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Obsidian: The Natural Glass Rock — Reading Comprehension

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MS-ESS2-3
MS-ESS3-2
RST.6-8.4

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This science reading passage explores obsidian, a volcanic glass that forms when lava cools so rapidly that crystals cannot develop. Written for middle school students, the content clearly explains that obsidian is an igneous rock, specifically a volcanic rock, answering the key question 'What type of rock is obsidian?' The passage aligns with NGSS standard MS-ESS2-3 (Earth's Systems) by examining how Earth materials form through specific geological processes. The text covers obsidian's formation process, unique glassy appearance, physical properties including conchoidal fracture and extremely sharp edges, geographic distribution near silica-rich volcanoes, and historical human uses from ancient tools to modern applications. The accompanying 8 multiple-choice questions assess students' comprehension of main ideas, key details, and vocabulary in alignment with Common Core Reading Informational Text standards. This resource helps students understand volcanic processes, rock classification, and the relationship between cooling rate and rock texture within Earth's systems.

CONTENT PREVIEW

Have you ever seen a rock that looks like black glass? If so, you might have found obsidian. This unique rock forms when lava cools so quickly that crystals don't have time to grow.

What Type of Rock Is Obsidian?

Obsidian is an igneous rock. Igneous rocks form when hot, liquid rock cools and hardens. There are two main groups of igneous rocks - volcanic and plutonic. Obsidian is a volcanic rock because it forms when lava (melted rock above ground. cools very fast.

Unlike most rocks, obsidian has no crystals that you can see. When lava cools slowly, minerals in the lava have time to form crystals. But when lava cools very quickly - like when it flows into water or is exposed to cold air - the minerals freeze in place before crystals can form. That's why obsidian looks like glass.

How Obsidian Forms

Obsidian forms most often when thick, sticky lava rich in silica cools rapidly. This usually happens when:

● Lava flows into water

● Thin edges of lava flows cool in air

● Lava sprays into the air during an eruption and cools before landing

The quick cooling is key. If the same lava cooled slowly underground, it would form a rock with visible crystals, like granite or rhyolite.

Properties of Obsidian

Obsidian has some special properties that make it different from other rocks:

● Color: Usually black, but can also be brown, tan, green, or even rainbow-colored

● Shine: Glassy and shiny

● Break Pattern: Breaks with curved surfaces (called conchoidal fracture)

● Edges: Can form extremely sharp edges when broken

● Hardness: About 5-6 on the mineral hardness scale

When obsidian breaks, it forms curved, shell-like surfaces and very sharp edges. In fact, obsidian edges can be sharper than steel - some surgeons even use obsidian scalpels for delicate operations!

Where Obsidian Is Found

You can find obsidian near volcanoes that have thick, silica-rich lava. Some well-known places for obsidian include:

● Glass Mountain in California

● Yellowstone National Park in Wyoming

● The islands of Iceland and Japan

● Various locations in Mexico, Greece, and Turkey

Obsidian doesn't last forever in nature. Over time, it slowly transforms into other minerals. Most obsidian found on Earth is less than 20 million years old - which is young in geologic time!

How People Have Used Obsidian

Throughout history, people have valued obsidian for its sharp edges and beautiful appearance. Ancient cultures used obsidian to make:

● Cutting tools and weapons

● Mirrors

● Arrowheads and spear points

● Jewelry and decorative items

Archaeologists can sometimes trace where ancient obsidian tools came from by analyzing their chemical makeup. This helps them understand ancient trade routes.

Key Points to Remember:

● Obsidian is an igneous rock that forms when lava cools very quickly

● It looks like glass because crystals don't have time to form

● Obsidian breaks with curved surfaces and extremely sharp edges

● People have used obsidian for tools and ornaments for thousands of years

Fun Fact: Obsidian edges can be so incredibly sharp that they create cutting edges as thin as 3 nanometers—that's about 500 times sharper than the best surgical steel scalpels! This extreme sharpness has led some modern surgeons to use obsidian scalpels for delicate operations, as they cause less tissue damage and can lead to faster healing.

What type of rock is obsidian classified as?

MetamorphicSedimentaryIgneousMineral

Why does obsidian look like glass?

High silica contentForms underwaterRapid coolingNatural polishing

Which is NOT mentioned as a place where obsidian can be found?

Glass MountainGrand CanyonYellowstone ParkIceland

What is the main idea of the passage?

Obsidian's unique formation and propertiesAncient uses of obsidianVarious colors of obsidianVolcanic rock classifications

In the passage, what does "conchoidal fracture" refer to?

Crystal patternsCurved breaking surfacesSharp edgesRainbow colors

Based on the passage, what does "silica-rich" mean?

Contains specific mineralsFlows quicklyExtremely hotPartially cooled