Praktikum Geologi Dasar - Pengenalan Tekstur dan Struktur Batuan Beku

Summary

This practicum session covers the fascinating world of igneous rocks, explaining their formation, textures, and structures. The definition of igneous rocks is explored, followed by an in-depth discussion on texture characteristics such as crystallization, grain size, and mineral shapes. The session also details various structures like vesicular and amygdaloidal, and provides step-by-step guidance on describing igneous rocks in the field, including color identification and sample measurements. A hands-on approach to classifying rocks is introduced, with a practical example of identifying and describing andesite, concluding with tips on photography and sketching for rock documentation.

Takeaways

• 😀 Igneous rocks are formed by the cooling or crystallization of magma, either underground (intrusive) or on the surface (extrusive).
• 😀 The texture of igneous rocks is categorized based on crystallization, granularity (grain size), and mineral shapes.
• 😀 The degree of crystallization is divided into three categories: holocrystalline (completely crystalline), hypocrystalline (a mix of crystals and glass), and holohyaline (completely glass).
• 😀 Granularity or grain size can be classified as phaneritic (large grains), porphyritic (mix of large and small grains), or aphanitic (fine grains).
• 😀 Mineral shapes in igneous rocks are categorized as anhedral, subhedral, or euhedral, based on the regularity of crystal boundaries.
• 😀 Texture also includes the presence of equigranular (uniform grain size) and inequigranular (uneven grain size) rock types.
• 😀 Igneous rock structures can be intrusive or extrusive, with examples like massive (uniform), vesicular (gas bubbles), and amygdaloidal (gas bubbles filled with other minerals).
• 😀 The cooling and crystallization process of magma can also lead to columnar jointing, sheet-shaped joints, or pillow lava structures.
• 😀 When describing igneous rocks, record the sample's color (weathered and fresh), crystallization type, grain size, and mineral shape.
• 😀 Photos of igneous rock samples should include a comparison object to show scale, with measurements for length, height, and width recorded accurately.
• 😀 The rock’s formation process and classification (extrusive or hypabyssal) should be noted, followed by a sketch of the sample at a scaled size.

Q & A

What are igneous rocks and how are they formed?

Igneous rocks are rocks formed from the cooling and crystallization of magma. This process can occur both inside the Earth (intrusive) or on the surface (extrusive).

What are the main textures of igneous rocks?

The main textures of igneous rocks are crystallization degree and granularity. The crystallization degree includes holocrystalline, hypocrystalline, and holohyaline, while granularity refers to the size of the mineral grains, which can be phaneritic, porphyritic, or aphanitic.

What is the difference between holocrystalline, hypocrystalline, and holohyaline textures?

Holocrystalline textures are formed entirely from crystals, hypocrystalline has a mixture of crystals and glass, and holohyaline is composed entirely of glass.

How are igneous rock textures categorized based on mineral grain sizes?

Textures are categorized into phaneritic (large grains), porphyritic (a mix of large and small grains), and aphanitic (fine-grained minerals).

What are the differences between the crystal shapes in igneous rocks?

The crystal shapes are categorized as anhedral (irregular), subhedral (transitional), and euhedral (perfectly formed crystals).

What is the significance of the granularity classification in igneous rocks?

Granularity helps determine the size of the rock-forming mineral grains, which can impact the rock's appearance and texture. Phaneritic granularity has large grains, porphyritic has a mix, and aphanitic has fine grains.

What does 'equigranular' and 'inequigranular' mean in terms of igneous rocks?

'Equigranular' means the grains are roughly the same size, while 'inequigranular' means the grains vary in size, with some being large and others small or fine.

What is the vesicular structure in igneous rocks?

The vesicular structure occurs when gas bubbles get trapped in the rock as it cools, creating holes in the rock.

What is the difference between intrusive and extrusive igneous rock structures?

Intrusive structures form from magma that cools beneath the Earth's surface, leading to rocks like granite. Extrusive structures form from lava that cools on the surface, such as basalt or pumice.

How can you describe igneous rocks in a field study?

To describe igneous rocks, you need to note the sample number, color (both weathered and fresh), texture (crystallization and granularity), and structure. The final classification includes naming the rock type, noting its formation process, and capturing images and sketches for documentation.