Earth Geology is a science interested with the solid part of earth, the types of rocks and minerals by which the earth is composed and the processes that change the earth over many millions of years. We took a look at earth like planets and discovered that they have to be of a certain size and distance from their sun (which should be g-type) as well as having a circular orbit with a tilt about their axis. We notice that Mercury, the moon and Mars have one distinguishing feature that is lacking on Earth. That is many impact craters. (Mars especially also has many large non active volcanoes a feature rare on earth) The planets and the moon formed about 4.6 billion years ago and after the first 500 million years the planets looked pretty much the same. Spherical bodies of rock with many craters and giant primary volcanoes. But now these planets look remarkably different. Erosion and a more geologically active earth created the difference. Erosion is a major component of earth geology. Erosion is created by flowing water not found on Mars Mercury and the moon. A more geologically active earth because of tectonic plates breaking apart is called faulting (eg San Andreas Fault California) and tectonic plates going under each other is called folding (eg. Indian subcontinent plate going under Asian Plate). These geological forces destroyed the crater marks and prim ary large volcanoes on earth. This happen after 3.6 billion years ago. This fact must be looked into when dealing with the structure and landscape of earth today for physical geographers.
If one takes away all the water and vegetation on earth and all the craters on the moon the two planetary bodies would look remarkably similar. The ocean basins and the continents on earth would correspond to the dark and light areas of the moon respectively. The visible surface of the earth (or any planetary body) is called the crust The ocean basins are one of two categories of crust. Basins are called the sima layer. Now this is an acronym for the elements silicon and magnesium. Now this layer contains high levels of magnesium and iron hence it’s darker colour and greater density. The ocean basins on earth can be divided into the: continental slopes and trenches which are a result of tectonic folding. The ocean plain, the product of settled igneous rocks. The mid oceanic ridge which is the consequence of faulting. The ocean basins make up about 65% of the earth’s surface geology. Therefore Some wonder why we call our planet earth and not water.
The crust of the continents are called the sial layer which is actually sits on top of the sima layer. Now sial is an acronym for the elements silicon and aluminum. Now this layer has little magnesium and iron, hence it’s lighter colour and lesser density. Granite is the typical rock of the continental crust/sial layer. The continental crust can be divided into the 1)continental shelf. Although this is covered by a shallow layer of sea water it is geologically part of the continental crust. In Canada an example of this would be the Grand Banks of Newfoundland. 2)the shield of a continent. This is believed to be the core that the continental land mass grows out from. 3)the orgenic belts which are the mountain ranges of continents. These organic belts are younger than the continental shelves, which are in turn younger still than the continental shields.
The mantle is the massive layer which is about 3700 km thick and lies just below the crust. It is made mostly of mafic silicate materials. Mafic is an acronym for magnesium and iron(ferric). Now the mantle as one can see from the diagram is subcategory-ed into the lithosphere being the first layer below the crust. This region is ridged and brittle. Some geologists include the crust as part of the lithosphere since “lith” is the Greek word for stone or rock. Now this lithosphere floats on the asthenosphere. which is a semi liquid or some say plastic layer. This asthenosphere is where molten rock forms and energies created in this sphere give rise to volcanoes, earth quakes and general plate tectonics that have formed land masses and oceans for billions of years. Now the mesosphere as we know from the diagram is below the asthenosphere and it is made up of silicate materials. These materials are more densely packed near the core than near the asthenosphere. But the temperature of the mesosphere near the core is about 2800 degrees centigrade and roughly 800 degrees less near the asthenosphere. Higher temperatures make matter less dense, but the increased weight from above more more than makes up for this. The core of the earth is below the mesosphere of the mantle. Now the core is the hottest part of the earth ranging from 3000 to 5000 degrees centigrade. The outer core is liquid iron with some nickel and sulfur and the inner core is solid iron(some nickel and sulfur). The inner core is hotter but here again more densely packed due to the greater mass above. Because the earth rotates, the liquid iron core swirls around the solid core and creates what some geologists call the dynamo effect. The iron liquid core is rotating, convecting and is a conductible fluid(electricity can flow through it). It therefore creates earth’s magnetic field. The earth’s magnetic field protects us from the solar winds electrically charged particles. These charged particles can strip away the atmosphere, especially the ozone layer that is essential to all life on the land.
The science of geology is not only the study of the structure of the earth but also the study of individual rocks and minerals as well. Now the most common elements in the earths crust are oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium. In that order. Minerals with one element are usually called native minerals, while multi elemental minerals are just called minerals. They are of a non organic solid material. And are defined by their specific composition and crystal structure. Minerals do not contain fossils.(ex. Quartz) Whereas rocks on the other hand are an aggregate of minerals and are more defined by the geological processes that made them.
The three main categories of rocks are igneous, sedimentary and metamorphic. Igneous rocks came into existence from solidified molten matter that has cooled. Igneous rocks that have cooled deep within the earth are called intrusive. Because their rate of cooling was slow they have crystals visible to the naked eye. Examples are your granites, diorites, and gabbros which range from light to dark in colour respectively, due to a decreasing amount of silicate. Extrusive igneous rocks are expelled from the earth quickly because of vulcanism. Their rapid cooling process makes the crystals of these rock samples very small. The most well known is basalt. Basalt has a dark colour because of its low silicate makeup and comprises most the ocean basins. Igneous rocks in addition to silicate materials contain metallic elements and are hard and strong because of the grains of minerals being firmly interlocked. Just ask any body who has a granite counter or table.
Erosion of any type of rock will lead to sediment particles. The erosion could be from wind, moving water, temperature change.(expanding and shrinking causes a break down of rocks) and ice erosion due to expanding or retreating glaciers. Sediment particles are often deposited in low lying areas or lakes, marshes and the shallow seas of continental shelves in the case of water erosion. Now sediment builds up in waves(not all at once) and becomes layered. This creates heat and pressure from above layers and the cementation process begins. Rocks formed through this erosion layering process and called detrital or clastic sedimentary rocks. They are typically shale and sandstone.
Some sedimentary rocks are produced by chemical process. These are typically your limestone and dolmite rocks. Limestone is formed from and by calcium deposits from living creatures (shells and bones) or calcium deposits directly from sea water. Dolmites are formed by some of the calcium in limestone being replaced by magnesium. When it comes to coal and peat however some geologists put them in the category of organic and not sedimentary rock. They are formed over millions of years of decaying plant and some animal material. These rocks contain hydrocarbon.
Metamorphic Rocks can be produced from either sedimentary or igneous rocks. The process of faulting can create such pressure and temperature that these rocks are transformed into a separate category. Magma from the asthenosphere can seep up through vents and metaphorphasize crust or resident rock. The result of either of these processes can transform granite into gneiss, shale into slate, sandstone into quartzite and limestone into marble. These metamorphic rocks are much more resistant to erosion.