"An altar of earth you shall make for Me" (Exodus 20:21): a distinct altar you shall make for Me, that you should not build it at the outset for the sake of something else. Rabbi Ishmael says: an altar joined to the ground you shall make for Me, that you should not build it upon arches nor upon pillars. Rabbi Nathan says: a hollow altar of earth you shall make for Me, as it is said, "Hollow with boards you shall make it" (Exodus 27:8). Issi ben Akiva says: an altar of bronze filled with earth, as it is said, "For the bronze altar that was before the LORD was too small to hold" and so on (1 Kings 8:64). And was it small? Has it not already said, "Solomon offered a thousand burnt offerings" (2 Chronicles 1:6)? And what does "too small" teach? Rather, it teaches that on the day this one was built, that one was hidden away.
The Earthen Altar Set Apart and Joined to the Ground
Yalkut Shimoni on Torah 303:4
מִזְבַּח אֲדָמָה תַּעֲשֶׂה לִי, מִזְבֵּחַ מְיֻחָד תַּעֲשֶׂה לִי, שֶׁלֹּא תִּבְנֵהוּ בִּתְחִלָּה לְשֵׁם דָּבָר אַחֵר. רַבִּי יִשְׁמָעֵאל אוֹמֵר, מִזְבַּח מְחֻבָּר בַּאֲדָמָה תַּעֲשֶׂה לִי, שֶׁלֹּא תִּבְנֵהוּ עַל גַּבֵּי כֵּפִים וְלֹא עַל גַּבֵּי עֲמוּדִים. רַבִּי נָתָן אוֹמֵר, מִזְבַּח חָלוּל בַּאֲדָמָה תַּעֲשֶׂה לִי, שֶׁנֶּאֱמַר (שמות כז, ח) "נְבוּב לֻחֹת תַּעֲשֶׂה אֹתוֹ". אִסִּי בֶּן עֲקִיבָא אוֹמֵר, מִזְבַּח נְחשֶׁת מָלֵא אֲדָמָה, שֶׁנֶּאֱמַר "כִּי מִזְבַּח הַנְּחֹשֶׁת אֲשֶׁר לִפְנֵי ה' קָטָן מֵהָכִיל" וְגוֹ', וְכִי קָטָן הָיָה, וַהֲלֹא כְּבָר נֶאֱמַר (שם ג, ד) "אֶלֶף עֹלוֹת יַעֲלֶה שְׁלֹמֹה", וּמַה תַּלְמוּד לוֹמַר "קָטָן", אֶלָּא מְלַמֵּד בְּיוֹם שֶׁנִּבְנָה זֶה נִגְנַז זֶה.