Question

obsidian, a fine - grained igneous rock, has a glass - like smooth texture (see picture below). which of the following is correct? it formed from quickly cooling magma above the surface it has been altered by high heat and high pressure from granite it formed from slowly cooling magma below the surface it is made of very fine rock fragments

Explanation:

Obsidian is an igneous rock. Igneous rocks form from magma or lava. Obsidian has a glass - like texture because it forms from quickly cooling magma (lava, since it's above the surface). When magma cools quickly, there is no time for large crystals to form, resulting in a glassy texture.

• Option 2: Alteration by high heat and high pressure refers to metamorphic rocks, not igneous rocks like obsidian.
• Option 3: Slowly cooling magma below the surface forms coarse - grained igneous rocks (e.g., granite), not obsidian.
• Option 4: Rocks made of very fine rock fragments are sedimentary rocks (e.g., shale), not igneous rocks like obsidian.

Answer:

it formed from quickly cooling magma above the surface