Introduction to Geology and Atmospheric Sciences
Geology
The Earth’s crust occupies 0.6% of the planet’s volume and 0.4% of its total mass. It is remarkably thin compared to the Earth’s radius. The oceanic crust averages about 7 km thick, while the continental crust increases to 35-40 km. Beneath the crust lie the mantle, outer core (both fluid), and the solid inner core at the planet’s center. These layers likely represent different chemical compositions and are classified based on their physical behavior.
The lithosphere, Earth’s rigid outer layer, comprises tectonic plates and extends to a depth of 75-100 km. These plates float on the partially molten asthenosphere, a layer about 200 km thick. The near-melting-point magma in this region allows for slow flow, enabling the lithospheric (tectonic) plates to move.
Until about 35 years ago, geologists believed the crust was stationary and continents formed early in Earth’s history. Thanks to the theory of continental drift, proposed by Alfred Wegener in 1912, we now know that continents move and the existing oceanic crust formed only within the last 200 million years. This realization revolutionized geological thought in the late 1960s.
Geological Processes
Processes shaping Earth’s surface can be categorized as constructive or destructive. Climate, erosion, and landslides degrade the landscape, leveling the relief by removing materials from heights and depositing them in lower areas.
- Climate: Refers to the physical, chemical, and biological breakdown and decomposition of rocks on the Earth’s surface.
- Erosion: The transportation of decomposed material by agents like water, wind, or ice.
- Mass Wasting: The downslope movement of material due to gravity.
Climate factors, particularly temperature and humidity patterns, significantly influence weathering rates. The resulting mineral fragments, combined with water, air, and organic matter (humus), form soil, which supports plant life.
Constructive processes, counteracting the destructive ones, include volcanism and mountain building (orogeny), driven by Earth’s internal heat. These processes are explained by plate tectonics, the accepted model of Earth’s inner workings, which emerged from Wegener’s theory of continental drift.
Minerals and Rocks
- Mineral: A naturally occurring, inorganic solid with a defined internal structure and specific chemical composition.
- Rock: An aggregate of one or more minerals.
The concept of the rock cycle, proposed by James Hutton in the late 18th century, helps understand the origin of the three basic rock types and the geological processes transforming them. It forms the foundation of geological science.
- Igneous rocks form from the crystallization (cooling and solidification) of magma. This can occur beneath the surface (intrusive igneous rocks) or on the surface as lava solidifies (extrusive igneous or volcanic rocks).
- Exposed to the surface by tectonic processes, igneous rocks undergo weathering and decompose. The resulting materials are transported and deposited by erosion agents, forming sediments.
- These sediments undergo lithification (turning into rock) through compaction or cementation, forming sedimentary rocks.
- Under high pressure and temperature, sedimentary rocks can transform into metamorphic rocks.
- At even higher temperatures and pressures, rocks melt, creating magma that can solidify into igneous rocks, completing the cycle.
The rock cycle doesn’t always run its full course; shortcuts exist. For example, igneous rocks can directly undergo metamorphism, and any rock type can experience weathering, contributing to sediment formation.
Uniformitarianism
Uniformitarianism, a fundamental concept in modern geology, states that the physical, chemical, and biological laws operating today operated the same way in the past. The forces and processes shaping our world have been at work for a long time. This principle is key to understanding Earth’s history and emerged as a response to the catastrophic model.
Atmospheric Sciences
Weather and Climate
- Weather: Refers to the atmospheric variables (temperature, precipitation, cloudiness, humidity, wind, pressure) at a specific location and time.
- Climate: Describes the average and variability of atmospheric conditions, seasonal patterns, and outliers over a long period (decades) in a particular place.
Climate significantly influences water availability and temperature, crucial factors in soil formation. It determines the vegetation types that can thrive, impacting water and nutrient cycles, photosynthesis, decomposition rates, and more.
Factors Influencing Climate
Climate results from the interplay of effects at different scales:
- Global: The general circulation of the atmosphere.
- Regional (continental): The influence of mountain ranges and large water bodies, modifying global patterns.
- Local (meso and micro): Valleys, slope orientation, and vegetation patterns affecting regional climates.
Atmospheric General Circulation
Due to Earth’s curvature, equatorial and tropical regions receive more solar radiation than temperate or polar regions. This uneven energy distribution drives a heat flow from the equator towards the poles, maintaining balance. This redistribution, primarily through water vapor transport (latent heat flux), drives the atmospheric general circulation.
As water vapor condenses at high latitudes, it releases heat absorbed during evaporation. This process creates a latitudinal pattern of circulation cells, each spanning approximately 30° of latitude. However, the distribution of continents and oceans disrupts these bands, resulting in semi-permanent high and low-pressure areas known as action centers.
Anticyclones, high-pressure areas, radiate counterclockwise winds in the Northern Hemisphere. These define a semi-permanent pressure and wind pattern that shifts seasonally, following the thermal equator. This pattern is overlaid by disturbances caused by weather fronts associated with extratropical cyclones. These cyclones originate from discontinuities between air masses with different characteristics, typically occurring around 60°N (the polar front).
Oceanic Circulation
Atmospheric general circulation drives oceanic circulation patterns (ocean currents). These currents redistribute energy across the planet, primarily as sensible heat. While atmospheric circulation accounts for about 85% of this energy exchange, ocean currents play a crucial role in shaping global climate.
Biomes
Climate determines the distribution of biomes, large-scale ecosystems characterized by similar plant communities. These biomes are arranged in latitudinal bands. In mountainous regions, altitudinal changes in temperature create similar zonation patterns over shorter distances.