5.2 - Introduction to Landforms: The Structure of the Earth and its Composition
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The Structure of the Earth
Our knowledge of the interior of the Earth is based on indirect evidence. Despite movies, books and cartoons, nobody has actually been to the center of the Earth. It is about 3,960 miles from the surface to the center of the Earth. the deepest mine shaft is about 2.4 miles and the deepest bore hole is about 8 miles. We have barely scratched the surface. Most of what we know has been learned by geophysicists who monitor the patterns of seismic waves generated by Earthquakes or man-made explosions as those waves pass through the Earth's interior. These waves speed up or slow down and change direction depending on the material they encounter.
From this information, geologists have developed a model of the Earth's internal structure.
The Crust
The crust, the outermost shell, averages about 3 miles thick beneath the oceans and about 25 miles beneath the continents. The density of the rocks increases with depth. The crust makes up less than 1% of the Earth's volume.
Oceanic crust is primarily mafic rock: rock rich in magnesium and iron, dark in color and dense. The continental crust is primarily felsic rock: rocks rich in feldspar and silicon, lighter in color and less dense than mafic rock.
At the base of the crust there is an abrupt change in mineral composition which separates the crust from the mantle. It is called the Mohorovicic Discontinuity or the Moho for short (no one can spell the full name!).
The Mantle
Beneath the Moho is the the mantle, which extends down to a depth of about 1800 miles. It is the largest of the layers with in the Earth. It makes up 84% of the total volume of the Earth. Earth Scientists think that there are three layers within the mantle.
I like to think of the mantle as a giant Oreo cookie. The upper part of the mantle is relatively thin but hard and rigid extending down about 40 to 60 miles. This rigid upper part of the mantle together with the crust is called the lithosphere. Beneath this rigid zone is the asthenosphere extending down about 200 miles. The rock here is under high pressure and is hot so that it is easily deformed and tar-like in consistency. I think of this as the fluffy white part of the Oreo (maybe not the best analogy). Below the asthenosphere is the lower mantle or mesosphere, where the rocks are believed to be rigid again (more Oreo cookie).
The Core
Beneath the mantle is the outer core, thought to be molten rock and extending to a depth of about 3100 miles. The Earth's innermost portion is the inner core, a solid and dense mass with a radius of about 900 miles. Both the inner and outer cores are thought to be made of iron and nickel. Together they account for about 15% of the Earth's volume. The Earth's magnetic field is generated in the outer core.
Minerals, Rocks and the Rock Cycle
There are about 100 natural chemical elements found in the Earth's crust, mantle and core. Most are bonded with one or more other elements to form compounds. These naturally formed compounds and elements are called minerals which are the building blocks of rocks.
In order for a substance to be a mineral it must:
- Be solid
- Be naturally found in nature
- Be inorganic
- Have a specific chemical composition
- Contain atoms arranged in a regular pattern to form solid crystals
Approximately 4400 minerals have been identified!! Of that more than 4000 minerals, 20 account for 90% of the all rock. The rock forming minerals can be grouped into 7 broad categories: silicates (eg. feldspar and quartz), oxides (eg hematite and magnetite), sulfides (eg. lead, zinc and copper), sulfates (eg. calcium), carbonates (eg. calcite), halides (eg. salt), and native elements (eg. gold and silver).
Rocks are aggregated mineral particles - sometimes just one kind of mineral, but usually several different minerals. There is huge variety and complexity in rocks way beyond the scope of this course. But lucky for us we can divide rocks into three major rock classes - igneous, sedimentary and metamorphic rocks.
Igneous Rocks
Igneous rocks are formed by the cooling and solidification of molten rock. Molten rock beneath the surface of the Earth is called magma, while molten rock at the surface of the Earth is lava. Igneous rocks are generally subdivided into two main categories based on where they form: extrusive igneous rocks (cooling of lava at the surface of Earth) and intrusive igneous rocks (cooling of magma below the surface).
Extrusive igneous rocks cool rapidly at the Earth's surface. Mineral crystals in these rocks don't have a chance to grow and are usually not visible to the naked eye. The most common extrusive igneous rock is basalt, a dense, dark gray or black, fine grained rock. The oceanic crust is primarily made of this mafic rock. See the photo below. Other extrusive igneous rocks include obsidian, pumice, tuff and andesite.
