Earth's Structure, Plate Tectonics, and Genetic Inheritance: Key Concepts in Geology and Biology

Earth's Internal Structure

Scientists cannot directly observe the interior of the Earth. Deep drilling has reached only about 12 km, far short of the planet's center. Instead, knowledge of the interior comes from the analysis of seismic waves produced by earthquakes. These waves reveal the internal layers, their compositions, densities, and the discontinuities that separate them.

The main layers are the crust, the mantle, and the core. The crust is composed mostly of rocks rich in silicon and aluminum. The mantle is dense and largely solid, but it behaves plastically over long periods. The core is mostly iron and nickel, with an outer liquid part and a solid inner part.

Lithosphere and Asthenosphere

The lithosphere includes the crust and the uppermost, rigid part of the mantle. It behaves as a rigid and brittle layer. Beneath it lies the asthenosphere, a plastic and ductile zone of the upper mantle that partially melts and allows the lithospheric plates to move.

Continental Drift and Plate Tectonics

Wegener's Hypothesis

In 1912, Alfred Wegener proposed the theory of continental drift. He noted that the continents fit together like pieces of a puzzle, particularly the coasts of South America and Africa. He suggested that all the continents were once joined in a single supercontinent, which he called Pangaea, and that they had gradually separated.

Wegener supported his idea with paleontological evidence, such as identical fossils of land organisms found on continents now separated by oceans. He also cited geological evidence, including matching rock formations, and paleoclimatic evidence from coal deposits and sediments showing climates unlike those of their current locations.

At the time, however, Wegener could not offer a convincing force capable of moving continents. His hypothesis that the continents plowed through the oceanic crust was rejected, and the theory was largely dismissed until the mid-twentieth century.

The Theory of Plate Tectonics

The lithosphere is divided into large fragments called tectonic plates, which float on the plastic asthenosphere. This principle is known as isostasy, the equilibrium of the lithosphere floating on the denser mantle.

The interaction between plates occurs at three types of boundaries:

• Constructive boundaries: plates move apart, and new crust forms, as at mid-ocean ridges.
• Destructive boundaries: plates collide, and one may subduct beneath the other, as occurs where continents converge.
• Transform boundaries: plates slide past each other horizontally, producing faults such as the San Andreas Fault.

Mid-ocean ridges, which form the largest mountain range on the planet, are constructive boundaries marked by a central rift and crossed by numerous fractures and deep trenches. Earthquakes and volcanoes are concentrated along plate boundaries.

The Rock Cycle

Rocks are continuously transformed. Exogenous processes, which act on the surface, include weathering, erosion, transport, sedimentation, and diagenesis, and they form sedimentary rocks. Endogenous processes, which originate within the Earth, include magmatism and metamorphism, and they form igneous and metamorphic rocks. Tectonic forces and the relief of the surface, along with climate and lithology, influence how rapidly these processes operate.

Dating Geological Events

Geologists use two approaches to date rocks and events:

• Absolute dating assigns a fixed numerical age to rocks or materials, typically using radioactive isotopes.
• Relative dating establishes the order of events without assigning precise ages. The principle of superposition states that in undisturbed strata, the lower layers are older and the upper layers are younger. Fossils are also used: index fossils, which are abundant, widely distributed, and restricted to short time spans, help correlate rock layers and identify geological periods.

Genetics and the Laws of Inheritance

Genetics is the branch of biology that studies how traits are transmitted from parents to offspring. The set of genes an organism carries is its genotype. Its observable characteristics, which result from the interaction of the genotype with the environment, make up its phenotype. When the two alleles for a trait are identical, the individual is homozygous; when they differ, it is heterozygous.

Mendel's Laws

1. First law (uniformity): When crossing two pure-bred individuals that differ in a trait, all the offspring of the first filial generation (F1) are identical.
2. Second law (segregation): When the F1 offspring are crossed, the second generation (F2) displays two phenotypes in a fixed ratio.
3. Third law (independent assortment): When individuals differing in more than one character are crossed, the transmission of each character is independent of the others.

The Chromosome Theory of Inheritance

The chromosome theory of inheritance establishes that:

1. Genes are located on chromosomes.
2. Each gene occupies a particular place, or locus, on a chromosome.
3. The two alleles for a character are found on two homologous chromosomes.