Fossils: Understanding Earth’s Past Life

Posted on Feb 19, 2025 in Geology

Fossils

A fossil is any remains of a living organism or its activity that lived in another era and has reached our days transformed into a rock. This generally includes remains such as shells, eggs, leaves, or traces.

Fossilization Process

  1. An organism dies and is covered by sediments.
  2. The soft parts rot, but the harder parts remain long enough for the outer surface to be imprinted in the sediment.
  3. Sometimes, these hard parts are mineralized and are well preserved.
  4. The hard parts are dissolved, and the hole is filled by materials provided by underground water. This will be the body’s outer mold.

What Can Fossils Tell Us?

Fossils indicate environmental conditions that existed at that location and in which era the living being in question lived, including the environment and climate.

The existence of marine fossils in places that are mountains today tells us that the mountains were formed after the Earth formed and that the rocks containing fossils were formed after fossilization.

How to Explain Their Presence?

Sea level has changed:

  1. The sea level has risen or fallen.
  2. The mainland has risen or fallen.
  3. Both processes have occurred.

Changes in Sea Level

There are two types of changes in sea level:

  • Daily Changes: Due to the gravitational attraction between the Moon and Earth.
  • Static Changes: These are very large changes that occur over long periods of time. They may be due to two causes: climate change and changes in the shape of ocean basins due to tectonic processes.

Isostatic Changes

Tectonic masses are subject to a dynamic equilibrium with respect to the materials below them. It is said that they float on fluid materials. The theory of isostasy attempts to explain these equilibria and their consequences.

  • When a continental area increases in weight, it will sink.
  • There are two types of costs: subsidence (due to erosion) and uplift.

A Large Relief with Two Steps

The solid surface layer of the Earth is called the crust. The part covered by the sea is the oceanic crust. The emerged part is the continental crust. The differences range from 10,000m in deeper areas up to 800m inland in the more elevated areas. Two steps are distinguished: one formed by thick continental crust and one by thin continental crust.

The Continental Puzzle

Theories on the geographical position of continents:

  • Fixed Theories: Suppose that continents have always occupied the same position since their origin.
  • Mobile Theories: Suppose that continents have changed position throughout Earth’s geographical history.

The Case of Wegener

Wegener proposed his mobile theory, called “Continental Drift,” supporting it with the following evidence:

  1. Geographic: Observing the contours of inland seas, he could verify that all the pieces fit together like a puzzle.
  2. Geological: By analyzing the distribution of rocks, he could verify that some identical rocks were now widely separated on different continents.
  3. Paleoclimatic: Studying ancient climates (paleoclimates), he could verify the distribution of land areas where there are now very different climates.
  4. Paleontological: Based on the study of fossils, he observed that terrestrial species were present on continents that are now separated by the sea.