Intrusive igneous rocks cool very slowly beneath the Earth's surface. It can take many thousands of years for this rock to form. Crystals in the rock often grow to be quite large, giving the rock a course grained appearance. The most common intrusive igneous rock is granite, a light colored, lower density, felsic rock. The continents are primarily granite. Other intrusive igneous rocks include diorite and gabbro.
Sedimentary Rocks
Sedimentary rocks are formed from pieces of other existing rock or organic material. There are three different types of sedimentary rocks: clastic, organic (biological), and chemical. Clastic sedimentary rocks, like sandstone, form from clasts, or pieces of other rock. Organic sedimentary rocks, like coal, form from biological material like plants, shells, and bones that are compressed into rock.
The formation of clastic sedimentary rocks begins with the weathering, or breaking down, of exposed rock into small fragments - sediment. Through the process of erosion, these fragments are removed from their source and transported by wind, water, ice, or biological activity to a new location. Once the sediment settles somewhere, and enough of it collects, the lowest layers become compacted so tightly that they form gradually form solid rock.
Organic sedimentary rocks form from the accumulation of organic debris, such as leaves, roots, sea shells and other plant or animal material. Rocks that were once swampy sediments or peat beds contain carbon and are black, soft and full of fossils. Rich enough in carbon to burn, coal is an organic sedimentary rock that is a widespread and important fuel source. Coquina limestone, and skeletal limestone are also organic sedimentary rocks, comprised of shells cemented together.
Chemical sedimentary rocks, like most limestone, halite, and flint, form from chemical precipitation. A chemical precipitate is a chemical compound—for instance, calcium carbonate, salt, and silica—that forms when it is dissolved in water, the water evaporates and leaves a chemical compound behind. This occurs as water travels through Earth’s crust, weathering the rock and dissolving some of its minerals, transporting it elsewhere. These dissolved minerals are precipitated when the water evaporates.
From left to right, below are photos of sandstone (a clastic sedimentary rock), coal (an organic sedimentary rock) and limestone (sometimes organic but often chemical)
Metamorphic Rocks
Metamorphic rocks started out as either sedimentary or igneous rocks. They have been changed by heat and/or pressure. The effects of heat and pressure are complex and vary depending on the length of time the rocks are heated and/or subjected to high pressure. Metamorphism heats the rock, causing its mineral components to be recrystallized and rearranged. The metamorphic result is very different from the original rock in terms of structure, texture, composition and appearance.
Metamorphism can occur beneath the Earth's surface where magma comes into contact with surrounding rock. This is called contact metamorphism. Where large volumes of rock deep with in the crust are subject to heat and pressure over long periods we have regional metamorphism. This is often found at the boundaries between lithospheric plates.
Foliated metamorphic rocks such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and direct pressure.
Non-foliated metamorphic rocks such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance.
Some rocks metamorphose predictably (limestone becomes marble, sandstone becomes quartzite, shale becomes slate) and others do not. (From left to right, photos of marble, quartzite and slate).
The Rock Cycle
Over long periods of time, the minerals in one rock might well end up in a different rock: igneous rocks can be broken down into sediments that might then form a sedimentary rock, which in turn might undergo metamorphism, only to be worn back again into sediment and reform a sedimentary rock. This is a giant rock recycling process known as the rock cycle.
Geologic Time
The concept of geologic time refers to vast periods of time over which geologic processes operate. These time frames are pretty tough for we mere mortals to grasp, so an analogy is in order.
If you squeezed all of Earth's history into one calendar year, the first 4 billion years (informally known as the Precambrian), would last from the first of January into late November. The oldest rocks discovered at Earth’s surface would date from mid-March. Geologists know very little about what happened during the Precambrian compared to later periods. They do know that very primitive life forms first appeared in the late Precambrian seas—on the calendar that would be late November.
The vast majority of what we know about Earth's history would fall into December. Dinosaurs became dominant in mid-December but disappeared the day after Christmas. At about the same time, the Rocky Mountains were uplifted. The first human ancestors appeared sometime during the evening of December 31. Everyone on Earth today would have been born in the last millisecond of New Year's Eve.