Evolution of the earth

The direct answer is A: repeated impacts increased extreme heat and melting on early Earth. During planetary formation, planetesimals and other bodies struck the growing Earth at high speed. Their motion energy changed largely into heat on impact. The growing mass also produced pressure and additional internal heating, while radioactive materials contributed heat. As a result, much of the early surface and interior became molten or partially molten, making Earth hot and unstable. The reasoning is: many high-energy impacts → impact energy becomes heat → temperature rises → rocks melt and the surface remains partly molten. Option A is correct because it identifies the physical result of repeated collisions. Option B is wrong because dense greenery requires a later stable environment, liquid water, soil, and life; it could not exist during this intensely hot stage. Option C is wrong because repeated impacts worked against a permanent ice sheet by supplying heat. Option D is wrong because humans appeared vastly later and could not live on the early molten planet. “Extreme heat and melting” does not mean every part was always completely liquid; it means impacts strongly promoted heating and melting. Memory cue: early impacts were like repeated giant blows, converting motion into heat.

Density difference arranged materials into different layers. Exam tip: treat it as the basis of differentiation.

The governing concept is differentiation of the early Earth. When the young Earth was extremely hot and partly molten, materials separated according to density. Dense metallic elements, especially iron and nickel, moved downward under gravity and accumulated at the centre, forming the core. Lighter silicate materials remained above and contributed to the mantle and crust. Therefore, option D is correct. The crust is the thin outer rocky layer, the ocean is a water body rather than an internal metallic layer, and the atmosphere is the gaseous envelope. This density-based separation explains the broad layered structure of Earth.

The governing process was cooling and solidification during the early evolution of Earth. The newly formed planet initially contained abundant heat from accretion, compression, radioactive decay, and impacts, so much of its outer material was molten or semi-molten. As the surface lost heat to space, it cooled and hardened into the first solid crust. Hence option A is the only scientifically appropriate answer. Atmospheric disappearance is not the direct mechanism, and farming occurred billions of years later. Ozone damage is a modern atmospheric issue and has no role in creating the original rocky outer layer.

Gases and water vapour from the interior formed a new atmosphere. Exam tip: remember Earth's interior as the source of secondary atmosphere.

Solar wind removed light gases like hydrogen and helium. Exam tip: remember the reason for loss of the first atmosphere.

Condensation of water vapour formed clouds and caused long rainfall. Exam tip: remember rainfall after condensation.

Continuous rainwater collected in low areas and formed oceans. Exam tip: link ocean formation with rainfall.

Lithosphere is linked with Earth's solid rocky part. Exam tip: connect it with crust and upper mantle.

Oceans provided a medium for pre-life chemical reactions. Exam tip: connect oceans with the beginning of life.

Early organisms are considered simple and microscopic. Exam tip: remember life evolution from simple to complex.

Free oxygen was very low before photosynthesis. Exam tip: connect oxygen rise with photosynthesis.

Photosynthetic organisms increased oxygen accumulation. Exam tip: treat it as a biological stage of atmospheric evolution.

The direct answer is D, oxygen. The ozone layer is made mainly of ozone, which contains three oxygen atoms. First, early Earth had very little free oxygen. Photosynthetic organisms later released oxygen into the atmosphere. Sunlight split some oxygen molecules, and the oxygen atoms joined other oxygen molecules to form ozone. Ozone then collected in the upper atmosphere and absorbed harmful ultraviolet radiation. A, helium, is a light noble gas and does not form ozone. B, methane, is a carbon compound and is not the main raw material for ozone. C, ammonia contains nitrogen and hydrogen and is not the required gas. D is correct because rising atmospheric oxygen made ozone formation possible. Memory cue: more free oxygen means the possibility of an ozone shield.

The ozone layer protects life by blocking ultraviolet rays. Exam tip: treat ozone as a protective shield.

Water and a favourable atmosphere were necessary for the biosphere. Exam tip: link biosphere with life-supporting conditions.

Crust formation created a solid surface on which landforms developed. Exam tip: treat crust as the base of surface features.

Water vapour cooled into rain and helped form the hydrosphere. Exam tip: remember the volcano vapour rain sequence.

Crust water atmosphere and life formed in gradual stages. Exam tip: check chronology in sequence questions.

Low free oxygen was a major difference from today's atmosphere. Exam tip: keep oxygen as the main point in comparison.

Internal heat kept volcanic activity active for a long time. Exam tip: connect it with gas release.

Formation of simple organic substances before life was chemical evolution. Exam tip: place chemical evolution before biological evolution.

Chemical reactions in oceans are linked with origin of life. Exam tip: connect seas with chemical evolution.

Early life developed in oxygen-poor conditions. Exam tip: link early life with low oxygen conditions.

Ocean water absorbs heat slowly and releases it slowly. Therefore, after oceans formed, temperature fluctuations at Earth’s surface were moderated and surface conditions became relatively more stable. Option A is incorrect because temperature variations do not disappear completely. Exam tip: link water’s high heat capacity with moderation of temperature.