Question

What distinguishes metamorphic rocks from igneous and sedimentary rocks?

• A. They are formed from the cooling of molten lava
• B. They are formed from the accumulation of sediments
• C. They are formed from the alteration of pre-existing rocks
• D. They are formed from the compression of organic remains

Hint:

• Metamorphic rocks are "changed form" rocks.
• They originate from pre-existing rocks (igneous, sedimentary, or other metamorphic rocks).
• The transformation occurs due to heat, pressure, and/or chemically active fluids, without complete melting.
• This changes the rock's mineralogy, texture, and chemical composition.

Answer 1

The Correct Option is C

The three main types of rocks are igneous, sedimentary, and metamorphic, distinguished by their mode of formation:

• Igneous Rocks (related to option a): Formed from the cooling and solidification of molten rock material (magma below the surface or lava on the surface). Examples: Granite, Basalt.
• Sedimentary Rocks (related to option b): Formed from the accumulation, compaction, and cementation (lithification) of sediments (e.g., rock fragments, mineral grains, shells, organic remains). Examples: Sandstone, Shale, Limestone. The compression of organic remains (option d) can lead to specific types of sedimentary rocks like coal.
• Metamorphic Rocks (option c): Formed when pre-existing rocks (igneous, sedimentary, or even other metamorphic rocks) are changed (metamorphosed) by exposure to high temperatures, high pressures, chemically active fluids, or a combination of these, without melting completely. This alteration changes the rock's mineralogy, texture, and sometimes chemical composition. Examples: Marble (from limestone), Slate (from shale), Gneiss (from granite or other rocks).

Therefore, the distinguishing characteristic of metamorphic rocks is that they are formed from the alteration (metamorphism) of pre-existing rocks. Option (a) describes igneous rock formation. Option (b) describes sedimentary rock formation. Option (d) describes the formation of specific organic sedimentary rocks (like coal). \[ \boxed{\text{They are formed from the alteration of pre-existing rocks}} \]