FOSSILS FOSSILIZATION AND ROCKS

 THIS BLOG BEGINS WITH 3 QUESTIONS:

1. Why can't fossils be found in igneous rocks?

2. Why can't fossils be found in metamorphic rocks?

3. So in what type of rocks can fossils be found? Why?

Before answering those three questions, I will first explain some concepts that readers can interpret as the basis for answering them. Please read carefully...

A. FOSSILS AND FOSSILIZATION

a. Definition

Fossils: Organisms or remains of organisms, including their traces, that are naturally preserved and are older than 10,000 years or pre-Holocene in age.

From this definition, we can conclude two things:

1. Fossils come from animals or plants, found in the form of whole bodies, certain parts, or traces (such as droppings, movement direction, movement patterns, etc.).

2. The >10,000 years age is used as a reference because fossilization was only recognized by humans around 10,000 years ago. Therefore, if a fossil is younger than 10,000 years, it might have been preserved by human intervention—this could potentially lead to misinterpretation of its environmental or climatic context.

Fossilization: The process of burial and accumulation of animal or plant remains in sediment or deposits, whether fully, partially preserved, or only in the form of traces.

Requirements for fossilization to occur:

• The organism has hard body parts, e.g., shells, bones, horns, tusks, etc.

• Low oxygen content.

• Natural preservation.

• Free from decomposing bacteria.

• Older than 10,000 years.

b. Classification

1. Based on fossil size

i. Megafossils

ii. Macrofossils

iii. Microfossils

iv. Nanofossils

2. Based on fossil form and associated fossilization

i. Body Fossils: Fossils in the form of the physical body of an organism, found either whole or partially. Here, "body" refers to the hard parts of the organism such as shells and bones. There are 2 types of body fossils:

- Unaltered Remains: Fossils whose body parts are found intact, with minimal physical or chemical alteration. There are four types of fossilization under this category

• Original Skeletal Material: The hard parts of the body are preserved without changes in their mineral composition. 

• Tar Impregnation: Preserved by tar or bitumen. 

• Amber Entombment: Preserved by plant resin. Typically, small animals such as insects are preserved after being trapped in the resin of a tree, which later hardens into a solid. It is yellow in color. 

• Refrigeration: Preserved by ice.

- Altered Remains: Fossils in which only part of the body is found, and significant physical and chemical changes have occurred. There are four types of fossilization in this category:

• Histometabasis: Replacement of plant tissue by other minerals.

• Permineralization: Some of the minerals contained within an organism have been replaced by other minerals. It is characterized by the filling of the pores or cavities of the organism with different minerals.

• Replacement: All the minerals originally present in the organism's body have been replaced by other minerals, making it appear stone-like. This is indicated by a noticeable difference between the weight of the fossil and the original organism.

• Recrystallization: All the minerals within an organism's body have changed into more complex mineral forms, such as carbonates transforming into calcite. Visually, it appears more beautiful, smooth, and regular. In terms of weight, it does not differ significantly from the original organism.

• Carbonization: The soft parts of an organism are buried by sediment containing high levels of carbon, resulting in preservation through carbon. Visually, this type of fossilization produces fossils that resemble imprints.

• Imprinting: Carbonization that occurs in plants. The most commonly found parts are the leaves.

ii. Trace Fossils: Fossils in the form of traces (not the body or whole body) of organisms that show how they lived, moved, their shape, etc. There are several types of trace fossils:

• Mold: An imprint of an organism's body, which can be either an internal mold (imprint of the inner body) or an external mold (imprint of the outer body).

• Cast: A mold that is filled with secondary minerals. A cast forms when the original minerals of the organism's body are dissolved, creating a mold that is then filled with other minerals.

• Burrow: A hole left by an organism's excavation in soft substrate.

• Boring: A hole left by an organism's excavation in hard substrate.

• Coprolite: Fossilized excrement or feces of an organism.

iii. Pseudofossils: False fossils, which appear to be fossils but are not actually fossils. There are several types of pseudofossils, such as: pseudomorphs, concretions, cone-in-cone structures, dendrites, root impressions, precipitate minerals, and more.

Dendrite

Rainwater that hits fine sediments

B. ROCKS

Rocks are collections of minerals that have solidified. There are three types of rocks:

a. Igneous Rocks: Rocks formed from the solidification of magma due to significant temperature differences. Igneous rocks can be classified as follows:

1. Based on acidity

• Ultrabasic Igneous Rocks: Igneous rocks with a SiO₂ composition of less than 45%. E.g., Peridotite.
• Basic Igneous Rocks: Igneous rocks with a SiO₂ composition ranging from 45% to 55%. E.g., Gabbro.
• Intermediate Igneous Rocks: Igneous rocks with a SiO₂ composition ranging from 55% to 65%. E.g., Diorite.

• Acidic Igneous Rocks: Igneous rocks with a SiO₂ composition greater than 65%. E.g., Granite.

2. Based on the place of formation

• Intrusive Igneous Rocks: Igneous rocks that form beneath the Earth's surface. They typically have a coarse texture due to the slow formation of their crystals, allowing the crystals to grow large and well-formed. E.g., Granite.
• Extrusive Igneous Rocks: Igneous rocks that form above the Earth's surface. They usually have a fine texture because their crystals form quickly, so they are not fully developed and are small in size. E.g., Obsidian.

b. Sedimentary Rocks: Rocks formed through the processes of weathering, erosion, transportation, and compaction (cementation and compaction). The composition of sedimentary rocks typically consists of fragments, a matrix, and cement. E.g., sandstone, salt rock, shale, etc.

c. Metamorphic Rocks: Rocks that undergo mineral recrystallization without melting, occurring beneath the Earth's surface and through isochemical processes (no change in chemical composition). Metamorphic rocks form due to high temperature, high pressure, or both. Metamorphic rocks can be classified as:

1. Contact Metamorphic Rocks: Formed due to the influence of high temperatures.

2. Dynamo Metamorphic Rocks: Formed due to the influence of high pressure.

3. Regional Metamorphic Rocks: Formed due to the influence of both high pressure and high temperature.