example of sedimentary material, since it is human landfill. Foster City and other sedimentary locations tend to experience magnified earthquake activity due to the sedimentary nature of the rock material sedimentary rock has very little clastic (or cementing) material unifying the rock together. This accentuates the tremors. If we compare the magnification of earthquakes in sedimentary, igneous and metamorphic materials, we would find much less magnification on igneous or metamorphic rock.

c. METAMORPHIC ROCK​: this material is created through heat and pressure within the earth's crust. This can be achieved through subduction or compaction over long periods of time. These materials are known as continental shields (see page 361 text). They are the ​oldest ​rock formations on the earth's surface, and many archeological findings correlate to these locations. Recall from the first tectonics lecture that the continental materials are composed of SIAL or silica and aluminum, also these lightweight materials do not subduct at points of collision. We are lucky that remnant features of past environments exist due to the buoyancy of sialatic materials. Examples of metamorphic materials are marble, coal, and diamonds.

2) Rock types allow us to rank the rocks according to their level of resistance and hardness​. Basically, the slower the rocks are in cooling, the harder and more resistant the rocks. Below I have listed the ​rock types above from the most resistant to the least resistant​.

1. Metamorphic 2. Intrusive Igneous 3. Extrusive Igneous 4. Sedimentary

A personal way that you can use this information is when purchasing a home. Especially in a tectonically active area such as the Bay Area, locating which materials are the most resistant will allow you to avoid quake related hazards and destruction from weathering less resistant materials over your lifetime, such as landslides. What type of rock would you want to build your house on if you lived in the foothills? Remember, this is a windward slope affected by wind, water, snow and ice.

Rock types also allows us to easily determine the age of specific regions. Basically, think of the earth at three distinct ages:

YOUTHFUL​​comprised of igneous or alpine materials. This is where current building is taking place. These areas appear very rugged, with high elevation and steep slope.

MATURE​​comprised of sedimentary rock which has been weathered, eroded and deposited. These areas are where we can observe current processes of degradation. Such areas appear as hill and valley topography.

OLD AGE​​comprised of metamorphic rock, also known as regions of continental shields. These areas are remnant features of past environments which have been weathered down to their base rock through long periods of degradation. These landscapes are usually flat, such as Africa, yet not worn down to sea level.

2.) The landform maps (6​7 atlas) also allow us to predict the type of degradation processes we can expect to find on a global basis.

We can overlay the climate map to determine which type of degradation processes will occur in a specific location (ice, water or wind), recall the resistance of rock materials in each location to predict the area's vulnerability to the elements. Below I have listed the ​types of degradation which can degrade each type of rock​.

a. Metamorphic ​ glaciation b. Intrusive igneous ​ glaciation c. Extrusive igneous ​ glaciation, fluvial and aeolian d. Sedimentary ​ glaciation, fluvial and aeolian

Metamorphic and intrusive igneous rock can be degraded by water and wind, but this happens so slowly that if these materials are located in a climatic region dominated by water or wind processes, what tends to appear are resistant rock formations such as aretes or erratics as seen on page 507, 9th ed or pages 400-401, 10th ed in Christopherson, and cataracts (rapids) or knickpoints as seen on page 441, 9th ed or page 424, 10th ed. Be sure to connect examples of regions characterized by igneous, metamorphic and sedimentary rock to degradation processes.

EFFUSIVE AND EXPLOSIVE VOLCANISM The text takes much time to discuss effusive and explosive volcanism. What I find to be most significant for you to understand are the differences in formation of each type of volcanism. Most significantly, volcanism occurs through the tectonic processes of divergence, convergence and hot spot activity.

Correlate page 6​7 in the atlas with page 8​9 in the atlas. From this you will be able to predict areas where volcanism is determined by subduction versus those areas where volcanism occurs through divergence and hot spot activity. If you also re​examine page 389 in the text you can see the reason for the different appearance and composition of effusive and explosive materials.

Explosive volcanism ​occurs as magma builds up under rigid continental plates. We find explosive volcanism in the following locations;

● areas of continental​oceanic collision ● areas of continental divergence ​ rifting

Once this energy becomes massive enough the volcano erupts and this eruption may tear away a large portion of the continental mass with it. Mount St. Helen's is a superb example. With explosive volcanism, we can now observe "bulges" in the mountain mass of the volcano with use of satellite images.

Intrusive materials ​which are created through the melting of the continental shield often form batholiths and plutons, once cooled. These are recognizable features of resistant rock that reflect the rounded shape of the magma chamber. This can be seen when you go to Yosemite, Half Dome is a good example of such a batholith that has been sheared by glaciation. Explosive volcanism occurs in areas where continental​oceanic collision occurs, such as the Andes, as well as in rifted shield areas, such as the East Africa Rift zone.

Effusive volcanism ​is much less destructive. Effusive volcanism occurs in the following locations:

● areas of oceanic divergence ​along ridges and rises ● areas of oceanic​oceanic collision ● hot spots

Often effusive volcanism is thought of as just flowing out of a shield volcano, which looks like a large depression in the landscape. If you've ever been to the big island of Hawaii, you can observe effusive volcanism at Mauna Loa and Mauna Kea. This material is mostly basalt, and is extremely fertile once it is weathered. Effusive volcanism occurs in areas of divergence, such as the Mid​Atlantic Ridge, in areas dominated by hot spot activity, such as Iceland and Hawaii, and regions of oceanic​oceanic collision, since these are simatic plates, made up of basaltic materials.