Earth's Landscape

The Structure of Earth

The physical environment and geologic can have an extremely large effect on living things. In order to properly understand these connections and the reason why it has the effects that it does, it is important to learn about both the physical environment itself and what leads to what we see in Earth's landscape.

We must first start with the composition of the Earth. It is the makeup of Earth that determines a lot of the geologic structures that form as well as how they form.

The innermost part of Earth is known as the core. This is the hot and dense center, consisting mainly of an iron-nickel alloy, which radiates heat due to the radioactive decay that is occurring. The inner core is solid due to the pressure applies to it from the rest of the planet. The outer core is liquid, and it is the movement of the iron here that creates Earth's magnetic field.

Most of Earth consists of the mantle - the solid (mostly) layer between the innermost core and outermost crust. The mantle varies in the strength of the rock - a term which in this context refers to how easily it deforms. It may be helpful to imagine candle wax here - hot wax is closer to its melting point, is more malleable, and therefore "weaker". The upper mantle has two parts - the lithosphere, which is the rigid, rocky outer shell, and the asthenosphere, which is a soft, weaker layer beneath the lithosphere. The top part of the asthenosphere experiences a small amount of melting, which allows for the lithosphere to move independently of the asthenosphere. The lower mantle strengthens with depth, is very hot, and flows very gradually.

The outermost part of Earth is known as the crust and consists of two types - oceanic crust and continental crust, of which continental is significantly thicker, older, and less dense. The crust is relatively thin and rocky.

Plate Tectonics

The lithosphere is broken into about twenty various irregularly shaped chunks known as "plates". As mentioned above, due to the melting that occurs in the asthenosphere, the lithosphere is capable of moving independently of the asthenosphere. It is this property that allows for the movement of continents and the creation of various landforms.

Imagine a lava lamp, where the light bulb at the bottom heats up the "lava," which floats to the top, cools off as it's farther away from the bulb, and then falls back towards the bottom. That process is known as convection and this process is currently happening within Earth. Mantle convection, the slow movement of Earth's mantle due to convection currents, results in the movement of tectonic plates on Earth's surface. It is the heat from radioactive decay within the core that causes the convection.

Plate Boundaries

The plates are constantly moving. The locations where they meet are known as plate boundaries and it is their relative movement and the interactions that occur at these boundaries that result in geologic landscapes and events. Maps of these boundaries, such as the one above that show the plates and their relative movement to each other can be used to determine locations of things such as volcanoes, earthquakes, faults, island arcs, hot spots, and mountains.

Convergent Boundaries

Convergent boundaries, also known as destructive margins because they result in the destruction of crust, are where two plates are moving toward each other. The prefix "con" means with. If you know any Spanish, just think of that. "Taylor va a comer con Husain" means "Taylor is going to eat with Husain". Similarly, the English word convene: "my friends convened for the murder mystery party" means that my friends came together with each other for the murder mystery.

At convergent boundaries, as the plates move toward each other, the less dense one will subduct or sink beneath the other one. Due to this, convergent boundaries are also known as subduction zones. As plates subduct, it is rather common for the forces to result in earthquake (and therefore also tsunami) activity. Earthquakes at this type of boundary tend to be the strongest and deepest.

It is important to note that continental crust is less dense and will therefore not subduct to significant depths within the Earth. Different geological events and landscapes will occur depending on which two plates are moving toward each other.

At an oceanic-oceanic convergent boundary, one plate will subduct under the other, forming a deep-ocean trench at the surface where one plate sank into the mantle. As the plate sinks, the pressure squeezes the water out from the oceanic crust (as it was at one point the ocean floor and therefore contains a lot of water). "Wet" rock melts at lower temperatures than "dry" rock does (similar to how we add salt to ice to make it melt sooner), so this results in melting in the mantle. As this molten rock is less dense than the other mantle, it rises. Oftentimes it solidifies and results in a thickened crust, but it can result in a volcanic eruption. Volcanoes form on the ocean floor and their eruptions can result in volcanic islands. These islands are usually spaced out by roughly 80km and will form an arc of islands known as an island arc.

Oceanic-continental convergence occurs when a denser oceanic plate subducts under a continental plate. Similarly to oceanic-oceanic, this can result in trenches and volcanic activity, except the volcanoes will form on the continent. Mountains can be formed from this volcanic activity, such as the Cascades, and such systems are known as continental volcanic arcs.

Continental-continental convergence occurs when two landmasses move towards each other. What happens historically is seafloor between them subduct and they gradually move towards each other, eventually resulting in a collision. As continental crust is far less dense, one does not sink under the other. Rather, the rock and sediments deform, resulting in the formation of mountains.

Divergent Boundaries

Divergent boundaries, also known as constructive margins, are where two plates move away from each other. Think about the famous line from a Robert Frost poem, "two roads diverged in a yellow wood". This is referring to a fork in the road where the roads separated from each other. Likewise, the root di means two - such as carbon dioxide (CO2) having two oxygens.

These are known as constructive margins because they are where new ocean floor is made.

The majority of divergent boundaries are found at mid-ocean ridges - areas where the seafloor is elevated there is high volcanic activity. At some ridges, there are lower elevation areas called rift valleys that are formed from the tension that pulls crust apart at the ridge.

As volcanic activity results in the creation of new oceanic crust, seafloor spreading occurs. Because this new crust is hot, it is less dense than the older crust; this is what leads to the elevation that occurs in the formation of a ridge. This new crust is continually, but very slowly, pushed away from the ridge as seafloor spreading continues to happen.

Divergent boundaries can occur on continents, although they are rarer. The lithosphere is pulled apart, which can result in the continent being split into multiple smaller landmasses. Long depressions, known as continental rifts, can form in the continent where the crust is being stretched and pulled apart.

Earthquakes are common along these boundaries, but they are relatively weak and shallow.

Transform Boundaries

Transform boundaries are the rarest of the boundaries. Convergent and divergent each make up roughly 40% of plate boundaries, with transform boundaries consisting of about 20% of boundaries. At these boundaries, the plates slide past each other. Lithosphere is not made or destroyed at these boundaries, but it can be broken. Faults, fractures within the crust, are common here. Pressure builds as the rough edges of plates get stuck on each other, eventually resulting in a sudden release stored energy. Strong but shallow earthquakes occur when this happens.

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