EESC 3000: Igneous and Metamorphic Petrology

IGNEOUS LAB 1: VOLCANIC ROCKS AND TEXTURES

Learning Objectives:  Students will be able to identify volcanic textures in hand-sample  Students will be able to classify volcanic rocks based on texture

Key Minerals: None

PYROCLASTIC ROCKS

Pyroclastic rocks are classified principally based on grain size, and secondarily on composition:

Ash <2mm Lapilli 2-64 mm Block/Bomb >64mm

Examine the samples of tephra of various sizes: ash (Mt. St. Helens, WA), laplli (Flagstaff, AZ), and a bomb (Death Valley, CA). Then examine the three pyroclastic rocks. Determine the percentage of components (ash, lapilli, blocks/bombs) and classify each based on grain size. If possible, determine the composition of the tephra (glass, crystals, rock fragments) and add a modifier to each rock name (e.g., lithic Lapilli-tuff). Use a binocular microscope to aid in tephra identification.

Pyroclastic Rock 1: ______

Pyroclastic Rock 2: ______

Pyroclastic Rock 3: ______

1 The temperature of tephra when deposited can affect the texture of the resulting pyroclastic rock. If the tephra is deposited while hot then the individual pieces will be welded together, and so is referred to as a welded tuff. Because the hot fragments are still ductile, the weight of the overlying rock material may cause the tephra to flatten to form a foliated texture referred to as eutaxitic texture.

Examine the samples of unwelded and welded tuff from the same volcanic sequence in Shoshone, CA. Identify the composition of the lapilli in the unwelded lapilli tuff and find the corresponding clasts in the welded tuff. Describe how the tephra changed in the welding process and make sketches.

Unwelded Tuff Welded Tuff

Description: Description:

VOLCANIC FLOWS

Lava flows may develop surface features or textures due to movement of the lava. In basaltic lavas, the top surface quenches and forms a solid crust. When the crust is thin and the underlying lava is flowing readily, then the surface will fold into a continuous, undulating, ropy form that is referred to as pahoehoe. If the lava is more viscous, and the surface rock thicker, then the quenched surface rock will break into rough, angular chunks, and is referred to as aa.

Examine the samples of pahoehoe and aa basalt and familiarize yourself with their appearance. Sketch these flow-top features.

Pahoehoe Aa

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Intermediate to felsic flows are much more viscous, and so flow is much slower and solid components are less likely to sink or mix. In addition, friction occurs along the contact with the underlying rock and this may result in laminar flow in which crystals, fragments, and irregularities may get entrained. This process develops a texture referred to as flow banding, and it is common in obsidian and rhyolite flows.

Examine the samples of planar, irregular and folded flow banding, and explain how you would differentiate the following:

Flow banding vs bedding ______

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Flow banding vs tectonic folding ______

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Volcanic flows may also contain non-planar features. Some of the most common are: Vesicles: Pits that form in lava flows due to the release of dissolved gases to form bubbles. Vesicles are most common at the tops of lava flows. A volcanic rock with vesicules is referred to as vesicular.

Amygdules: Vesicles that have been filled by growth of secondary minerals (commonly calcite, quartz, chlorite, zeolites, prehnite, pumpellyite, epidote). A volcanic rock with amygdules is referred to as amygduloidal.

Phenocrysts: Crystals that are distinctly larger than the groundmass of the igneous rock that host them. Phenocrysts crystallize early from the magma. An igneous rock with phenocrysts is referred to as porphyritic.

Spherules: Radial aggregates of cristobalite and feldspar that form around nucleation points in glassy felsic rocks. They are a devitrification texture that may overprint primary igneous textures. An igneous rock with spherules is referred to as spherulitic.

Examine the samples of vesicular and amygduloidal basalt, prophyritic dacite, and spherulitic rhyolite and obsidian (commonly called snowflake obsidian). Describe how you would distinguish these features from each other (and clasts) in the following table:

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Clast Amygdule Phenocryst Spherule Clast

Amygdule

Phenocryst

Spherule

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